xlog.c

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/*-------------------------------------------------------------------------
 *
 * xlog.c
 *      PostgreSQL transaction log manager
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * src/backend/access/transam/xlog.c
 *
 *-------------------------------------------------------------------------
 */

#include "postgres.h"

#include <ctype.h>
#include <time.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <unistd.h>

#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
#include "access/timeline.h"
#include "access/transam.h"
#include "access/tuptoaster.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog_internal.h"
#include "access/xlogreader.h"
#include "access/xlogutils.h"
#include "catalog/catversion.h"
#include "catalog/pg_control.h"
#include "catalog/pg_database.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "postmaster/bgwriter.h"
#include "postmaster/startup.h"
#include "replication/walreceiver.h"
#include "replication/walsender.h"
#include "storage/bufmgr.h"
#include "storage/fd.h"
#include "storage/ipc.h"
#include "storage/latch.h"
#include "storage/pmsignal.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/reinit.h"
#include "storage/smgr.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/guc.h"
#include "utils/ps_status.h"
#include "utils/relmapper.h"
#include "utils/snapmgr.h"
#include "utils/timestamp.h"
#include "pg_trace.h"

extern uint32 bootstrap_data_checksum_version;

/* File path names (all relative to $PGDATA) */
#define RECOVERY_COMMAND_FILE   "recovery.conf"
#define RECOVERY_COMMAND_DONE   "recovery.done"
#define PROMOTE_SIGNAL_FILE "promote"
#define FAST_PROMOTE_SIGNAL_FILE "fast_promote"


/* User-settable parameters */
int         CheckPointSegments = 3;
int         wal_keep_segments = 0;
int         XLOGbuffers = -1;
int         XLogArchiveTimeout = 0;
bool        XLogArchiveMode = false;
char       *XLogArchiveCommand = NULL;
bool        EnableHotStandby = false;
bool        fullPageWrites = true;
bool        log_checkpoints = false;
int         sync_method = DEFAULT_SYNC_METHOD;
int         wal_level = WAL_LEVEL_MINIMAL;
int         CommitDelay = 0;    /* precommit delay in microseconds */
int         CommitSiblings = 5; /* # concurrent xacts needed to sleep */

#ifdef WAL_DEBUG
bool        XLOG_DEBUG = false;
#endif

/*
 * XLOGfileslop is the maximum number of preallocated future XLOG segments.
 * When we are done with an old XLOG segment file, we will recycle it as a
 * future XLOG segment as long as there aren't already XLOGfileslop future
 * segments; else we'll delete it.  This could be made a separate GUC
 * variable, but at present I think it's sufficient to hardwire it as
 * 2*CheckPointSegments+1.  Under normal conditions, a checkpoint will free
 * no more than 2*CheckPointSegments log segments, and we want to recycle all
 * of them; the +1 allows boundary cases to happen without wasting a
 * delete/create-segment cycle.
 */
#define XLOGfileslop    (2*CheckPointSegments + 1)


/*
 * GUC support
 */
const struct config_enum_entry sync_method_options[] = {
    {"fsync", SYNC_METHOD_FSYNC, false},
#ifdef HAVE_FSYNC_WRITETHROUGH
    {"fsync_writethrough", SYNC_METHOD_FSYNC_WRITETHROUGH, false},
#endif
#ifdef HAVE_FDATASYNC
    {"fdatasync", SYNC_METHOD_FDATASYNC, false},
#endif
#ifdef OPEN_SYNC_FLAG
    {"open_sync", SYNC_METHOD_OPEN, false},
#endif
#ifdef OPEN_DATASYNC_FLAG
    {"open_datasync", SYNC_METHOD_OPEN_DSYNC, false},
#endif
    {NULL, 0, false}
};

/*
 * Statistics for current checkpoint are collected in this global struct.
 * Because only the background writer or a stand-alone backend can perform
 * checkpoints, this will be unused in normal backends.
 */
CheckpointStatsData CheckpointStats;

/*
 * ThisTimeLineID will be same in all backends --- it identifies current
 * WAL timeline for the database system.
 */
TimeLineID  ThisTimeLineID = 0;

/*
 * Are we doing recovery from XLOG?
 *
 * This is only ever true in the startup process; it should be read as meaning
 * "this process is replaying WAL records", rather than "the system is in
 * recovery mode".  It should be examined primarily by functions that need
 * to act differently when called from a WAL redo function (e.g., to skip WAL
 * logging).  To check whether the system is in recovery regardless of which
 * process you're running in, use RecoveryInProgress() but only after shared
 * memory startup and lock initialization.
 */
bool        InRecovery = false;

/* Are we in Hot Standby mode? Only valid in startup process, see xlog.h */
HotStandbyState standbyState = STANDBY_DISABLED;

static XLogRecPtr LastRec;

/* Local copy of WalRcv->receivedUpto */
static XLogRecPtr receivedUpto = 0;
static TimeLineID receiveTLI = 0;

/*
 * During recovery, lastFullPageWrites keeps track of full_page_writes that
 * the replayed WAL records indicate. It's initialized with full_page_writes
 * that the recovery starting checkpoint record indicates, and then updated
 * each time XLOG_FPW_CHANGE record is replayed.
 */
static bool lastFullPageWrites;

/*
 * Local copy of SharedRecoveryInProgress variable. True actually means "not
 * known, need to check the shared state".
 */
static bool LocalRecoveryInProgress = true;

/*
 * Local copy of SharedHotStandbyActive variable. False actually means "not
 * known, need to check the shared state".
 */
static bool LocalHotStandbyActive = false;

/*
 * Local state for XLogInsertAllowed():
 *      1: unconditionally allowed to insert XLOG
 *      0: unconditionally not allowed to insert XLOG
 *      -1: must check RecoveryInProgress(); disallow until it is false
 * Most processes start with -1 and transition to 1 after seeing that recovery
 * is not in progress.  But we can also force the value for special cases.
 * The coding in XLogInsertAllowed() depends on the first two of these states
 * being numerically the same as bool true and false.
 */
static int  LocalXLogInsertAllowed = -1;

/*
 * When ArchiveRecoveryRequested is set, archive recovery was requested,
 * ie. recovery.conf file was present. When InArchiveRecovery is set, we are
 * currently recovering using offline XLOG archives. These variables are only
 * valid in the startup process.
 *
 * When ArchiveRecoveryRequested is true, but InArchiveRecovery is false, we're
 * currently performing crash recovery using only XLOG files in pg_xlog, but
 * will switch to using offline XLOG archives as soon as we reach the end of
 * WAL in pg_xlog.
*/
bool ArchiveRecoveryRequested = false;
bool InArchiveRecovery = false;

/* Was the last xlog file restored from archive, or local? */
static bool restoredFromArchive = false;

/* options taken from recovery.conf for archive recovery */
char *recoveryRestoreCommand = NULL;
static char *recoveryEndCommand = NULL;
static char *archiveCleanupCommand = NULL;
static RecoveryTargetType recoveryTarget = RECOVERY_TARGET_UNSET;
static bool recoveryTargetInclusive = true;
static bool recoveryPauseAtTarget = true;
static TransactionId recoveryTargetXid;
static TimestampTz recoveryTargetTime;
static char *recoveryTargetName;

/* options taken from recovery.conf for XLOG streaming */
static bool StandbyModeRequested = false;
static char *PrimaryConnInfo = NULL;
static char *TriggerFile = NULL;

/* are we currently in standby mode? */
bool StandbyMode = false;

/* whether request for fast promotion has been made yet */
static bool fast_promote = false;

/* if recoveryStopsHere returns true, it saves actual stop xid/time/name here */
static TransactionId recoveryStopXid;
static TimestampTz recoveryStopTime;
static char recoveryStopName[MAXFNAMELEN];
static bool recoveryStopAfter;

/*
 * During normal operation, the only timeline we care about is ThisTimeLineID.
 * During recovery, however, things are more complicated.  To simplify life
 * for rmgr code, we keep ThisTimeLineID set to the "current" timeline as we
 * scan through the WAL history (that is, it is the line that was active when
 * the currently-scanned WAL record was generated).  We also need these
 * timeline values:
 *
 * recoveryTargetTLI: the desired timeline that we want to end in.
 *
 * recoveryTargetIsLatest: was the requested target timeline 'latest'?
 *
 * expectedTLEs: a list of TimeLineHistoryEntries for recoveryTargetTLI and the timelines of
 * its known parents, newest first (so recoveryTargetTLI is always the
 * first list member).  Only these TLIs are expected to be seen in the WAL
 * segments we read, and indeed only these TLIs will be considered as
 * candidate WAL files to open at all.
 *
 * curFileTLI: the TLI appearing in the name of the current input WAL file.
 * (This is not necessarily the same as ThisTimeLineID, because we could
 * be scanning data that was copied from an ancestor timeline when the current
 * file was created.)  During a sequential scan we do not allow this value
 * to decrease.
 */
static TimeLineID recoveryTargetTLI;
static bool recoveryTargetIsLatest = false;
static List *expectedTLEs;
static TimeLineID curFileTLI;

/*
 * ProcLastRecPtr points to the start of the last XLOG record inserted by the
 * current backend.  It is updated for all inserts.  XactLastRecEnd points to
 * end+1 of the last record, and is reset when we end a top-level transaction,
 * or start a new one; so it can be used to tell if the current transaction has
 * created any XLOG records.
 */
static XLogRecPtr ProcLastRecPtr = InvalidXLogRecPtr;

XLogRecPtr  XactLastRecEnd = InvalidXLogRecPtr;

/*
 * RedoRecPtr is this backend's local copy of the REDO record pointer
 * (which is almost but not quite the same as a pointer to the most recent
 * CHECKPOINT record).  We update this from the shared-memory copy,
 * XLogCtl->Insert.RedoRecPtr, whenever we can safely do so (ie, when we
 * hold the Insert lock).  See XLogInsert for details.  We are also allowed
 * to update from XLogCtl->Insert.RedoRecPtr if we hold the info_lck;
 * see GetRedoRecPtr.  A freshly spawned backend obtains the value during
 * InitXLOGAccess.
 */
static XLogRecPtr RedoRecPtr;

/*
 * RedoStartLSN points to the checkpoint's REDO location which is specified
 * in a backup label file, backup history file or control file. In standby
 * mode, XLOG streaming usually starts from the position where an invalid
 * record was found. But if we fail to read even the initial checkpoint
 * record, we use the REDO location instead of the checkpoint location as
 * the start position of XLOG streaming. Otherwise we would have to jump
 * backwards to the REDO location after reading the checkpoint record,
 * because the REDO record can precede the checkpoint record.
 */
static XLogRecPtr RedoStartLSN = InvalidXLogRecPtr;

/*----------
 * Shared-memory data structures for XLOG control
 *
 * LogwrtRqst indicates a byte position that we need to write and/or fsync
 * the log up to (all records before that point must be written or fsynced).
 * LogwrtResult indicates the byte positions we have already written/fsynced.
 * These structs are identical but are declared separately to indicate their
 * slightly different functions.
 *
 * To read XLogCtl->LogwrtResult, you must hold either info_lck or
 * WALWriteLock.  To update it, you need to hold both locks.  The point of
 * this arrangement is that the value can be examined by code that already
 * holds WALWriteLock without needing to grab info_lck as well.  In addition
 * to the shared variable, each backend has a private copy of LogwrtResult,
 * which is updated when convenient.
 *
 * The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
 * (protected by info_lck), but we don't need to cache any copies of it.
 *
 * info_lck is only held long enough to read/update the protected variables,
 * so it's a plain spinlock.  The other locks are held longer (potentially
 * over I/O operations), so we use LWLocks for them.  These locks are:
 *
 * WALInsertLock: must be held to insert a record into the WAL buffers.
 *
 * WALWriteLock: must be held to write WAL buffers to disk (XLogWrite or
 * XLogFlush).
 *
 * ControlFileLock: must be held to read/update control file or create
 * new log file.
 *
 * CheckpointLock: must be held to do a checkpoint or restartpoint (ensures
 * only one checkpointer at a time; currently, with all checkpoints done by
 * the checkpointer, this is just pro forma).
 *
 *----------
 */

typedef struct XLogwrtRqst
{
    XLogRecPtr  Write;          /* last byte + 1 to write out */
    XLogRecPtr  Flush;          /* last byte + 1 to flush */
} XLogwrtRqst;

typedef struct XLogwrtResult
{
    XLogRecPtr  Write;          /* last byte + 1 written out */
    XLogRecPtr  Flush;          /* last byte + 1 flushed */
} XLogwrtResult;

/*
 * Shared state data for XLogInsert.
 */
typedef struct XLogCtlInsert
{
    XLogRecPtr  PrevRecord;     /* start of previously-inserted record */
    int         curridx;        /* current block index in cache */
    XLogPageHeader currpage;    /* points to header of block in cache */
    char       *currpos;        /* current insertion point in cache */
    XLogRecPtr  RedoRecPtr;     /* current redo point for insertions */
    bool        forcePageWrites;    /* forcing full-page writes for PITR? */

    /*
     * fullPageWrites is the master copy used by all backends to determine
     * whether to write full-page to WAL, instead of using process-local one.
     * This is required because, when full_page_writes is changed by SIGHUP,
     * we must WAL-log it before it actually affects WAL-logging by backends.
     * Checkpointer sets at startup or after SIGHUP.
     */
    bool        fullPageWrites;

    /*
     * exclusiveBackup is true if a backup started with pg_start_backup() is
     * in progress, and nonExclusiveBackups is a counter indicating the number
     * of streaming base backups currently in progress. forcePageWrites is set
     * to true when either of these is non-zero. lastBackupStart is the latest
     * checkpoint redo location used as a starting point for an online backup.
     */
    bool        exclusiveBackup;
    int         nonExclusiveBackups;
    XLogRecPtr  lastBackupStart;
} XLogCtlInsert;

/*
 * Shared state data for XLogWrite/XLogFlush.
 */
typedef struct XLogCtlWrite
{
    int         curridx;        /* cache index of next block to write */
    pg_time_t   lastSegSwitchTime;      /* time of last xlog segment switch */
} XLogCtlWrite;

/*
 * Total shared-memory state for XLOG.
 */
typedef struct XLogCtlData
{
    /* Protected by WALInsertLock: */
    XLogCtlInsert Insert;

    /* Protected by info_lck: */
    XLogwrtRqst LogwrtRqst;
    uint32      ckptXidEpoch;   /* nextXID & epoch of latest checkpoint */
    TransactionId ckptXid;
    XLogRecPtr  asyncXactLSN;   /* LSN of newest async commit/abort */
    XLogSegNo   lastRemovedSegNo; /* latest removed/recycled XLOG segment */

    /* Fake LSN counter, for unlogged relations. Protected by ulsn_lck */
    XLogRecPtr  unloggedLSN;
    slock_t     ulsn_lck;

    /* Protected by WALWriteLock: */
    XLogCtlWrite Write;

    /*
     * Protected by info_lck and WALWriteLock (you must hold either lock to
     * read it, but both to update)
     */
    XLogwrtResult LogwrtResult;

    /*
     * These values do not change after startup, although the pointed-to pages
     * and xlblocks values certainly do.  Permission to read/write the pages
     * and xlblocks values depends on WALInsertLock and WALWriteLock.
     */
    char       *pages;          /* buffers for unwritten XLOG pages */
    XLogRecPtr *xlblocks;       /* 1st byte ptr-s + XLOG_BLCKSZ */
    int         XLogCacheBlck;  /* highest allocated xlog buffer index */

    /*
     * Shared copy of ThisTimeLineID. Does not change after end-of-recovery.
     * If we created a new timeline when the system was started up,
     * PrevTimeLineID is the old timeline's ID that we forked off from.
     * Otherwise it's equal to ThisTimeLineID.
     */
    TimeLineID  ThisTimeLineID;
    TimeLineID  PrevTimeLineID;

    /*
     * archiveCleanupCommand is read from recovery.conf but needs to be in
     * shared memory so that the checkpointer process can access it.
     */
    char        archiveCleanupCommand[MAXPGPATH];

    /*
     * SharedRecoveryInProgress indicates if we're still in crash or archive
     * recovery.  Protected by info_lck.
     */
    bool        SharedRecoveryInProgress;

    /*
     * SharedHotStandbyActive indicates if we're still in crash or archive
     * recovery.  Protected by info_lck.
     */
    bool        SharedHotStandbyActive;

    /*
     * WalWriterSleeping indicates whether the WAL writer is currently in
     * low-power mode (and hence should be nudged if an async commit occurs).
     * Protected by info_lck.
     */
    bool        WalWriterSleeping;

    /*
     * recoveryWakeupLatch is used to wake up the startup process to continue
     * WAL replay, if it is waiting for WAL to arrive or failover trigger file
     * to appear.
     */
    Latch       recoveryWakeupLatch;

    /*
     * During recovery, we keep a copy of the latest checkpoint record here.
     * Used by the background writer when it wants to create a restartpoint.
     *
     * Protected by info_lck.
     */
    XLogRecPtr  lastCheckPointRecPtr;
    CheckPoint  lastCheckPoint;

    /*
     * lastReplayedEndRecPtr points to end+1 of the last record successfully
     * replayed. When we're currently replaying a record, ie. in a redo
     * function, replayEndRecPtr points to the end+1 of the record being
     * replayed, otherwise it's equal to lastReplayedEndRecPtr.
     */
    XLogRecPtr  lastReplayedEndRecPtr;
    TimeLineID  lastReplayedTLI;
    XLogRecPtr  replayEndRecPtr;
    TimeLineID  replayEndTLI;
    /* timestamp of last COMMIT/ABORT record replayed (or being replayed) */
    TimestampTz recoveryLastXTime;
    /* current effective recovery target timeline */
    TimeLineID  RecoveryTargetTLI;

    /*
     * timestamp of when we started replaying the current chunk of WAL data,
     * only relevant for replication or archive recovery
     */
    TimestampTz currentChunkStartTime;
    /* Are we requested to pause recovery? */
    bool        recoveryPause;

    /*
     * lastFpwDisableRecPtr points to the start of the last replayed
     * XLOG_FPW_CHANGE record that instructs full_page_writes is disabled.
     */
    XLogRecPtr  lastFpwDisableRecPtr;

    slock_t     info_lck;       /* locks shared variables shown above */
} XLogCtlData;

static XLogCtlData *XLogCtl = NULL;

/*
 * We maintain an image of pg_control in shared memory.
 */
static ControlFileData *ControlFile = NULL;

/*
 * Macros for managing XLogInsert state.  In most cases, the calling routine
 * has local copies of XLogCtl->Insert and/or XLogCtl->Insert->curridx,
 * so these are passed as parameters instead of being fetched via XLogCtl.
 */

/* Free space remaining in the current xlog page buffer */
#define INSERT_FREESPACE(Insert)  \
    (XLOG_BLCKSZ - ((Insert)->currpos - (char *) (Insert)->currpage))

/* Construct XLogRecPtr value for current insertion point */
#define INSERT_RECPTR(recptr,Insert,curridx)  \
        (recptr) = XLogCtl->xlblocks[curridx] - INSERT_FREESPACE(Insert)

#define PrevBufIdx(idx)     \
        (((idx) == 0) ? XLogCtl->XLogCacheBlck : ((idx) - 1))

#define NextBufIdx(idx)     \
        (((idx) == XLogCtl->XLogCacheBlck) ? 0 : ((idx) + 1))

/*
 * Private, possibly out-of-date copy of shared LogwrtResult.
 * See discussion above.
 */
static XLogwrtResult LogwrtResult = {0, 0};

/*
 * Codes indicating where we got a WAL file from during recovery, or where
 * to attempt to get one.
 */
typedef enum
{
    XLOG_FROM_ANY = 0,      /* request to read WAL from any source */
    XLOG_FROM_ARCHIVE,      /* restored using restore_command */
    XLOG_FROM_PG_XLOG,      /* existing file in pg_xlog */
    XLOG_FROM_STREAM,       /* streamed from master */
} XLogSource;

/* human-readable names for XLogSources, for debugging output */
static const char *xlogSourceNames[] = { "any", "archive", "pg_xlog", "stream" };

/*
 * openLogFile is -1 or a kernel FD for an open log file segment.
 * When it's open, openLogOff is the current seek offset in the file.
 * openLogSegNo identifies the segment.  These variables are only
 * used to write the XLOG, and so will normally refer to the active segment.
 */
static int  openLogFile = -1;
static XLogSegNo openLogSegNo = 0;
static uint32 openLogOff = 0;

/*
 * These variables are used similarly to the ones above, but for reading
 * the XLOG.  Note, however, that readOff generally represents the offset
 * of the page just read, not the seek position of the FD itself, which
 * will be just past that page. readLen indicates how much of the current
 * page has been read into readBuf, and readSource indicates where we got
 * the currently open file from.
 */
static int  readFile = -1;
static XLogSegNo readSegNo = 0;
static uint32 readOff = 0;
static uint32 readLen = 0;
static XLogSource readSource = 0;       /* XLOG_FROM_* code */

/*
 * Keeps track of which source we're currently reading from. This is
 * different from readSource in that this is always set, even when we don't
 * currently have a WAL file open. If lastSourceFailed is set, our last
 * attempt to read from currentSource failed, and we should try another source
 * next.
 */
static XLogSource currentSource = 0;    /* XLOG_FROM_* code */
static bool lastSourceFailed = false;

typedef struct XLogPageReadPrivate
{
    int         emode;
    bool        fetching_ckpt;  /* are we fetching a checkpoint record? */
    bool        randAccess;
} XLogPageReadPrivate;

/*
 * These variables track when we last obtained some WAL data to process,
 * and where we got it from.  (XLogReceiptSource is initially the same as
 * readSource, but readSource gets reset to zero when we don't have data
 * to process right now.  It is also different from currentSource, which
 * also changes when we try to read from a source and fail, while
 * XLogReceiptSource tracks where we last successfully read some WAL.)
 */
static TimestampTz XLogReceiptTime = 0;
static XLogSource XLogReceiptSource = 0;    /* XLOG_FROM_* code */

/* State information for XLOG reading */
static XLogRecPtr ReadRecPtr;   /* start of last record read */
static XLogRecPtr EndRecPtr;    /* end+1 of last record read */

static XLogRecPtr minRecoveryPoint;     /* local copy of
                                         * ControlFile->minRecoveryPoint */
static TimeLineID minRecoveryPointTLI;
static bool updateMinRecoveryPoint = true;

/*
 * Have we reached a consistent database state? In crash recovery, we have
 * to replay all the WAL, so reachedConsistency is never set. During archive
 * recovery, the database is consistent once minRecoveryPoint is reached.
 */
bool        reachedConsistency = false;

static bool InRedo = false;

/* Have we launched bgwriter during recovery? */
static bool bgwriterLaunched = false;


static void readRecoveryCommandFile(void);
static void exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo);
static bool recoveryStopsHere(XLogRecord *record, bool *includeThis);
static void recoveryPausesHere(void);
static void SetLatestXTime(TimestampTz xtime);
static void SetCurrentChunkStartTime(TimestampTz xtime);
static void CheckRequiredParameterValues(void);
static void XLogReportParameters(void);
static void checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI,
                    TimeLineID prevTLI);
static void LocalSetXLogInsertAllowed(void);
static void CreateEndOfRecoveryRecord(void);
static void CheckPointGuts(XLogRecPtr checkPointRedo, int flags);
static void KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo);

static bool XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
                XLogRecPtr *lsn, BkpBlock *bkpb);
static Buffer RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb,
                char *blk, bool get_cleanup_lock, bool keep_buffer);
static bool AdvanceXLInsertBuffer(bool new_segment);
static bool XLogCheckpointNeeded(XLogSegNo new_segno);
static void XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch);
static bool InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                       bool find_free, int *max_advance,
                       bool use_lock);
static int XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
             int source, bool notexistOk);
static int XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source);
static int XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr,
             int reqLen, XLogRecPtr targetRecPtr, char *readBuf,
             TimeLineID *readTLI);
static bool WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
                            bool fetching_ckpt, XLogRecPtr tliRecPtr);
static int  emode_for_corrupt_record(int emode, XLogRecPtr RecPtr);
static void XLogFileClose(void);
static void PreallocXlogFiles(XLogRecPtr endptr);
static void RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr);
static void UpdateLastRemovedPtr(char *filename);
static void ValidateXLOGDirectoryStructure(void);
static void CleanupBackupHistory(void);
static void UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force);
static XLogRecord *ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
           int emode, bool fetching_ckpt);
static void CheckRecoveryConsistency(void);
static XLogRecord *ReadCheckpointRecord(XLogReaderState *xlogreader,
                     XLogRecPtr RecPtr, int whichChkpti, bool report);
static bool rescanLatestTimeLine(void);
static void WriteControlFile(void);
static void ReadControlFile(void);
static char *str_time(pg_time_t tnow);
static bool CheckForStandbyTrigger(void);

#ifdef WAL_DEBUG
static void xlog_outrec(StringInfo buf, XLogRecord *record);
#endif
static void pg_start_backup_callback(int code, Datum arg);
static bool read_backup_label(XLogRecPtr *checkPointLoc,
                  bool *backupEndRequired, bool *backupFromStandby);
static void rm_redo_error_callback(void *arg);
static int  get_sync_bit(int method);


/*
 * Insert an XLOG record having the specified RMID and info bytes,
 * with the body of the record being the data chunk(s) described by
 * the rdata chain (see xlog.h for notes about rdata).
 *
 * Returns XLOG pointer to end of record (beginning of next record).
 * This can be used as LSN for data pages affected by the logged action.
 * (LSN is the XLOG point up to which the XLOG must be flushed to disk
 * before the data page can be written out.  This implements the basic
 * WAL rule "write the log before the data".)
 *
 * NB: this routine feels free to scribble on the XLogRecData structs,
 * though not on the data they reference.  This is OK since the XLogRecData
 * structs are always just temporaries in the calling code.
 */
XLogRecPtr
XLogInsert(RmgrId rmid, uint8 info, XLogRecData *rdata)
{
    XLogCtlInsert *Insert = &XLogCtl->Insert;
    XLogRecPtr  RecPtr;
    XLogRecPtr  WriteRqst;
    uint32      freespace;
    int         curridx;
    XLogRecData *rdt;
    XLogRecData *rdt_lastnormal;
    Buffer      dtbuf[XLR_MAX_BKP_BLOCKS];
    bool        dtbuf_bkp[XLR_MAX_BKP_BLOCKS];
    BkpBlock    dtbuf_xlg[XLR_MAX_BKP_BLOCKS];
    XLogRecPtr  dtbuf_lsn[XLR_MAX_BKP_BLOCKS];
    XLogRecData dtbuf_rdt1[XLR_MAX_BKP_BLOCKS];
    XLogRecData dtbuf_rdt2[XLR_MAX_BKP_BLOCKS];
    XLogRecData dtbuf_rdt3[XLR_MAX_BKP_BLOCKS];
    XLogRecData hdr_rdt;
    pg_crc32    rdata_crc;
    uint32      len,
                write_len;
    unsigned    i;
    bool        updrqst;
    bool        doPageWrites;
    bool        isLogSwitch = (rmid == RM_XLOG_ID && info == XLOG_SWITCH);
    uint8       info_orig = info;
    static XLogRecord *rechdr;

    if (rechdr == NULL)
    {
        rechdr = malloc(SizeOfXLogRecord);
        if (rechdr == NULL)
            elog(ERROR, "out of memory");
        MemSet(rechdr, 0, SizeOfXLogRecord);
    }

    /* cross-check on whether we should be here or not */
    if (!XLogInsertAllowed())
        elog(ERROR, "cannot make new WAL entries during recovery");

    /* info's high bits are reserved for use by me */
    if (info & XLR_INFO_MASK)
        elog(PANIC, "invalid xlog info mask %02X", info);

    TRACE_POSTGRESQL_XLOG_INSERT(rmid, info);

    /*
     * In bootstrap mode, we don't actually log anything but XLOG resources;
     * return a phony record pointer.
     */
    if (IsBootstrapProcessingMode() && rmid != RM_XLOG_ID)
    {
        RecPtr = SizeOfXLogLongPHD;     /* start of 1st chkpt record */
        return RecPtr;
    }

    /*
     * Here we scan the rdata chain, to determine which buffers must be backed
     * up.
     *
     * We may have to loop back to here if a race condition is detected below.
     * We could prevent the race by doing all this work while holding the
     * insert lock, but it seems better to avoid doing CRC calculations while
     * holding the lock.
     *
     * We add entries for backup blocks to the chain, so that they don't need
     * any special treatment in the critical section where the chunks are
     * copied into the WAL buffers. Those entries have to be unlinked from the
     * chain if we have to loop back here.
     */
begin:;
    for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
    {
        dtbuf[i] = InvalidBuffer;
        dtbuf_bkp[i] = false;
    }

    /*
     * Decide if we need to do full-page writes in this XLOG record: true if
     * full_page_writes is on or we have a PITR request for it.  Since we
     * don't yet have the insert lock, fullPageWrites and forcePageWrites
     * could change under us, but we'll recheck them once we have the lock.
     */
    doPageWrites = Insert->fullPageWrites || Insert->forcePageWrites;

    len = 0;
    for (rdt = rdata;;)
    {
        if (rdt->buffer == InvalidBuffer)
        {
            /* Simple data, just include it */
            len += rdt->len;
        }
        else
        {
            /* Find info for buffer */
            for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
            {
                if (rdt->buffer == dtbuf[i])
                {
                    /* Buffer already referenced by earlier chain item */
                    if (dtbuf_bkp[i])
                    {
                        rdt->data = NULL;
                        rdt->len = 0;
                    }
                    else if (rdt->data)
                        len += rdt->len;
                    break;
                }
                if (dtbuf[i] == InvalidBuffer)
                {
                    /* OK, put it in this slot */
                    dtbuf[i] = rdt->buffer;
                    if (doPageWrites && XLogCheckBuffer(rdt, true,
                                        &(dtbuf_lsn[i]), &(dtbuf_xlg[i])))
                    {
                        dtbuf_bkp[i] = true;
                        rdt->data = NULL;
                        rdt->len = 0;
                    }
                    else if (rdt->data)
                        len += rdt->len;
                    break;
                }
            }
            if (i >= XLR_MAX_BKP_BLOCKS)
                elog(PANIC, "can backup at most %d blocks per xlog record",
                     XLR_MAX_BKP_BLOCKS);
        }
        /* Break out of loop when rdt points to last chain item */
        if (rdt->next == NULL)
            break;
        rdt = rdt->next;
    }

    /*
     * NOTE: We disallow len == 0 because it provides a useful bit of extra
     * error checking in ReadRecord.  This means that all callers of
     * XLogInsert must supply at least some not-in-a-buffer data.  However, we
     * make an exception for XLOG SWITCH records because we don't want them to
     * ever cross a segment boundary.
     */
    if (len == 0 && !isLogSwitch)
        elog(PANIC, "invalid xlog record length %u", len);

    /*
     * Make additional rdata chain entries for the backup blocks, so that we
     * don't need to special-case them in the write loop.  This modifies the
     * original rdata chain, but we keep a pointer to the last regular entry,
     * rdt_lastnormal, so that we can undo this if we have to loop back to the
     * beginning.
     *
     * At the exit of this loop, write_len includes the backup block data.
     *
     * Also set the appropriate info bits to show which buffers were backed
     * up. The XLR_BKP_BLOCK(N) bit corresponds to the N'th distinct buffer
     * value (ignoring InvalidBuffer) appearing in the rdata chain.
     */
    rdt_lastnormal = rdt;
    write_len = len;
    for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
    {
        BkpBlock   *bkpb;
        char       *page;

        if (!dtbuf_bkp[i])
            continue;

        info |= XLR_BKP_BLOCK(i);

        bkpb = &(dtbuf_xlg[i]);
        page = (char *) BufferGetBlock(dtbuf[i]);

        rdt->next = &(dtbuf_rdt1[i]);
        rdt = rdt->next;

        rdt->data = (char *) bkpb;
        rdt->len = sizeof(BkpBlock);
        write_len += sizeof(BkpBlock);

        rdt->next = &(dtbuf_rdt2[i]);
        rdt = rdt->next;

        if (bkpb->hole_length == 0)
        {
            rdt->data = page;
            rdt->len = BLCKSZ;
            write_len += BLCKSZ;
            rdt->next = NULL;
        }
        else
        {
            /* must skip the hole */
            rdt->data = page;
            rdt->len = bkpb->hole_offset;
            write_len += bkpb->hole_offset;

            rdt->next = &(dtbuf_rdt3[i]);
            rdt = rdt->next;

            rdt->data = page + (bkpb->hole_offset + bkpb->hole_length);
            rdt->len = BLCKSZ - (bkpb->hole_offset + bkpb->hole_length);
            write_len += rdt->len;
            rdt->next = NULL;
        }
    }

    /*
     * Calculate CRC of the data, including all the backup blocks
     *
     * Note that the record header isn't added into the CRC initially since we
     * don't know the prev-link yet.  Thus, the CRC will represent the CRC of
     * the whole record in the order: rdata, then backup blocks, then record
     * header.
     */
    INIT_CRC32(rdata_crc);
    for (rdt = rdata; rdt != NULL; rdt = rdt->next)
        COMP_CRC32(rdata_crc, rdt->data, rdt->len);

    /*
     * Construct record header (prev-link and CRC are filled in later), and
     * make that the first chunk in the chain.
     */
    rechdr->xl_xid = GetCurrentTransactionIdIfAny();
    rechdr->xl_tot_len = SizeOfXLogRecord + write_len;
    rechdr->xl_len = len;       /* doesn't include backup blocks */
    rechdr->xl_info = info;
    rechdr->xl_rmid = rmid;

    hdr_rdt.next = rdata;
    hdr_rdt.data = (char *) rechdr;
    hdr_rdt.len = SizeOfXLogRecord;

    write_len += SizeOfXLogRecord;

    START_CRIT_SECTION();

    /* Now wait to get insert lock */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

    /*
     * Check to see if my RedoRecPtr is out of date.  If so, may have to go
     * back and recompute everything.  This can only happen just after a
     * checkpoint, so it's better to be slow in this case and fast otherwise.
     *
     * If we aren't doing full-page writes then RedoRecPtr doesn't actually
     * affect the contents of the XLOG record, so we'll update our local copy
     * but not force a recomputation.
     */
    if (RedoRecPtr != Insert->RedoRecPtr)
    {
        Assert(RedoRecPtr < Insert->RedoRecPtr);
        RedoRecPtr = Insert->RedoRecPtr;

        if (doPageWrites)
        {
            for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
            {
                if (dtbuf[i] == InvalidBuffer)
                    continue;
                if (dtbuf_bkp[i] == false &&
                    dtbuf_lsn[i] <= RedoRecPtr)
                {
                    /*
                     * Oops, this buffer now needs to be backed up, but we
                     * didn't think so above.  Start over.
                     */
                    LWLockRelease(WALInsertLock);
                    END_CRIT_SECTION();
                    rdt_lastnormal->next = NULL;
                    info = info_orig;
                    goto begin;
                }
            }
        }
    }

    /*
     * Also check to see if fullPageWrites or forcePageWrites was just turned
     * on; if we weren't already doing full-page writes then go back and
     * recompute. (If it was just turned off, we could recompute the record
     * without full pages, but we choose not to bother.)
     */
    if ((Insert->fullPageWrites || Insert->forcePageWrites) && !doPageWrites)
    {
        /* Oops, must redo it with full-page data. */
        LWLockRelease(WALInsertLock);
        END_CRIT_SECTION();
        rdt_lastnormal->next = NULL;
        info = info_orig;
        goto begin;
    }

    /*
     * If the current page is completely full, the record goes to the next
     * page, right after the page header.
     */
    updrqst = false;
    freespace = INSERT_FREESPACE(Insert);
    if (freespace == 0)
    {
        updrqst = AdvanceXLInsertBuffer(false);
        freespace = INSERT_FREESPACE(Insert);
    }

    /* Compute record's XLOG location */
    curridx = Insert->curridx;
    INSERT_RECPTR(RecPtr, Insert, curridx);

    /*
     * If the record is an XLOG_SWITCH, and we are exactly at the start of a
     * segment, we need not insert it (and don't want to because we'd like
     * consecutive switch requests to be no-ops).  Instead, make sure
     * everything is written and flushed through the end of the prior segment,
     * and return the prior segment's end address.
     */
    if (isLogSwitch && (RecPtr % XLogSegSize) == SizeOfXLogLongPHD)
    {
        /* We can release insert lock immediately */
        LWLockRelease(WALInsertLock);

        RecPtr -= SizeOfXLogLongPHD;

        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
        LogwrtResult = XLogCtl->LogwrtResult;
        if (LogwrtResult.Flush < RecPtr)
        {
            XLogwrtRqst FlushRqst;

            FlushRqst.Write = RecPtr;
            FlushRqst.Flush = RecPtr;
            XLogWrite(FlushRqst, false, false);
        }
        LWLockRelease(WALWriteLock);

        END_CRIT_SECTION();

        /* wake up walsenders now that we've released heavily contended locks */
        WalSndWakeupProcessRequests();
        return RecPtr;
    }

    /* Finish the record header */
    rechdr->xl_prev = Insert->PrevRecord;

    /* Now we can finish computing the record's CRC */
    COMP_CRC32(rdata_crc, (char *) rechdr, offsetof(XLogRecord, xl_crc));
    FIN_CRC32(rdata_crc);
    rechdr->xl_crc = rdata_crc;

#ifdef WAL_DEBUG
    if (XLOG_DEBUG)
    {
        StringInfoData buf;

        initStringInfo(&buf);
        appendStringInfo(&buf, "INSERT @ %X/%X: ",
                         (uint32) (RecPtr >> 32), (uint32) RecPtr);
        xlog_outrec(&buf, rechdr);
        if (rdata->data != NULL)
        {
            appendStringInfo(&buf, " - ");
            RmgrTable[rechdr->xl_rmid].rm_desc(&buf, rechdr->xl_info, rdata->data);
        }
        elog(LOG, "%s", buf.data);
        pfree(buf.data);
    }
#endif

    /* Record begin of record in appropriate places */
    ProcLastRecPtr = RecPtr;
    Insert->PrevRecord = RecPtr;

    /*
     * Append the data, including backup blocks if any
     */
    rdata = &hdr_rdt;
    while (write_len)
    {
        while (rdata->data == NULL)
            rdata = rdata->next;

        if (freespace > 0)
        {
            if (rdata->len > freespace)
            {
                memcpy(Insert->currpos, rdata->data, freespace);
                rdata->data += freespace;
                rdata->len -= freespace;
                write_len -= freespace;
            }
            else
            {
                memcpy(Insert->currpos, rdata->data, rdata->len);
                freespace -= rdata->len;
                write_len -= rdata->len;
                Insert->currpos += rdata->len;
                rdata = rdata->next;
                continue;
            }
        }

        /* Use next buffer */
        updrqst = AdvanceXLInsertBuffer(false);
        curridx = Insert->curridx;
        /* Mark page header to indicate this record continues on the page */
        Insert->currpage->xlp_info |= XLP_FIRST_IS_CONTRECORD;
        Insert->currpage->xlp_rem_len = write_len;
        freespace = INSERT_FREESPACE(Insert);
    }

    /* Ensure next record will be properly aligned */
    Insert->currpos = (char *) Insert->currpage +
        MAXALIGN(Insert->currpos - (char *) Insert->currpage);
    freespace = INSERT_FREESPACE(Insert);

    /*
     * The recptr I return is the beginning of the *next* record. This will be
     * stored as LSN for changed data pages...
     */
    INSERT_RECPTR(RecPtr, Insert, curridx);

    /*
     * If the record is an XLOG_SWITCH, we must now write and flush all the
     * existing data, and then forcibly advance to the start of the next
     * segment.  It's not good to do this I/O while holding the insert lock,
     * but there seems too much risk of confusion if we try to release the
     * lock sooner.  Fortunately xlog switch needn't be a high-performance
     * operation anyway...
     */
    if (isLogSwitch)
    {
        XLogwrtRqst FlushRqst;
        XLogRecPtr  OldSegEnd;

        TRACE_POSTGRESQL_XLOG_SWITCH();

        LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);

        /*
         * Flush through the end of the page containing XLOG_SWITCH, and
         * perform end-of-segment actions (eg, notifying archiver).
         */
        WriteRqst = XLogCtl->xlblocks[curridx];
        FlushRqst.Write = WriteRqst;
        FlushRqst.Flush = WriteRqst;
        XLogWrite(FlushRqst, false, true);

        /* Set up the next buffer as first page of next segment */
        /* Note: AdvanceXLInsertBuffer cannot need to do I/O here */
        (void) AdvanceXLInsertBuffer(true);

        /* There should be no unwritten data */
        curridx = Insert->curridx;
        Assert(curridx == XLogCtl->Write.curridx);

        /* Compute end address of old segment */
        OldSegEnd = XLogCtl->xlblocks[curridx];
        OldSegEnd -= XLOG_BLCKSZ;

        /* Make it look like we've written and synced all of old segment */
        LogwrtResult.Write = OldSegEnd;
        LogwrtResult.Flush = OldSegEnd;

        /*
         * Update shared-memory status --- this code should match XLogWrite
         */
        {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->LogwrtResult = LogwrtResult;
            if (xlogctl->LogwrtRqst.Write < LogwrtResult.Write)
                xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
            if (xlogctl->LogwrtRqst.Flush < LogwrtResult.Flush)
                xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
            SpinLockRelease(&xlogctl->info_lck);
        }

        LWLockRelease(WALWriteLock);

        updrqst = false;        /* done already */
    }
    else
    {
        /* normal case, ie not xlog switch */

        /* Need to update shared LogwrtRqst if some block was filled up */
        if (freespace == 0)
        {
            /* curridx is filled and available for writing out */
            updrqst = true;
        }
        else
        {
            /* if updrqst already set, write through end of previous buf */
            curridx = PrevBufIdx(curridx);
        }
        WriteRqst = XLogCtl->xlblocks[curridx];
    }

    LWLockRelease(WALInsertLock);

    if (updrqst)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        /* advance global request to include new block(s) */
        if (xlogctl->LogwrtRqst.Write < WriteRqst)
            xlogctl->LogwrtRqst.Write = WriteRqst;
        /* update local result copy while I have the chance */
        LogwrtResult = xlogctl->LogwrtResult;
        SpinLockRelease(&xlogctl->info_lck);
    }

    XactLastRecEnd = RecPtr;

    END_CRIT_SECTION();

    /* wake up walsenders now that we've released heavily contended locks */
    WalSndWakeupProcessRequests();

    return RecPtr;
}

/*
 * Determine whether the buffer referenced by an XLogRecData item has to
 * be backed up, and if so fill a BkpBlock struct for it.  In any case
 * save the buffer's LSN at *lsn.
 */
static bool
XLogCheckBuffer(XLogRecData *rdata, bool holdsExclusiveLock,
                XLogRecPtr *lsn, BkpBlock *bkpb)
{
    Page        page;

    page = BufferGetPage(rdata->buffer);

    /*
     * We assume page LSN is first data on *every* page that can be passed
     * to XLogInsert, whether it has the standard page layout or not. We
     * don't need to take the buffer header lock for PageGetLSN if we hold
     * an exclusive lock on the page and/or the relation.
     */
    if (holdsExclusiveLock)
        *lsn = PageGetLSN(page);
    else
        *lsn = BufferGetLSNAtomic(rdata->buffer);

    if (*lsn <= RedoRecPtr)
    {
        /*
         * The page needs to be backed up, so set up *bkpb
         */
        BufferGetTag(rdata->buffer, &bkpb->node, &bkpb->fork, &bkpb->block);

        if (rdata->buffer_std)
        {
            /* Assume we can omit data between pd_lower and pd_upper */
            uint16      lower = ((PageHeader) page)->pd_lower;
            uint16      upper = ((PageHeader) page)->pd_upper;

            if (lower >= SizeOfPageHeaderData &&
                upper > lower &&
                upper <= BLCKSZ)
            {
                bkpb->hole_offset = lower;
                bkpb->hole_length = upper - lower;
            }
            else
            {
                /* No "hole" to compress out */
                bkpb->hole_offset = 0;
                bkpb->hole_length = 0;
            }
        }
        else
        {
            /* Not a standard page header, don't try to eliminate "hole" */
            bkpb->hole_offset = 0;
            bkpb->hole_length = 0;
        }

        return true;            /* buffer requires backup */
    }

    return false;               /* buffer does not need to be backed up */
}

/*
 * Advance the Insert state to the next buffer page, writing out the next
 * buffer if it still contains unwritten data.
 *
 * If new_segment is TRUE then we set up the next buffer page as the first
 * page of the next xlog segment file, possibly but not usually the next
 * consecutive file page.
 *
 * The global LogwrtRqst.Write pointer needs to be advanced to include the
 * just-filled page.  If we can do this for free (without an extra lock),
 * we do so here.  Otherwise the caller must do it.  We return TRUE if the
 * request update still needs to be done, FALSE if we did it internally.
 *
 * Must be called with WALInsertLock held.
 */
static bool
AdvanceXLInsertBuffer(bool new_segment)
{
    XLogCtlInsert *Insert = &XLogCtl->Insert;
    int         nextidx = NextBufIdx(Insert->curridx);
    bool        update_needed = true;
    XLogRecPtr  OldPageRqstPtr;
    XLogwrtRqst WriteRqst;
    XLogRecPtr  NewPageEndPtr;
    XLogRecPtr  NewPageBeginPtr;
    XLogPageHeader NewPage;

    /*
     * Get ending-offset of the buffer page we need to replace (this may be
     * zero if the buffer hasn't been used yet).  Fall through if it's already
     * written out.
     */
    OldPageRqstPtr = XLogCtl->xlblocks[nextidx];
    if (LogwrtResult.Write < OldPageRqstPtr)
    {
        /* nope, got work to do... */
        XLogRecPtr  FinishedPageRqstPtr;

        FinishedPageRqstPtr = XLogCtl->xlblocks[Insert->curridx];

        /* Before waiting, get info_lck and update LogwrtResult */
        {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            if (xlogctl->LogwrtRqst.Write < FinishedPageRqstPtr)
                xlogctl->LogwrtRqst.Write = FinishedPageRqstPtr;
            LogwrtResult = xlogctl->LogwrtResult;
            SpinLockRelease(&xlogctl->info_lck);
        }

        update_needed = false;  /* Did the shared-request update */

        /*
         * Now that we have an up-to-date LogwrtResult value, see if we still
         * need to write it or if someone else already did.
         */
        if (LogwrtResult.Write < OldPageRqstPtr)
        {
            /* Must acquire write lock */
            LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
            LogwrtResult = XLogCtl->LogwrtResult;
            if (LogwrtResult.Write >= OldPageRqstPtr)
            {
                /* OK, someone wrote it already */
                LWLockRelease(WALWriteLock);
            }
            else
            {
                /*
                 * Have to write buffers while holding insert lock. This is
                 * not good, so only write as much as we absolutely must.
                 */
                TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_START();
                WriteRqst.Write = OldPageRqstPtr;
                WriteRqst.Flush = 0;
                XLogWrite(WriteRqst, false, false);
                LWLockRelease(WALWriteLock);
                TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
            }
        }
    }

    /*
     * Now the next buffer slot is free and we can set it up to be the next
     * output page.
     */
    NewPageBeginPtr = XLogCtl->xlblocks[Insert->curridx];

    if (new_segment)
    {
        /* force it to a segment start point */
        if (NewPageBeginPtr % XLogSegSize != 0)
            NewPageBeginPtr += XLogSegSize - NewPageBeginPtr % XLogSegSize;
    }

    NewPageEndPtr = NewPageBeginPtr;
    NewPageEndPtr += XLOG_BLCKSZ;
    XLogCtl->xlblocks[nextidx] = NewPageEndPtr;
    NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);

    Insert->curridx = nextidx;
    Insert->currpage = NewPage;

    Insert->currpos = ((char *) NewPage) +SizeOfXLogShortPHD;

    /*
     * Be sure to re-zero the buffer so that bytes beyond what we've written
     * will look like zeroes and not valid XLOG records...
     */
    MemSet((char *) NewPage, 0, XLOG_BLCKSZ);

    /*
     * Fill the new page's header
     */
    NewPage   ->xlp_magic = XLOG_PAGE_MAGIC;

    /* NewPage->xlp_info = 0; */    /* done by memset */
    NewPage   ->xlp_tli = ThisTimeLineID;
    NewPage   ->xlp_pageaddr = NewPageBeginPtr;

    /*
     * If online backup is not in progress, mark the header to indicate that
     * WAL records beginning in this page have removable backup blocks.  This
     * allows the WAL archiver to know whether it is safe to compress archived
     * WAL data by transforming full-block records into the non-full-block
     * format.  It is sufficient to record this at the page level because we
     * force a page switch (in fact a segment switch) when starting a backup,
     * so the flag will be off before any records can be written during the
     * backup.  At the end of a backup, the last page will be marked as all
     * unsafe when perhaps only part is unsafe, but at worst the archiver
     * would miss the opportunity to compress a few records.
     */
    if (!Insert->forcePageWrites)
        NewPage   ->xlp_info |= XLP_BKP_REMOVABLE;

    /*
     * If first page of an XLOG segment file, make it a long header.
     */
    if ((NewPage->xlp_pageaddr % XLogSegSize) == 0)
    {
        XLogLongPageHeader NewLongPage = (XLogLongPageHeader) NewPage;

        NewLongPage->xlp_sysid = ControlFile->system_identifier;
        NewLongPage->xlp_seg_size = XLogSegSize;
        NewLongPage->xlp_xlog_blcksz = XLOG_BLCKSZ;
        NewPage   ->xlp_info |= XLP_LONG_HEADER;

        Insert->currpos = ((char *) NewPage) +SizeOfXLogLongPHD;
    }

    return update_needed;
}

/*
 * Check whether we've consumed enough xlog space that a checkpoint is needed.
 *
 * new_segno indicates a log file that has just been filled up (or read
 * during recovery). We measure the distance from RedoRecPtr to new_segno
 * and see if that exceeds CheckPointSegments.
 *
 * Note: it is caller's responsibility that RedoRecPtr is up-to-date.
 */
static bool
XLogCheckpointNeeded(XLogSegNo new_segno)
{
    XLogSegNo   old_segno;

    XLByteToSeg(RedoRecPtr, old_segno);

    if (new_segno >= old_segno + (uint64) (CheckPointSegments - 1))
        return true;
    return false;
}

/*
 * Write and/or fsync the log at least as far as WriteRqst indicates.
 *
 * If flexible == TRUE, we don't have to write as far as WriteRqst, but
 * may stop at any convenient boundary (such as a cache or logfile boundary).
 * This option allows us to avoid uselessly issuing multiple writes when a
 * single one would do.
 *
 * If xlog_switch == TRUE, we are intending an xlog segment switch, so
 * perform end-of-segment actions after writing the last page, even if
 * it's not physically the end of its segment.  (NB: this will work properly
 * only if caller specifies WriteRqst == page-end and flexible == false,
 * and there is some data to write.)
 *
 * Must be called with WALWriteLock held.
 */
static void
XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
{
    XLogCtlWrite *Write = &XLogCtl->Write;
    bool        ispartialpage;
    bool        last_iteration;
    bool        finishing_seg;
    bool        use_existent;
    int         curridx;
    int         npages;
    int         startidx;
    uint32      startoffset;

    /* We should always be inside a critical section here */
    Assert(CritSectionCount > 0);

    /*
     * Update local LogwrtResult (caller probably did this already, but...)
     */
    LogwrtResult = XLogCtl->LogwrtResult;

    /*
     * Since successive pages in the xlog cache are consecutively allocated,
     * we can usually gather multiple pages together and issue just one
     * write() call.  npages is the number of pages we have determined can be
     * written together; startidx is the cache block index of the first one,
     * and startoffset is the file offset at which it should go. The latter
     * two variables are only valid when npages > 0, but we must initialize
     * all of them to keep the compiler quiet.
     */
    npages = 0;
    startidx = 0;
    startoffset = 0;

    /*
     * Within the loop, curridx is the cache block index of the page to
     * consider writing.  We advance Write->curridx only after successfully
     * writing pages.  (Right now, this refinement is useless since we are
     * going to PANIC if any error occurs anyway; but someday it may come in
     * useful.)
     */
    curridx = Write->curridx;

    while (LogwrtResult.Write < WriteRqst.Write)
    {
        /*
         * Make sure we're not ahead of the insert process.  This could happen
         * if we're passed a bogus WriteRqst.Write that is past the end of the
         * last page that's been initialized by AdvanceXLInsertBuffer.
         */
        if (LogwrtResult.Write >= XLogCtl->xlblocks[curridx])
            elog(PANIC, "xlog write request %X/%X is past end of log %X/%X",
                 (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
                 (uint32) (XLogCtl->xlblocks[curridx] >> 32),
                 (uint32) XLogCtl->xlblocks[curridx]);

        /* Advance LogwrtResult.Write to end of current buffer page */
        LogwrtResult.Write = XLogCtl->xlblocks[curridx];
        ispartialpage = WriteRqst.Write < LogwrtResult.Write;

        if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
        {
            /*
             * Switch to new logfile segment.  We cannot have any pending
             * pages here (since we dump what we have at segment end).
             */
            Assert(npages == 0);
            if (openLogFile >= 0)
                XLogFileClose();
            XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);

            /* create/use new log file */
            use_existent = true;
            openLogFile = XLogFileInit(openLogSegNo, &use_existent, true);
            openLogOff = 0;
        }

        /* Make sure we have the current logfile open */
        if (openLogFile < 0)
        {
            XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
            openLogFile = XLogFileOpen(openLogSegNo);
            openLogOff = 0;
        }

        /* Add current page to the set of pending pages-to-dump */
        if (npages == 0)
        {
            /* first of group */
            startidx = curridx;
            startoffset = (LogwrtResult.Write - XLOG_BLCKSZ) % XLogSegSize;
        }
        npages++;

        /*
         * Dump the set if this will be the last loop iteration, or if we are
         * at the last page of the cache area (since the next page won't be
         * contiguous in memory), or if we are at the end of the logfile
         * segment.
         */
        last_iteration = WriteRqst.Write <= LogwrtResult.Write;

        finishing_seg = !ispartialpage &&
            (startoffset + npages * XLOG_BLCKSZ) >= XLogSegSize;

        if (last_iteration ||
            curridx == XLogCtl->XLogCacheBlck ||
            finishing_seg)
        {
            char       *from;
            Size        nbytes;

            /* Need to seek in the file? */
            if (openLogOff != startoffset)
            {
                if (lseek(openLogFile, (off_t) startoffset, SEEK_SET) < 0)
                    ereport(PANIC,
                            (errcode_for_file_access(),
                             errmsg("could not seek in log file %s to offset %u: %m",
                                    XLogFileNameP(ThisTimeLineID, openLogSegNo),
                                    startoffset)));
                openLogOff = startoffset;
            }

            /* OK to write the page(s) */
            from = XLogCtl->pages + startidx * (Size) XLOG_BLCKSZ;
            nbytes = npages * (Size) XLOG_BLCKSZ;
            errno = 0;
            if (write(openLogFile, from, nbytes) != nbytes)
            {
                /* if write didn't set errno, assume no disk space */
                if (errno == 0)
                    errno = ENOSPC;
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not write to log file %s "
                                "at offset %u, length %lu: %m",
                                XLogFileNameP(ThisTimeLineID, openLogSegNo),
                                openLogOff, (unsigned long) nbytes)));
            }

            /* Update state for write */
            openLogOff += nbytes;
            Write->curridx = ispartialpage ? curridx : NextBufIdx(curridx);
            npages = 0;

            /*
             * If we just wrote the whole last page of a logfile segment,
             * fsync the segment immediately.  This avoids having to go back
             * and re-open prior segments when an fsync request comes along
             * later. Doing it here ensures that one and only one backend will
             * perform this fsync.
             *
             * We also do this if this is the last page written for an xlog
             * switch.
             *
             * This is also the right place to notify the Archiver that the
             * segment is ready to copy to archival storage, and to update the
             * timer for archive_timeout, and to signal for a checkpoint if
             * too many logfile segments have been used since the last
             * checkpoint.
             */
            if (finishing_seg || (xlog_switch && last_iteration))
            {
                issue_xlog_fsync(openLogFile, openLogSegNo);

                /* signal that we need to wakeup walsenders later */
                WalSndWakeupRequest();

                LogwrtResult.Flush = LogwrtResult.Write;        /* end of page */

                if (XLogArchivingActive())
                    XLogArchiveNotifySeg(openLogSegNo);

                Write->lastSegSwitchTime = (pg_time_t) time(NULL);

                /*
                 * Request a checkpoint if we've consumed too much xlog since
                 * the last one.  For speed, we first check using the local
                 * copy of RedoRecPtr, which might be out of date; if it looks
                 * like a checkpoint is needed, forcibly update RedoRecPtr and
                 * recheck.
                 */
                if (IsUnderPostmaster && XLogCheckpointNeeded(openLogSegNo))
                {
                    (void) GetRedoRecPtr();
                    if (XLogCheckpointNeeded(openLogSegNo))
                        RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
                }
            }
        }

        if (ispartialpage)
        {
            /* Only asked to write a partial page */
            LogwrtResult.Write = WriteRqst.Write;
            break;
        }
        curridx = NextBufIdx(curridx);

        /* If flexible, break out of loop as soon as we wrote something */
        if (flexible && npages == 0)
            break;
    }

    Assert(npages == 0);
    Assert(curridx == Write->curridx);

    /*
     * If asked to flush, do so
     */
    if (LogwrtResult.Flush < WriteRqst.Flush &&
        LogwrtResult.Flush < LogwrtResult.Write)

    {
        /*
         * Could get here without iterating above loop, in which case we might
         * have no open file or the wrong one.  However, we do not need to
         * fsync more than one file.
         */
        if (sync_method != SYNC_METHOD_OPEN &&
            sync_method != SYNC_METHOD_OPEN_DSYNC)
        {
            if (openLogFile >= 0 &&
                !XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
                XLogFileClose();
            if (openLogFile < 0)
            {
                XLByteToPrevSeg(LogwrtResult.Write, openLogSegNo);
                openLogFile = XLogFileOpen(openLogSegNo);
                openLogOff = 0;
            }

            issue_xlog_fsync(openLogFile, openLogSegNo);
        }

        /* signal that we need to wakeup walsenders later */
        WalSndWakeupRequest();

        LogwrtResult.Flush = LogwrtResult.Write;
    }

    /*
     * Update shared-memory status
     *
     * We make sure that the shared 'request' values do not fall behind the
     * 'result' values.  This is not absolutely essential, but it saves some
     * code in a couple of places.
     */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->LogwrtResult = LogwrtResult;
        if (xlogctl->LogwrtRqst.Write < LogwrtResult.Write)
            xlogctl->LogwrtRqst.Write = LogwrtResult.Write;
        if (xlogctl->LogwrtRqst.Flush < LogwrtResult.Flush)
            xlogctl->LogwrtRqst.Flush = LogwrtResult.Flush;
        SpinLockRelease(&xlogctl->info_lck);
    }
}

/*
 * Record the LSN for an asynchronous transaction commit/abort
 * and nudge the WALWriter if there is work for it to do.
 * (This should not be called for synchronous commits.)
 */
void
XLogSetAsyncXactLSN(XLogRecPtr asyncXactLSN)
{
    XLogRecPtr  WriteRqstPtr = asyncXactLSN;
    bool        sleeping;

    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    LogwrtResult = xlogctl->LogwrtResult;
    sleeping = xlogctl->WalWriterSleeping;
    if (xlogctl->asyncXactLSN < asyncXactLSN)
        xlogctl->asyncXactLSN = asyncXactLSN;
    SpinLockRelease(&xlogctl->info_lck);

    /*
     * If the WALWriter is sleeping, we should kick it to make it come out of
     * low-power mode.  Otherwise, determine whether there's a full page of
     * WAL available to write.
     */
    if (!sleeping)
    {
        /* back off to last completed page boundary */
        WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

        /* if we have already flushed that far, we're done */
        if (WriteRqstPtr <= LogwrtResult.Flush)
            return;
    }

    /*
     * Nudge the WALWriter: it has a full page of WAL to write, or we want it
     * to come out of low-power mode so that this async commit will reach disk
     * within the expected amount of time.
     */
    if (ProcGlobal->walwriterLatch)
        SetLatch(ProcGlobal->walwriterLatch);
}

/*
 * Advance minRecoveryPoint in control file.
 *
 * If we crash during recovery, we must reach this point again before the
 * database is consistent.
 *
 * If 'force' is true, 'lsn' argument is ignored. Otherwise, minRecoveryPoint
 * is only updated if it's not already greater than or equal to 'lsn'.
 */
static void
UpdateMinRecoveryPoint(XLogRecPtr lsn, bool force)
{
    /* Quick check using our local copy of the variable */
    if (!updateMinRecoveryPoint || (!force && lsn <= minRecoveryPoint))
        return;

    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

    /* update local copy */
    minRecoveryPoint = ControlFile->minRecoveryPoint;
    minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

    /*
     * An invalid minRecoveryPoint means that we need to recover all the WAL,
     * i.e., we're doing crash recovery.  We never modify the control file's
     * value in that case, so we can short-circuit future checks here too.
     */
    if (minRecoveryPoint == 0)
        updateMinRecoveryPoint = false;
    else if (force || minRecoveryPoint < lsn)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr  newMinRecoveryPoint;
        TimeLineID  newMinRecoveryPointTLI;

        /*
         * To avoid having to update the control file too often, we update it
         * all the way to the last record being replayed, even though 'lsn'
         * would suffice for correctness.  This also allows the 'force' case
         * to not need a valid 'lsn' value.
         *
         * Another important reason for doing it this way is that the passed
         * 'lsn' value could be bogus, i.e., past the end of available WAL, if
         * the caller got it from a corrupted heap page.  Accepting such a
         * value as the min recovery point would prevent us from coming up at
         * all.  Instead, we just log a warning and continue with recovery.
         * (See also the comments about corrupt LSNs in XLogFlush.)
         */
        SpinLockAcquire(&xlogctl->info_lck);
        newMinRecoveryPoint = xlogctl->replayEndRecPtr;
        newMinRecoveryPointTLI = xlogctl->replayEndTLI;
        SpinLockRelease(&xlogctl->info_lck);

        if (!force && newMinRecoveryPoint < lsn)
            elog(WARNING,
               "xlog min recovery request %X/%X is past current point %X/%X",
                 (uint32) (lsn >> 32) , (uint32) lsn,
                 (uint32) (newMinRecoveryPoint >> 32),
                 (uint32) newMinRecoveryPoint);

        /* update control file */
        if (ControlFile->minRecoveryPoint < newMinRecoveryPoint)
        {
            ControlFile->minRecoveryPoint = newMinRecoveryPoint;
            ControlFile->minRecoveryPointTLI = newMinRecoveryPointTLI;
            UpdateControlFile();
            minRecoveryPoint = newMinRecoveryPoint;
            minRecoveryPointTLI = newMinRecoveryPointTLI;

            ereport(DEBUG2,
                    (errmsg("updated min recovery point to %X/%X on timeline %u",
                            (uint32) (minRecoveryPoint >> 32),
                            (uint32) minRecoveryPoint,
                            newMinRecoveryPointTLI)));
        }
    }
    LWLockRelease(ControlFileLock);
}

/*
 * Ensure that all XLOG data through the given position is flushed to disk.
 *
 * NOTE: this differs from XLogWrite mainly in that the WALWriteLock is not
 * already held, and we try to avoid acquiring it if possible.
 */
void
XLogFlush(XLogRecPtr record)
{
    XLogRecPtr  WriteRqstPtr;
    XLogwrtRqst WriteRqst;

    /*
     * During REDO, we are reading not writing WAL.  Therefore, instead of
     * trying to flush the WAL, we should update minRecoveryPoint instead. We
     * test XLogInsertAllowed(), not InRecovery, because we need checkpointer
     * to act this way too, and because when it tries to write the
     * end-of-recovery checkpoint, it should indeed flush.
     */
    if (!XLogInsertAllowed())
    {
        UpdateMinRecoveryPoint(record, false);
        return;
    }

    /* Quick exit if already known flushed */
    if (record <= LogwrtResult.Flush)
        return;

#ifdef WAL_DEBUG
    if (XLOG_DEBUG)
        elog(LOG, "xlog flush request %X/%X; write %X/%X; flush %X/%X",
             (uint32) (record >> 32), (uint32) record,
             (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
             (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

    START_CRIT_SECTION();

    /*
     * Since fsync is usually a horribly expensive operation, we try to
     * piggyback as much data as we can on each fsync: if we see any more data
     * entered into the xlog buffer, we'll write and fsync that too, so that
     * the final value of LogwrtResult.Flush is as large as possible. This
     * gives us some chance of avoiding another fsync immediately after.
     */

    /* initialize to given target; may increase below */
    WriteRqstPtr = record;

    /*
     * Now wait until we get the write lock, or someone else does the flush
     * for us.
     */
    for (;;)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /* read LogwrtResult and update local state */
        SpinLockAcquire(&xlogctl->info_lck);
        if (WriteRqstPtr < xlogctl->LogwrtRqst.Write)
            WriteRqstPtr = xlogctl->LogwrtRqst.Write;
        LogwrtResult = xlogctl->LogwrtResult;
        SpinLockRelease(&xlogctl->info_lck);

        /* done already? */
        if (record <= LogwrtResult.Flush)
            break;

        /*
         * Try to get the write lock. If we can't get it immediately, wait
         * until it's released, and recheck if we still need to do the flush
         * or if the backend that held the lock did it for us already. This
         * helps to maintain a good rate of group committing when the system
         * is bottlenecked by the speed of fsyncing.
         */
        if (!LWLockAcquireOrWait(WALWriteLock, LW_EXCLUSIVE))
        {
            /*
             * The lock is now free, but we didn't acquire it yet. Before we
             * do, loop back to check if someone else flushed the record for
             * us already.
             */
            continue;
        }

        /* Got the lock; recheck whether request is satisfied */
        LogwrtResult = XLogCtl->LogwrtResult;
        if (record <= LogwrtResult.Flush)
        {
            LWLockRelease(WALWriteLock);
            break;
        }

        /*
         * Sleep before flush! By adding a delay here, we may give further
         * backends the opportunity to join the backlog of group commit
         * followers; this can significantly improve transaction throughput, at
         * the risk of increasing transaction latency.
         *
         * We do not sleep if enableFsync is not turned on, nor if there are
         * fewer than CommitSiblings other backends with active transactions.
         */
        if (CommitDelay > 0 && enableFsync &&
            MinimumActiveBackends(CommitSiblings))
            pg_usleep(CommitDelay);

        /* try to write/flush later additions to XLOG as well */
        if (LWLockConditionalAcquire(WALInsertLock, LW_EXCLUSIVE))
        {
            XLogCtlInsert *Insert = &XLogCtl->Insert;
            uint32      freespace = INSERT_FREESPACE(Insert);

            if (freespace == 0)     /* buffer is full */
                WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
            else
            {
                WriteRqstPtr = XLogCtl->xlblocks[Insert->curridx];
                WriteRqstPtr -= freespace;
            }
            LWLockRelease(WALInsertLock);
            WriteRqst.Write = WriteRqstPtr;
            WriteRqst.Flush = WriteRqstPtr;
        }
        else
        {
            WriteRqst.Write = WriteRqstPtr;
            WriteRqst.Flush = record;
        }
        XLogWrite(WriteRqst, false, false);

        LWLockRelease(WALWriteLock);
        /* done */
        break;
    }

    END_CRIT_SECTION();

    /* wake up walsenders now that we've released heavily contended locks */
    WalSndWakeupProcessRequests();

    /*
     * If we still haven't flushed to the request point then we have a
     * problem; most likely, the requested flush point is past end of XLOG.
     * This has been seen to occur when a disk page has a corrupted LSN.
     *
     * Formerly we treated this as a PANIC condition, but that hurts the
     * system's robustness rather than helping it: we do not want to take down
     * the whole system due to corruption on one data page.  In particular, if
     * the bad page is encountered again during recovery then we would be
     * unable to restart the database at all!  (This scenario actually
     * happened in the field several times with 7.1 releases.)  As of 8.4, bad
     * LSNs encountered during recovery are UpdateMinRecoveryPoint's problem;
     * the only time we can reach here during recovery is while flushing the
     * end-of-recovery checkpoint record, and we don't expect that to have a
     * bad LSN.
     *
     * Note that for calls from xact.c, the ERROR will be promoted to PANIC
     * since xact.c calls this routine inside a critical section.  However,
     * calls from bufmgr.c are not within critical sections and so we will not
     * force a restart for a bad LSN on a data page.
     */
    if (LogwrtResult.Flush < record)
        elog(ERROR,
        "xlog flush request %X/%X is not satisfied --- flushed only to %X/%X",
             (uint32) (record >> 32), (uint32) record,
             (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
}

/*
 * Flush xlog, but without specifying exactly where to flush to.
 *
 * We normally flush only completed blocks; but if there is nothing to do on
 * that basis, we check for unflushed async commits in the current incomplete
 * block, and flush through the latest one of those.  Thus, if async commits
 * are not being used, we will flush complete blocks only.  We can guarantee
 * that async commits reach disk after at most three cycles; normally only
 * one or two.  (When flushing complete blocks, we allow XLogWrite to write
 * "flexibly", meaning it can stop at the end of the buffer ring; this makes a
 * difference only with very high load or long wal_writer_delay, but imposes
 * one extra cycle for the worst case for async commits.)
 *
 * This routine is invoked periodically by the background walwriter process.
 *
 * Returns TRUE if we flushed anything.
 */
bool
XLogBackgroundFlush(void)
{
    XLogRecPtr  WriteRqstPtr;
    bool        flexible = true;
    bool        wrote_something = false;

    /* XLOG doesn't need flushing during recovery */
    if (RecoveryInProgress())
        return false;

    /* read LogwrtResult and update local state */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        LogwrtResult = xlogctl->LogwrtResult;
        WriteRqstPtr = xlogctl->LogwrtRqst.Write;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /* back off to last completed page boundary */
    WriteRqstPtr -= WriteRqstPtr % XLOG_BLCKSZ;

    /* if we have already flushed that far, consider async commit records */
    if (WriteRqstPtr <= LogwrtResult.Flush)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        WriteRqstPtr = xlogctl->asyncXactLSN;
        SpinLockRelease(&xlogctl->info_lck);
        flexible = false;       /* ensure it all gets written */
    }

    /*
     * If already known flushed, we're done. Just need to check if we are
     * holding an open file handle to a logfile that's no longer in use,
     * preventing the file from being deleted.
     */
    if (WriteRqstPtr <= LogwrtResult.Flush)
    {
        if (openLogFile >= 0)
        {
            if (!XLByteInPrevSeg(LogwrtResult.Write, openLogSegNo))
            {
                XLogFileClose();
            }
        }
        return false;
    }

#ifdef WAL_DEBUG
    if (XLOG_DEBUG)
        elog(LOG, "xlog bg flush request %X/%X; write %X/%X; flush %X/%X",
             (uint32) (WriteRqstPtr >> 32), (uint32) WriteRqstPtr,
             (uint32) (LogwrtResult.Write >> 32), (uint32) LogwrtResult.Write,
             (uint32) (LogwrtResult.Flush >> 32), (uint32) LogwrtResult.Flush);
#endif

    START_CRIT_SECTION();

    /* now wait for the write lock */
    LWLockAcquire(WALWriteLock, LW_EXCLUSIVE);
    LogwrtResult = XLogCtl->LogwrtResult;
    if (WriteRqstPtr > LogwrtResult.Flush)
    {
        XLogwrtRqst WriteRqst;

        WriteRqst.Write = WriteRqstPtr;
        WriteRqst.Flush = WriteRqstPtr;
        XLogWrite(WriteRqst, flexible, false);
        wrote_something = true;
    }
    LWLockRelease(WALWriteLock);

    END_CRIT_SECTION();

    /* wake up walsenders now that we've released heavily contended locks */
    WalSndWakeupProcessRequests();

    return wrote_something;
}

/*
 * Test whether XLOG data has been flushed up to (at least) the given position.
 *
 * Returns true if a flush is still needed.  (It may be that someone else
 * is already in process of flushing that far, however.)
 */
bool
XLogNeedsFlush(XLogRecPtr record)
{
    /*
     * During recovery, we don't flush WAL but update minRecoveryPoint
     * instead. So "needs flush" is taken to mean whether minRecoveryPoint
     * would need to be updated.
     */
    if (RecoveryInProgress())
    {
        /* Quick exit if already known updated */
        if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
            return false;

        /*
         * Update local copy of minRecoveryPoint. But if the lock is busy,
         * just return a conservative guess.
         */
        if (!LWLockConditionalAcquire(ControlFileLock, LW_SHARED))
            return true;
        minRecoveryPoint = ControlFile->minRecoveryPoint;
        minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
        LWLockRelease(ControlFileLock);

        /*
         * An invalid minRecoveryPoint means that we need to recover all the
         * WAL, i.e., we're doing crash recovery.  We never modify the control
         * file's value in that case, so we can short-circuit future checks
         * here too.
         */
        if (minRecoveryPoint == 0)
            updateMinRecoveryPoint = false;

        /* check again */
        if (record <= minRecoveryPoint || !updateMinRecoveryPoint)
            return false;
        else
            return true;
    }

    /* Quick exit if already known flushed */
    if (record <= LogwrtResult.Flush)
        return false;

    /* read LogwrtResult and update local state */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        LogwrtResult = xlogctl->LogwrtResult;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /* check again */
    if (record <= LogwrtResult.Flush)
        return false;

    return true;
}

/*
 * Create a new XLOG file segment, or open a pre-existing one.
 *
 * log, seg: identify segment to be created/opened.
 *
 * *use_existent: if TRUE, OK to use a pre-existing file (else, any
 * pre-existing file will be deleted).  On return, TRUE if a pre-existing
 * file was used.
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns FD of opened file.
 *
 * Note: errors here are ERROR not PANIC because we might or might not be
 * inside a critical section (eg, during checkpoint there is no reason to
 * take down the system on failure).  They will promote to PANIC if we are
 * in a critical section.
 */
int
XLogFileInit(XLogSegNo logsegno, bool *use_existent, bool use_lock)
{
    char        path[MAXPGPATH];
    char        tmppath[MAXPGPATH];
    char       *zbuffer;
    XLogSegNo   installed_segno;
    int         max_advance;
    int         fd;
    int         nbytes;

    XLogFilePath(path, ThisTimeLineID, logsegno);

    /*
     * Try to use existent file (checkpoint maker may have created it already)
     */
    if (*use_existent)
    {
        fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                           S_IRUSR | S_IWUSR);
        if (fd < 0)
        {
            if (errno != ENOENT)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not open file \"%s\": %m", path)));
        }
        else
            return fd;
    }

    /*
     * Initialize an empty (all zeroes) segment.  NOTE: it is possible that
     * another process is doing the same thing.  If so, we will end up
     * pre-creating an extra log segment.  That seems OK, and better than
     * holding the lock throughout this lengthy process.
     */
    elog(DEBUG2, "creating and filling new WAL file");

    snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

    unlink(tmppath);

    /*
     * Allocate a buffer full of zeros. This is done before opening the file
     * so that we don't leak the file descriptor if palloc fails.
     *
     * Note: palloc zbuffer, instead of just using a local char array, to
     * ensure it is reasonably well-aligned; this may save a few cycles
     * transferring data to the kernel.
     */
    zbuffer = (char *) palloc0(XLOG_BLCKSZ);

    /* do not use get_sync_bit() here --- want to fsync only at end of fill */
    fd = BasicOpenFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create file \"%s\": %m", tmppath)));

    /*
     * Zero-fill the file.  We have to do this the hard way to ensure that all
     * the file space has really been allocated --- on platforms that allow
     * "holes" in files, just seeking to the end doesn't allocate intermediate
     * space.  This way, we know that we have all the space and (after the
     * fsync below) that all the indirect blocks are down on disk.  Therefore,
     * fdatasync(2) or O_DSYNC will be sufficient to sync future writes to the
     * log file.
     */
    for (nbytes = 0; nbytes < XLogSegSize; nbytes += XLOG_BLCKSZ)
    {
        errno = 0;
        if ((int) write(fd, zbuffer, XLOG_BLCKSZ) != (int) XLOG_BLCKSZ)
        {
            int         save_errno = errno;

            /*
             * If we fail to make the file, delete it to release disk space
             */
            unlink(tmppath);

            close(fd);

            /* if write didn't set errno, assume problem is no disk space */
            errno = save_errno ? save_errno : ENOSPC;

            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not write to file \"%s\": %m", tmppath)));
        }
    }
    pfree(zbuffer);

    if (pg_fsync(fd) != 0)
    {
        close(fd);
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not fsync file \"%s\": %m", tmppath)));
    }

    if (close(fd))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m", tmppath)));

    /*
     * Now move the segment into place with its final name.
     *
     * If caller didn't want to use a pre-existing file, get rid of any
     * pre-existing file.  Otherwise, cope with possibility that someone else
     * has created the file while we were filling ours: if so, use ours to
     * pre-create a future log segment.
     */
    installed_segno = logsegno;
    max_advance = XLOGfileslop;
    if (!InstallXLogFileSegment(&installed_segno, tmppath,
                                *use_existent, &max_advance,
                                use_lock))
    {
        /*
         * No need for any more future segments, or InstallXLogFileSegment()
         * failed to rename the file into place. If the rename failed, opening
         * the file below will fail.
         */
        unlink(tmppath);
    }

    /* Set flag to tell caller there was no existent file */
    *use_existent = false;

    /* Now open original target segment (might not be file I just made) */
    fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
           errmsg("could not open file \"%s\": %m", path)));

    elog(DEBUG2, "done creating and filling new WAL file");

    return fd;
}

/*
 * Create a new XLOG file segment by copying a pre-existing one.
 *
 * destsegno: identify segment to be created.
 *
 * srcTLI, srclog, srcseg: identify segment to be copied (could be from
 *      a different timeline)
 *
 * Currently this is only used during recovery, and so there are no locking
 * considerations.  But we should be just as tense as XLogFileInit to avoid
 * emplacing a bogus file.
 */
static void
XLogFileCopy(XLogSegNo destsegno, TimeLineID srcTLI, XLogSegNo srcsegno)
{
    char        path[MAXPGPATH];
    char        tmppath[MAXPGPATH];
    char        buffer[XLOG_BLCKSZ];
    int         srcfd;
    int         fd;
    int         nbytes;

    /*
     * Open the source file
     */
    XLogFilePath(path, srcTLI, srcsegno);
    srcfd = OpenTransientFile(path, O_RDONLY | PG_BINARY, 0);
    if (srcfd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", path)));

    /*
     * Copy into a temp file name.
     */
    snprintf(tmppath, MAXPGPATH, XLOGDIR "/xlogtemp.%d", (int) getpid());

    unlink(tmppath);

    /* do not use get_sync_bit() here --- want to fsync only at end of fill */
    fd = OpenTransientFile(tmppath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                           S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create file \"%s\": %m", tmppath)));

    /*
     * Do the data copying.
     */
    for (nbytes = 0; nbytes < XLogSegSize; nbytes += sizeof(buffer))
    {
        errno = 0;
        if ((int) read(srcfd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
        {
            if (errno != 0)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not read file \"%s\": %m", path)));
            else
                ereport(ERROR,
                        (errmsg("not enough data in file \"%s\"", path)));
        }
        errno = 0;
        if ((int) write(fd, buffer, sizeof(buffer)) != (int) sizeof(buffer))
        {
            int         save_errno = errno;

            /*
             * If we fail to make the file, delete it to release disk space
             */
            unlink(tmppath);
            /* if write didn't set errno, assume problem is no disk space */
            errno = save_errno ? save_errno : ENOSPC;

            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not write to file \"%s\": %m", tmppath)));
        }
    }

    if (pg_fsync(fd) != 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not fsync file \"%s\": %m", tmppath)));

    if (CloseTransientFile(fd))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not close file \"%s\": %m", tmppath)));

    CloseTransientFile(srcfd);

    /*
     * Now move the segment into place with its final name.
     */
    if (!InstallXLogFileSegment(&destsegno, tmppath, false, NULL, false))
        elog(ERROR, "InstallXLogFileSegment should not have failed");
}

/*
 * Install a new XLOG segment file as a current or future log segment.
 *
 * This is used both to install a newly-created segment (which has a temp
 * filename while it's being created) and to recycle an old segment.
 *
 * *segno: identify segment to install as (or first possible target).
 * When find_free is TRUE, this is modified on return to indicate the
 * actual installation location or last segment searched.
 *
 * tmppath: initial name of file to install.  It will be renamed into place.
 *
 * find_free: if TRUE, install the new segment at the first empty segno
 * number at or after the passed numbers.  If FALSE, install the new segment
 * exactly where specified, deleting any existing segment file there.
 *
 * *max_advance: maximum number of segno slots to advance past the starting
 * point.  Fail if no free slot is found in this range.  On return, reduced
 * by the number of slots skipped over.  (Irrelevant, and may be NULL,
 * when find_free is FALSE.)
 *
 * use_lock: if TRUE, acquire ControlFileLock while moving file into
 * place.  This should be TRUE except during bootstrap log creation.  The
 * caller must *not* hold the lock at call.
 *
 * Returns TRUE if the file was installed successfully.  FALSE indicates that
 * max_advance limit was exceeded, or an error occurred while renaming the
 * file into place.
 */
static bool
InstallXLogFileSegment(XLogSegNo *segno, char *tmppath,
                       bool find_free, int *max_advance,
                       bool use_lock)
{
    char        path[MAXPGPATH];
    struct stat stat_buf;

    XLogFilePath(path, ThisTimeLineID, *segno);

    /*
     * We want to be sure that only one process does this at a time.
     */
    if (use_lock)
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

    if (!find_free)
    {
        /* Force installation: get rid of any pre-existing segment file */
        unlink(path);
    }
    else
    {
        /* Find a free slot to put it in */
        while (stat(path, &stat_buf) == 0)
        {
            if (*max_advance <= 0)
            {
                /* Failed to find a free slot within specified range */
                if (use_lock)
                    LWLockRelease(ControlFileLock);
                return false;
            }
            (*segno)++;
            (*max_advance)--;
            XLogFilePath(path, ThisTimeLineID, *segno);
        }
    }

    /*
     * Prefer link() to rename() here just to be really sure that we don't
     * overwrite an existing logfile.  However, there shouldn't be one, so
     * rename() is an acceptable substitute except for the truly paranoid.
     */
#if HAVE_WORKING_LINK
    if (link(tmppath, path) < 0)
    {
        if (use_lock)
            LWLockRelease(ControlFileLock);
        ereport(LOG,
                (errcode_for_file_access(),
                 errmsg("could not link file \"%s\" to \"%s\" (initialization of log file): %m",
                        tmppath, path)));
        return false;
    }
    unlink(tmppath);
#else
    if (rename(tmppath, path) < 0)
    {
        if (use_lock)
            LWLockRelease(ControlFileLock);
        ereport(LOG,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\" (initialization of log file): %m",
                        tmppath, path)));
        return false;
    }
#endif

    if (use_lock)
        LWLockRelease(ControlFileLock);

    return true;
}

/*
 * Open a pre-existing logfile segment for writing.
 */
int
XLogFileOpen(XLogSegNo segno)
{
    char        path[MAXPGPATH];
    int         fd;

    XLogFilePath(path, ThisTimeLineID, segno);

    fd = BasicOpenFile(path, O_RDWR | PG_BINARY | get_sync_bit(sync_method),
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not open xlog file \"%s\": %m", path)));

    return fd;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * If source == XLOG_FROM_ARCHIVE, the segment is retrieved from archive.
 * Otherwise, it's assumed to be already available in pg_xlog.
 */
static int
XLogFileRead(XLogSegNo segno, int emode, TimeLineID tli,
             int source, bool notfoundOk)
{
    char        xlogfname[MAXFNAMELEN];
    char        activitymsg[MAXFNAMELEN + 16];
    char        path[MAXPGPATH];
    int         fd;

    XLogFileName(xlogfname, tli, segno);

    switch (source)
    {
        case XLOG_FROM_ARCHIVE:
            /* Report recovery progress in PS display */
            snprintf(activitymsg, sizeof(activitymsg), "waiting for %s",
                     xlogfname);
            set_ps_display(activitymsg, false);

            restoredFromArchive = RestoreArchivedFile(path, xlogfname,
                                                      "RECOVERYXLOG",
                                                      XLogSegSize,
                                                      InRedo);
            if (!restoredFromArchive)
                return -1;
            break;

        case XLOG_FROM_PG_XLOG:
        case XLOG_FROM_STREAM:
            XLogFilePath(path, tli, segno);
            restoredFromArchive = false;
            break;

        default:
            elog(ERROR, "invalid XLogFileRead source %d", source);
    }

    /*
     * If the segment was fetched from archival storage, replace the existing
     * xlog segment (if any) with the archival version.
     */
    if (source == XLOG_FROM_ARCHIVE)
    {
        KeepFileRestoredFromArchive(path, xlogfname);

        /*
         * Set path to point at the new file in pg_xlog.
         */
        snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlogfname);
    }

    fd = BasicOpenFile(path, O_RDONLY | PG_BINARY, 0);
    if (fd >= 0)
    {
        /* Success! */
        curFileTLI = tli;

        /* Report recovery progress in PS display */
        snprintf(activitymsg, sizeof(activitymsg), "recovering %s",
                 xlogfname);
        set_ps_display(activitymsg, false);

        /* Track source of data in assorted state variables */
        readSource = source;
        XLogReceiptSource = source;
        /* In FROM_STREAM case, caller tracks receipt time, not me */
        if (source != XLOG_FROM_STREAM)
            XLogReceiptTime = GetCurrentTimestamp();

        return fd;
    }
    if (errno != ENOENT || !notfoundOk) /* unexpected failure? */
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not open file \"%s\": %m", path)));
    return -1;
}

/*
 * Open a logfile segment for reading (during recovery).
 *
 * This version searches for the segment with any TLI listed in expectedTLEs.
 */
static int
XLogFileReadAnyTLI(XLogSegNo segno, int emode, int source)
{
    char        path[MAXPGPATH];
    ListCell   *cell;
    int         fd;
    List       *tles;

    /*
     * Loop looking for a suitable timeline ID: we might need to read any of
     * the timelines listed in expectedTLEs.
     *
     * We expect curFileTLI on entry to be the TLI of the preceding file in
     * sequence, or 0 if there was no predecessor.  We do not allow curFileTLI
     * to go backwards; this prevents us from picking up the wrong file when a
     * parent timeline extends to higher segment numbers than the child we
     * want to read.
     *
     * If we haven't read the timeline history file yet, read it now, so that
     * we know which TLIs to scan.  We don't save the list in expectedTLEs,
     * however, unless we actually find a valid segment.  That way if there is
     * neither a timeline history file nor a WAL segment in the archive, and
     * streaming replication is set up, we'll read the timeline history file
     * streamed from the master when we start streaming, instead of recovering
     * with a dummy history generated here.
     */
    if (expectedTLEs)
        tles = expectedTLEs;
    else
        tles = readTimeLineHistory(recoveryTargetTLI);

    foreach(cell, tles)
    {
        TimeLineID  tli = ((TimeLineHistoryEntry *) lfirst(cell))->tli;

        if (tli < curFileTLI)
            break;              /* don't bother looking at too-old TLIs */

        if (source == XLOG_FROM_ANY || source == XLOG_FROM_ARCHIVE)
        {
            fd = XLogFileRead(segno, emode, tli,
                              XLOG_FROM_ARCHIVE, true);
            if (fd != -1)
            {
                elog(DEBUG1, "got WAL segment from archive");
                if (!expectedTLEs)
                    expectedTLEs = tles;
                return fd;
            }
        }

        if (source == XLOG_FROM_ANY || source == XLOG_FROM_PG_XLOG)
        {
            fd = XLogFileRead(segno, emode, tli,
                              XLOG_FROM_PG_XLOG, true);
            if (fd != -1)
            {
                if (!expectedTLEs)
                    expectedTLEs = tles;
                return fd;
            }
        }
    }

    /* Couldn't find it.  For simplicity, complain about front timeline */
    XLogFilePath(path, recoveryTargetTLI, segno);
    errno = ENOENT;
    ereport(emode,
            (errcode_for_file_access(),
             errmsg("could not open file \"%s\": %m", path)));
    return -1;
}

/*
 * Close the current logfile segment for writing.
 */
static void
XLogFileClose(void)
{
    Assert(openLogFile >= 0);

    /*
     * WAL segment files will not be re-read in normal operation, so we advise
     * the OS to release any cached pages.  But do not do so if WAL archiving
     * or streaming is active, because archiver and walsender process could
     * use the cache to read the WAL segment.
     */
#if defined(USE_POSIX_FADVISE) && defined(POSIX_FADV_DONTNEED)
    if (!XLogIsNeeded())
        (void) posix_fadvise(openLogFile, 0, 0, POSIX_FADV_DONTNEED);
#endif

    if (close(openLogFile))
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not close log file %s: %m",
                        XLogFileNameP(ThisTimeLineID, openLogSegNo))));
    openLogFile = -1;
}

/*
 * Preallocate log files beyond the specified log endpoint.
 *
 * XXX this is currently extremely conservative, since it forces only one
 * future log segment to exist, and even that only if we are 75% done with
 * the current one.  This is only appropriate for very low-WAL-volume systems.
 * High-volume systems will be OK once they've built up a sufficient set of
 * recycled log segments, but the startup transient is likely to include
 * a lot of segment creations by foreground processes, which is not so good.
 */
static void
PreallocXlogFiles(XLogRecPtr endptr)
{
    XLogSegNo   _logSegNo;
    int         lf;
    bool        use_existent;

    XLByteToPrevSeg(endptr, _logSegNo);
    if ((endptr - 1) % XLogSegSize >= (uint32) (0.75 * XLogSegSize))
    {
        _logSegNo++;
        use_existent = true;
        lf = XLogFileInit(_logSegNo, &use_existent, true);
        close(lf);
        if (!use_existent)
            CheckpointStats.ckpt_segs_added++;
    }
}

/*
 * Throws an error if the given log segment has already been removed or
 * recycled. The caller should only pass a segment that it knows to have
 * existed while the server has been running, as this function always
 * succeeds if no WAL segments have been removed since startup.
 * 'tli' is only used in the error message.
 */
void
CheckXLogRemoved(XLogSegNo segno, TimeLineID tli)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    XLogSegNo   lastRemovedSegNo;

    SpinLockAcquire(&xlogctl->info_lck);
    lastRemovedSegNo = xlogctl->lastRemovedSegNo;
    SpinLockRelease(&xlogctl->info_lck);

    if (segno <= lastRemovedSegNo)
    {
        char        filename[MAXFNAMELEN];

        XLogFileName(filename, tli, segno);
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("requested WAL segment %s has already been removed",
                        filename)));
    }
}

/*
 * Update the last removed segno pointer in shared memory, to reflect
 * that the given XLOG file has been removed.
 */
static void
UpdateLastRemovedPtr(char *filename)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    uint32      tli;
    XLogSegNo   segno;

    XLogFromFileName(filename, &tli, &segno);

    SpinLockAcquire(&xlogctl->info_lck);
    if (segno > xlogctl->lastRemovedSegNo)
        xlogctl->lastRemovedSegNo = segno;
    SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Recycle or remove all log files older or equal to passed segno
 *
 * endptr is current (or recent) end of xlog; this is used to determine
 * whether we want to recycle rather than delete no-longer-wanted log files.
 */
static void
RemoveOldXlogFiles(XLogSegNo segno, XLogRecPtr endptr)
{
    XLogSegNo   endlogSegNo;
    int         max_advance;
    DIR        *xldir;
    struct dirent *xlde;
    char        lastoff[MAXFNAMELEN];
    char        path[MAXPGPATH];

#ifdef WIN32
    char        newpath[MAXPGPATH];
#endif
    struct stat statbuf;

    /*
     * Initialize info about where to try to recycle to.  We allow recycling
     * segments up to XLOGfileslop segments beyond the current XLOG location.
     */
    XLByteToPrevSeg(endptr, endlogSegNo);
    max_advance = XLOGfileslop;

    xldir = AllocateDir(XLOGDIR);
    if (xldir == NULL)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open transaction log directory \"%s\": %m",
                        XLOGDIR)));

    /*
     * Construct a filename of the last segment to be kept. The timeline ID
     * doesn't matter, we ignore that in the comparison. (During recovery,
     * ThisTimeLineID isn't set, so we can't use that.)
     */
    XLogFileName(lastoff, 0, segno);

    elog(DEBUG2, "attempting to remove WAL segments older than log file %s",
         lastoff);

    while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
    {
        /*
         * We ignore the timeline part of the XLOG segment identifiers in
         * deciding whether a segment is still needed.  This ensures that we
         * won't prematurely remove a segment from a parent timeline. We could
         * probably be a little more proactive about removing segments of
         * non-parent timelines, but that would be a whole lot more
         * complicated.
         *
         * We use the alphanumeric sorting property of the filenames to decide
         * which ones are earlier than the lastoff segment.
         */
        if (strlen(xlde->d_name) == 24 &&
            strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
            strcmp(xlde->d_name + 8, lastoff + 8) <= 0)
        {
            if (XLogArchiveCheckDone(xlde->d_name))
            {
                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);

                /* Update the last removed location in shared memory first */
                UpdateLastRemovedPtr(xlde->d_name);

                /*
                 * Before deleting the file, see if it can be recycled as a
                 * future log segment. Only recycle normal files, pg_standby
                 * for example can create symbolic links pointing to a
                 * separate archive directory.
                 */
                if (lstat(path, &statbuf) == 0 && S_ISREG(statbuf.st_mode) &&
                    InstallXLogFileSegment(&endlogSegNo, path,
                                           true, &max_advance, true))
                {
                    ereport(DEBUG2,
                            (errmsg("recycled transaction log file \"%s\"",
                                    xlde->d_name)));
                    CheckpointStats.ckpt_segs_recycled++;
                    /* Needn't recheck that slot on future iterations */
                    if (max_advance > 0)
                    {
                        endlogSegNo++;
                        max_advance--;
                    }
                }
                else
                {
                    /* No need for any more future segments... */
                    int         rc;

                    ereport(DEBUG2,
                            (errmsg("removing transaction log file \"%s\"",
                                    xlde->d_name)));

#ifdef WIN32

                    /*
                     * On Windows, if another process (e.g another backend)
                     * holds the file open in FILE_SHARE_DELETE mode, unlink
                     * will succeed, but the file will still show up in
                     * directory listing until the last handle is closed. To
                     * avoid confusing the lingering deleted file for a live
                     * WAL file that needs to be archived, rename it before
                     * deleting it.
                     *
                     * If another process holds the file open without
                     * FILE_SHARE_DELETE flag, rename will fail. We'll try
                     * again at the next checkpoint.
                     */
                    snprintf(newpath, MAXPGPATH, "%s.deleted", path);
                    if (rename(path, newpath) != 0)
                    {
                        ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not rename old transaction log file \"%s\": %m",
                                        path)));
                        continue;
                    }
                    rc = unlink(newpath);
#else
                    rc = unlink(path);
#endif
                    if (rc != 0)
                    {
                        ereport(LOG,
                                (errcode_for_file_access(),
                                 errmsg("could not remove old transaction log file \"%s\": %m",
                                        path)));
                        continue;
                    }
                    CheckpointStats.ckpt_segs_removed++;
                }

                XLogArchiveCleanup(xlde->d_name);
            }
        }
    }

    FreeDir(xldir);
}

/*
 * Verify whether pg_xlog and pg_xlog/archive_status exist.
 * If the latter does not exist, recreate it.
 *
 * It is not the goal of this function to verify the contents of these
 * directories, but to help in cases where someone has performed a cluster
 * copy for PITR purposes but omitted pg_xlog from the copy.
 *
 * We could also recreate pg_xlog if it doesn't exist, but a deliberate
 * policy decision was made not to.  It is fairly common for pg_xlog to be
 * a symlink, and if that was the DBA's intent then automatically making a
 * plain directory would result in degraded performance with no notice.
 */
static void
ValidateXLOGDirectoryStructure(void)
{
    char        path[MAXPGPATH];
    struct stat stat_buf;

    /* Check for pg_xlog; if it doesn't exist, error out */
    if (stat(XLOGDIR, &stat_buf) != 0 ||
        !S_ISDIR(stat_buf.st_mode))
        ereport(FATAL,
                (errmsg("required WAL directory \"%s\" does not exist",
                        XLOGDIR)));

    /* Check for archive_status */
    snprintf(path, MAXPGPATH, XLOGDIR "/archive_status");
    if (stat(path, &stat_buf) == 0)
    {
        /* Check for weird cases where it exists but isn't a directory */
        if (!S_ISDIR(stat_buf.st_mode))
            ereport(FATAL,
                    (errmsg("required WAL directory \"%s\" does not exist",
                            path)));
    }
    else
    {
        ereport(LOG,
                (errmsg("creating missing WAL directory \"%s\"", path)));
        if (mkdir(path, S_IRWXU) < 0)
            ereport(FATAL,
                    (errmsg("could not create missing directory \"%s\": %m",
                            path)));
    }
}

/*
 * Remove previous backup history files.  This also retries creation of
 * .ready files for any backup history files for which XLogArchiveNotify
 * failed earlier.
 */
static void
CleanupBackupHistory(void)
{
    DIR        *xldir;
    struct dirent *xlde;
    char        path[MAXPGPATH];

    xldir = AllocateDir(XLOGDIR);
    if (xldir == NULL)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not open transaction log directory \"%s\": %m",
                        XLOGDIR)));

    while ((xlde = ReadDir(xldir, XLOGDIR)) != NULL)
    {
        if (strlen(xlde->d_name) > 24 &&
            strspn(xlde->d_name, "0123456789ABCDEF") == 24 &&
            strcmp(xlde->d_name + strlen(xlde->d_name) - strlen(".backup"),
                   ".backup") == 0)
        {
            if (XLogArchiveCheckDone(xlde->d_name))
            {
                ereport(DEBUG2,
                (errmsg("removing transaction log backup history file \"%s\"",
                        xlde->d_name)));
                snprintf(path, MAXPGPATH, XLOGDIR "/%s", xlde->d_name);
                unlink(path);
                XLogArchiveCleanup(xlde->d_name);
            }
        }
    }

    FreeDir(xldir);
}

/*
 * Restore a full-page image from a backup block attached to an XLOG record.
 *
 * lsn: LSN of the XLOG record being replayed
 * record: the complete XLOG record
 * block_index: which backup block to restore (0 .. XLR_MAX_BKP_BLOCKS - 1)
 * get_cleanup_lock: TRUE to get a cleanup rather than plain exclusive lock
 * keep_buffer: TRUE to return the buffer still locked and pinned
 *
 * Returns the buffer number containing the page.  Note this is not terribly
 * useful unless keep_buffer is specified as TRUE.
 *
 * Note: when a backup block is available in XLOG, we restore it
 * unconditionally, even if the page in the database appears newer.
 * This is to protect ourselves against database pages that were partially
 * or incorrectly written during a crash.  We assume that the XLOG data
 * must be good because it has passed a CRC check, while the database
 * page might not be.  This will force us to replay all subsequent
 * modifications of the page that appear in XLOG, rather than possibly
 * ignoring them as already applied, but that's not a huge drawback.
 *
 * If 'get_cleanup_lock' is true, a cleanup lock is obtained on the buffer,
 * else a normal exclusive lock is used.  During crash recovery, that's just
 * pro forma because there can't be any regular backends in the system, but
 * in hot standby mode the distinction is important.
 *
 * If 'keep_buffer' is true, return without releasing the buffer lock and pin;
 * then caller is responsible for doing UnlockReleaseBuffer() later.  This
 * is needed in some cases when replaying XLOG records that touch multiple
 * pages, to prevent inconsistent states from being visible to other backends.
 * (Again, that's only important in hot standby mode.)
 */
Buffer
RestoreBackupBlock(XLogRecPtr lsn, XLogRecord *record, int block_index,
                   bool get_cleanup_lock, bool keep_buffer)
{
    BkpBlock    bkpb;
    char       *blk;
    int         i;

    /* Locate requested BkpBlock in the record */
    blk = (char *) XLogRecGetData(record) + record->xl_len;
    for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
    {
        if (!(record->xl_info & XLR_BKP_BLOCK(i)))
            continue;

        memcpy(&bkpb, blk, sizeof(BkpBlock));
        blk += sizeof(BkpBlock);

        if (i == block_index)
        {
            /* Found it, apply the update */
            return RestoreBackupBlockContents(lsn, bkpb, blk, get_cleanup_lock,
                                              keep_buffer);
        }

        blk += BLCKSZ - bkpb.hole_length;
    }

    /* Caller specified a bogus block_index */
    elog(ERROR, "failed to restore block_index %d", block_index);
    return InvalidBuffer;       /* keep compiler quiet */
}

/*
 * Workhorse for RestoreBackupBlock usable without an xlog record
 *
 * Restores a full-page image from BkpBlock and a data pointer.
 */
static Buffer
RestoreBackupBlockContents(XLogRecPtr lsn, BkpBlock bkpb, char *blk,
                           bool get_cleanup_lock, bool keep_buffer)
{
    Buffer      buffer;
    Page        page;

    buffer = XLogReadBufferExtended(bkpb.node, bkpb.fork, bkpb.block,
                                    RBM_ZERO);
    Assert(BufferIsValid(buffer));
    if (get_cleanup_lock)
        LockBufferForCleanup(buffer);
    else
        LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);

    page = (Page) BufferGetPage(buffer);

    if (bkpb.hole_length == 0)
    {
        memcpy((char *) page, blk, BLCKSZ);
    }
    else
    {
        memcpy((char *) page, blk, bkpb.hole_offset);
        /* must zero-fill the hole */
        MemSet((char *) page + bkpb.hole_offset, 0, bkpb.hole_length);
        memcpy((char *) page + (bkpb.hole_offset + bkpb.hole_length),
               blk + bkpb.hole_offset,
               BLCKSZ - (bkpb.hole_offset + bkpb.hole_length));
    }

    /*
     * The checksum value on this page is currently invalid. We don't
     * need to reset it here since it will be set before being written.
     */

    PageSetLSN(page, lsn);
    MarkBufferDirty(buffer);

    if (!keep_buffer)
        UnlockReleaseBuffer(buffer);

    return buffer;
}

/*
 * Attempt to read an XLOG record.
 *
 * If RecPtr is not NULL, try to read a record at that position.  Otherwise
 * try to read a record just after the last one previously read.
 *
 * If no valid record is available, returns NULL, or fails if emode is PANIC.
 * (emode must be either PANIC, LOG). In standby mode, retries until a valid
 * record is available.
 *
 * The record is copied into readRecordBuf, so that on successful return,
 * the returned record pointer always points there.
 */
static XLogRecord *
ReadRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr, int emode,
           bool fetching_ckpt)
{
    XLogRecord *record;
    XLogPageReadPrivate *private = (XLogPageReadPrivate *) xlogreader->private_data;

    /* Pass through parameters to XLogPageRead */
    private->fetching_ckpt = fetching_ckpt;
    private->emode = emode;
    private->randAccess = (RecPtr != InvalidXLogRecPtr);

    /* This is the first attempt to read this page. */
    lastSourceFailed = false;

    for (;;)
    {
        char   *errormsg;

        record = XLogReadRecord(xlogreader, RecPtr, &errormsg);
        ReadRecPtr = xlogreader->ReadRecPtr;
        EndRecPtr = xlogreader->EndRecPtr;
        if (record == NULL)
        {
            if (readFile >= 0)
            {
                close(readFile);
                readFile = -1;
            }

            /*
             * We only end up here without a message when XLogPageRead() failed
             * - in that case we already logged something.
             * In StandbyMode that only happens if we have been triggered, so
             * we shouldn't loop anymore in that case.
             */
            if (errormsg)
                ereport(emode_for_corrupt_record(emode,
                                                 RecPtr ? RecPtr : EndRecPtr),
                        (errmsg_internal("%s", errormsg) /* already translated */));
        }
        /*
         * Check page TLI is one of the expected values.
         */
        else if (!tliInHistory(xlogreader->latestPageTLI, expectedTLEs))
        {
            char        fname[MAXFNAMELEN];
            XLogSegNo segno;
            int32 offset;

            XLByteToSeg(xlogreader->latestPagePtr, segno);
            offset = xlogreader->latestPagePtr % XLogSegSize;
            XLogFileName(fname, xlogreader->readPageTLI, segno);
            ereport(emode_for_corrupt_record(emode,
                                             RecPtr ? RecPtr : EndRecPtr),
                    (errmsg("unexpected timeline ID %u in log segment %s, offset %u",
                            xlogreader->latestPageTLI,
                            fname,
                            offset)));
            record = NULL;
        }

        if (record)
        {
            /* Great, got a record */
            return record;
        }
        else
        {
            /* No valid record available from this source */
            lastSourceFailed = true;

            /*
             * If archive recovery was requested, but we were still doing crash
             * recovery, switch to archive recovery and retry using the offline
             * archive. We have now replayed all the valid WAL in pg_xlog, so
             * we are presumably now consistent.
             *
             * We require that there's at least some valid WAL present in
             * pg_xlog, however (!fetch_ckpt). We could recover using the WAL
             * from the archive, even if pg_xlog is completely empty, but we'd
             * have no idea how far we'd have to replay to reach consistency.
             * So err on the safe side and give up.
             */
            if (!InArchiveRecovery && ArchiveRecoveryRequested &&
                !fetching_ckpt)
            {
                ereport(DEBUG1,
                        (errmsg_internal("reached end of WAL in pg_xlog, entering archive recovery")));
                InArchiveRecovery = true;
                if (StandbyModeRequested)
                    StandbyMode = true;

                /* initialize minRecoveryPoint to this record */
                LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
                ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
                if (ControlFile->minRecoveryPoint < EndRecPtr)
                {
                    ControlFile->minRecoveryPoint = EndRecPtr;
                    ControlFile->minRecoveryPointTLI = ThisTimeLineID;
                }
                /* update local copy */
                minRecoveryPoint = ControlFile->minRecoveryPoint;
                minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

                UpdateControlFile();
                LWLockRelease(ControlFileLock);

                CheckRecoveryConsistency();

                /*
                 * Before we retry, reset lastSourceFailed and currentSource
                 * so that we will check the archive next.
                 */
                lastSourceFailed = false;
                currentSource = 0;

                continue;
            }

            /* In standby mode, loop back to retry. Otherwise, give up. */
            if (StandbyMode && !CheckForStandbyTrigger())
                continue;
            else
                return NULL;
        }
    }
}

/*
 * Scan for new timelines that might have appeared in the archive since we
 * started recovery.
 *
 * If there are any, the function changes recovery target TLI to the latest
 * one and returns 'true'.
 */
static bool
rescanLatestTimeLine(void)
{
    List       *newExpectedTLEs;
    bool        found;
    ListCell   *cell;
    TimeLineID  newtarget;
    TimeLineID  oldtarget = recoveryTargetTLI;
    TimeLineHistoryEntry *currentTle = NULL;

    newtarget = findNewestTimeLine(recoveryTargetTLI);
    if (newtarget == recoveryTargetTLI)
    {
        /* No new timelines found */
        return false;
    }

    /*
     * Determine the list of expected TLIs for the new TLI
     */

    newExpectedTLEs = readTimeLineHistory(newtarget);

    /*
     * If the current timeline is not part of the history of the new
     * timeline, we cannot proceed to it.
     */
    found = false;
    foreach (cell, newExpectedTLEs)
    {
        currentTle = (TimeLineHistoryEntry *) lfirst(cell);

        if (currentTle->tli == recoveryTargetTLI)
        {
            found = true;
            break;
        }
    }
    if (!found)
    {
        ereport(LOG,
                (errmsg("new timeline %u is not a child of database system timeline %u",
                        newtarget,
                        ThisTimeLineID)));
        return false;
    }

    /*
     * The current timeline was found in the history file, but check that the
     * next timeline was forked off from it *after* the current recovery
     * location.
     */
    if (currentTle->end < EndRecPtr)
    {
        ereport(LOG,
                (errmsg("new timeline %u forked off current database system timeline %u before current recovery point %X/%X",
                        newtarget,
                        ThisTimeLineID,
                        (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr)));
        return false;
    }

    /* The new timeline history seems valid. Switch target */
    recoveryTargetTLI = newtarget;
    list_free_deep(expectedTLEs);
    expectedTLEs = newExpectedTLEs;

    /*
     * As in StartupXLOG(), try to ensure we have all the history files
     * between the old target and new target in pg_xlog.
     */
    restoreTimeLineHistoryFiles(oldtarget + 1, newtarget);

    ereport(LOG,
            (errmsg("new target timeline is %u",
                    recoveryTargetTLI)));

    return true;
}

/*
 * I/O routines for pg_control
 *
 * *ControlFile is a buffer in shared memory that holds an image of the
 * contents of pg_control.  WriteControlFile() initializes pg_control
 * given a preloaded buffer, ReadControlFile() loads the buffer from
 * the pg_control file (during postmaster or standalone-backend startup),
 * and UpdateControlFile() rewrites pg_control after we modify xlog state.
 *
 * For simplicity, WriteControlFile() initializes the fields of pg_control
 * that are related to checking backend/database compatibility, and
 * ReadControlFile() verifies they are correct.  We could split out the
 * I/O and compatibility-check functions, but there seems no need currently.
 */
static void
WriteControlFile(void)
{
    int         fd;
    char        buffer[PG_CONTROL_SIZE];        /* need not be aligned */

    /*
     * Initialize version and compatibility-check fields
     */
    ControlFile->pg_control_version = PG_CONTROL_VERSION;
    ControlFile->catalog_version_no = CATALOG_VERSION_NO;

    ControlFile->maxAlign = MAXIMUM_ALIGNOF;
    ControlFile->floatFormat = FLOATFORMAT_VALUE;

    ControlFile->blcksz = BLCKSZ;
    ControlFile->relseg_size = RELSEG_SIZE;
    ControlFile->xlog_blcksz = XLOG_BLCKSZ;
    ControlFile->xlog_seg_size = XLOG_SEG_SIZE;

    ControlFile->nameDataLen = NAMEDATALEN;
    ControlFile->indexMaxKeys = INDEX_MAX_KEYS;

    ControlFile->toast_max_chunk_size = TOAST_MAX_CHUNK_SIZE;

#ifdef HAVE_INT64_TIMESTAMP
    ControlFile->enableIntTimes = true;
#else
    ControlFile->enableIntTimes = false;
#endif
    ControlFile->float4ByVal = FLOAT4PASSBYVAL;
    ControlFile->float8ByVal = FLOAT8PASSBYVAL;

    /* Contents are protected with a CRC */
    INIT_CRC32(ControlFile->crc);
    COMP_CRC32(ControlFile->crc,
               (char *) ControlFile,
               offsetof(ControlFileData, crc));
    FIN_CRC32(ControlFile->crc);

    /*
     * We write out PG_CONTROL_SIZE bytes into pg_control, zero-padding the
     * excess over sizeof(ControlFileData).  This reduces the odds of
     * premature-EOF errors when reading pg_control.  We'll still fail when we
     * check the contents of the file, but hopefully with a more specific
     * error than "couldn't read pg_control".
     */
    if (sizeof(ControlFileData) > PG_CONTROL_SIZE)
        elog(PANIC, "sizeof(ControlFileData) is larger than PG_CONTROL_SIZE; fix either one");

    memset(buffer, 0, PG_CONTROL_SIZE);
    memcpy(buffer, ControlFile, sizeof(ControlFileData));

    fd = BasicOpenFile(XLOG_CONTROL_FILE,
                       O_RDWR | O_CREAT | O_EXCL | PG_BINARY,
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not create control file \"%s\": %m",
                        XLOG_CONTROL_FILE)));

    errno = 0;
    if (write(fd, buffer, PG_CONTROL_SIZE) != PG_CONTROL_SIZE)
    {
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not write to control file: %m")));
    }

    if (pg_fsync(fd) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not fsync control file: %m")));

    if (close(fd))
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not close control file: %m")));
}

static void
ReadControlFile(void)
{
    pg_crc32    crc;
    int         fd;

    /*
     * Read data...
     */
    fd = BasicOpenFile(XLOG_CONTROL_FILE,
                       O_RDWR | PG_BINARY,
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not open control file \"%s\": %m",
                        XLOG_CONTROL_FILE)));

    if (read(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not read from control file: %m")));

    close(fd);

    /*
     * Check for expected pg_control format version.  If this is wrong, the
     * CRC check will likely fail because we'll be checking the wrong number
     * of bytes.  Complaining about wrong version will probably be more
     * enlightening than complaining about wrong CRC.
     */

    if (ControlFile->pg_control_version != PG_CONTROL_VERSION && ControlFile->pg_control_version % 65536 == 0 && ControlFile->pg_control_version / 65536 != 0)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d (0x%08x),"
         " but the server was compiled with PG_CONTROL_VERSION %d (0x%08x).",
            ControlFile->pg_control_version, ControlFile->pg_control_version,
                           PG_CONTROL_VERSION, PG_CONTROL_VERSION),
                 errhint("This could be a problem of mismatched byte ordering.  It looks like you need to initdb.")));

    if (ControlFile->pg_control_version != PG_CONTROL_VERSION)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with PG_CONTROL_VERSION %d,"
                  " but the server was compiled with PG_CONTROL_VERSION %d.",
                        ControlFile->pg_control_version, PG_CONTROL_VERSION),
                 errhint("It looks like you need to initdb.")));

    /* Now check the CRC. */
    INIT_CRC32(crc);
    COMP_CRC32(crc,
               (char *) ControlFile,
               offsetof(ControlFileData, crc));
    FIN_CRC32(crc);

    if (!EQ_CRC32(crc, ControlFile->crc))
        ereport(FATAL,
                (errmsg("incorrect checksum in control file")));

    /*
     * Do compatibility checking immediately.  If the database isn't
     * compatible with the backend executable, we want to abort before we can
     * possibly do any damage.
     */
    if (ControlFile->catalog_version_no != CATALOG_VERSION_NO)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with CATALOG_VERSION_NO %d,"
                  " but the server was compiled with CATALOG_VERSION_NO %d.",
                        ControlFile->catalog_version_no, CATALOG_VERSION_NO),
                 errhint("It looks like you need to initdb.")));
    if (ControlFile->maxAlign != MAXIMUM_ALIGNOF)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
           errdetail("The database cluster was initialized with MAXALIGN %d,"
                     " but the server was compiled with MAXALIGN %d.",
                     ControlFile->maxAlign, MAXIMUM_ALIGNOF),
                 errhint("It looks like you need to initdb.")));
    if (ControlFile->floatFormat != FLOATFORMAT_VALUE)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster appears to use a different floating-point number format than the server executable."),
                 errhint("It looks like you need to initdb.")));
    if (ControlFile->blcksz != BLCKSZ)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
             errdetail("The database cluster was initialized with BLCKSZ %d,"
                       " but the server was compiled with BLCKSZ %d.",
                       ControlFile->blcksz, BLCKSZ),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->relseg_size != RELSEG_SIZE)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
        errdetail("The database cluster was initialized with RELSEG_SIZE %d,"
                  " but the server was compiled with RELSEG_SIZE %d.",
                  ControlFile->relseg_size, RELSEG_SIZE),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->xlog_blcksz != XLOG_BLCKSZ)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
        errdetail("The database cluster was initialized with XLOG_BLCKSZ %d,"
                  " but the server was compiled with XLOG_BLCKSZ %d.",
                  ControlFile->xlog_blcksz, XLOG_BLCKSZ),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->xlog_seg_size != XLOG_SEG_SIZE)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with XLOG_SEG_SIZE %d,"
                       " but the server was compiled with XLOG_SEG_SIZE %d.",
                           ControlFile->xlog_seg_size, XLOG_SEG_SIZE),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->nameDataLen != NAMEDATALEN)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
        errdetail("The database cluster was initialized with NAMEDATALEN %d,"
                  " but the server was compiled with NAMEDATALEN %d.",
                  ControlFile->nameDataLen, NAMEDATALEN),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->indexMaxKeys != INDEX_MAX_KEYS)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with INDEX_MAX_KEYS %d,"
                      " but the server was compiled with INDEX_MAX_KEYS %d.",
                           ControlFile->indexMaxKeys, INDEX_MAX_KEYS),
                 errhint("It looks like you need to recompile or initdb.")));
    if (ControlFile->toast_max_chunk_size != TOAST_MAX_CHUNK_SIZE)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with TOAST_MAX_CHUNK_SIZE %d,"
                " but the server was compiled with TOAST_MAX_CHUNK_SIZE %d.",
              ControlFile->toast_max_chunk_size, (int) TOAST_MAX_CHUNK_SIZE),
                 errhint("It looks like you need to recompile or initdb.")));

#ifdef HAVE_INT64_TIMESTAMP
    if (ControlFile->enableIntTimes != true)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized without HAVE_INT64_TIMESTAMP"
                  " but the server was compiled with HAVE_INT64_TIMESTAMP."),
                 errhint("It looks like you need to recompile or initdb.")));
#else
    if (ControlFile->enableIntTimes != false)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized with HAVE_INT64_TIMESTAMP"
               " but the server was compiled without HAVE_INT64_TIMESTAMP."),
                 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT4_BYVAL
    if (ControlFile->float4ByVal != true)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized without USE_FLOAT4_BYVAL"
                      " but the server was compiled with USE_FLOAT4_BYVAL."),
                 errhint("It looks like you need to recompile or initdb.")));
#else
    if (ControlFile->float4ByVal != false)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
        errdetail("The database cluster was initialized with USE_FLOAT4_BYVAL"
                  " but the server was compiled without USE_FLOAT4_BYVAL."),
                 errhint("It looks like you need to recompile or initdb.")));
#endif

#ifdef USE_FLOAT8_BYVAL
    if (ControlFile->float8ByVal != true)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
                 errdetail("The database cluster was initialized without USE_FLOAT8_BYVAL"
                      " but the server was compiled with USE_FLOAT8_BYVAL."),
                 errhint("It looks like you need to recompile or initdb.")));
#else
    if (ControlFile->float8ByVal != false)
        ereport(FATAL,
                (errmsg("database files are incompatible with server"),
        errdetail("The database cluster was initialized with USE_FLOAT8_BYVAL"
                  " but the server was compiled without USE_FLOAT8_BYVAL."),
                 errhint("It looks like you need to recompile or initdb.")));
#endif
}

void
UpdateControlFile(void)
{
    int         fd;

    INIT_CRC32(ControlFile->crc);
    COMP_CRC32(ControlFile->crc,
               (char *) ControlFile,
               offsetof(ControlFileData, crc));
    FIN_CRC32(ControlFile->crc);

    fd = BasicOpenFile(XLOG_CONTROL_FILE,
                       O_RDWR | PG_BINARY,
                       S_IRUSR | S_IWUSR);
    if (fd < 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not open control file \"%s\": %m",
                        XLOG_CONTROL_FILE)));

    errno = 0;
    if (write(fd, ControlFile, sizeof(ControlFileData)) != sizeof(ControlFileData))
    {
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not write to control file: %m")));
    }

    if (pg_fsync(fd) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not fsync control file: %m")));

    if (close(fd))
        ereport(PANIC,
                (errcode_for_file_access(),
                 errmsg("could not close control file: %m")));
}

/*
 * Returns the unique system identifier from control file.
 */
uint64
GetSystemIdentifier(void)
{
    Assert(ControlFile != NULL);
    return ControlFile->system_identifier;
}

/*
 * Are checksums enabled for data pages?
 */
bool
DataChecksumsEnabled(void)
{
    Assert(ControlFile != NULL);
    return (ControlFile->data_checksum_version > 0);
}

/*
 * Returns a fake LSN for unlogged relations.
 *
 * Each call generates an LSN that is greater than any previous value
 * returned. The current counter value is saved and restored across clean
 * shutdowns, but like unlogged relations, does not survive a crash. This can
 * be used in lieu of real LSN values returned by XLogInsert, if you need an
 * LSN-like increasing sequence of numbers without writing any WAL.
 */
XLogRecPtr
GetFakeLSNForUnloggedRel(void)
{
    XLogRecPtr nextUnloggedLSN;

    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    /* increment the unloggedLSN counter, need SpinLock */
    SpinLockAcquire(&xlogctl->ulsn_lck);
    nextUnloggedLSN = xlogctl->unloggedLSN++;
    SpinLockRelease(&xlogctl->ulsn_lck);

    return nextUnloggedLSN;
}

/*
 * Auto-tune the number of XLOG buffers.
 *
 * The preferred setting for wal_buffers is about 3% of shared_buffers, with
 * a maximum of one XLOG segment (there is little reason to think that more
 * is helpful, at least so long as we force an fsync when switching log files)
 * and a minimum of 8 blocks (which was the default value prior to PostgreSQL
 * 9.1, when auto-tuning was added).
 *
 * This should not be called until NBuffers has received its final value.
 */
static int
XLOGChooseNumBuffers(void)
{
    int         xbuffers;

    xbuffers = NBuffers / 32;
    if (xbuffers > XLOG_SEG_SIZE / XLOG_BLCKSZ)
        xbuffers = XLOG_SEG_SIZE / XLOG_BLCKSZ;
    if (xbuffers < 8)
        xbuffers = 8;
    return xbuffers;
}

/*
 * GUC check_hook for wal_buffers
 */
bool
check_wal_buffers(int *newval, void **extra, GucSource source)
{
    /*
     * -1 indicates a request for auto-tune.
     */
    if (*newval == -1)
    {
        /*
         * If we haven't yet changed the boot_val default of -1, just let it
         * be.  We'll fix it when XLOGShmemSize is called.
         */
        if (XLOGbuffers == -1)
            return true;

        /* Otherwise, substitute the auto-tune value */
        *newval = XLOGChooseNumBuffers();
    }

    /*
     * We clamp manually-set values to at least 4 blocks.  Prior to PostgreSQL
     * 9.1, a minimum of 4 was enforced by guc.c, but since that is no longer
     * the case, we just silently treat such values as a request for the
     * minimum.  (We could throw an error instead, but that doesn't seem very
     * helpful.)
     */
    if (*newval < 4)
        *newval = 4;

    return true;
}

/*
 * Initialization of shared memory for XLOG
 */
Size
XLOGShmemSize(void)
{
    Size        size;

    /*
     * If the value of wal_buffers is -1, use the preferred auto-tune value.
     * This isn't an amazingly clean place to do this, but we must wait till
     * NBuffers has received its final value, and must do it before using the
     * value of XLOGbuffers to do anything important.
     */
    if (XLOGbuffers == -1)
    {
        char        buf[32];

        snprintf(buf, sizeof(buf), "%d", XLOGChooseNumBuffers());
        SetConfigOption("wal_buffers", buf, PGC_POSTMASTER, PGC_S_OVERRIDE);
    }
    Assert(XLOGbuffers > 0);

    /* XLogCtl */
    size = sizeof(XLogCtlData);
    /* xlblocks array */
    size = add_size(size, mul_size(sizeof(XLogRecPtr), XLOGbuffers));
    /* extra alignment padding for XLOG I/O buffers */
    size = add_size(size, ALIGNOF_XLOG_BUFFER);
    /* and the buffers themselves */
    size = add_size(size, mul_size(XLOG_BLCKSZ, XLOGbuffers));

    /*
     * Note: we don't count ControlFileData, it comes out of the "slop factor"
     * added by CreateSharedMemoryAndSemaphores.  This lets us use this
     * routine again below to compute the actual allocation size.
     */

    return size;
}

void
XLOGShmemInit(void)
{
    bool        foundCFile,
                foundXLog;
    char       *allocptr;

    ControlFile = (ControlFileData *)
        ShmemInitStruct("Control File", sizeof(ControlFileData), &foundCFile);
    XLogCtl = (XLogCtlData *)
        ShmemInitStruct("XLOG Ctl", XLOGShmemSize(), &foundXLog);

    if (foundCFile || foundXLog)
    {
        /* both should be present or neither */
        Assert(foundCFile && foundXLog);
        return;
    }

    memset(XLogCtl, 0, sizeof(XLogCtlData));

    /*
     * Since XLogCtlData contains XLogRecPtr fields, its sizeof should be a
     * multiple of the alignment for same, so no extra alignment padding is
     * needed here.
     */
    allocptr = ((char *) XLogCtl) + sizeof(XLogCtlData);
    XLogCtl->xlblocks = (XLogRecPtr *) allocptr;
    memset(XLogCtl->xlblocks, 0, sizeof(XLogRecPtr) * XLOGbuffers);
    allocptr += sizeof(XLogRecPtr) * XLOGbuffers;

    /*
     * Align the start of the page buffers to an ALIGNOF_XLOG_BUFFER boundary.
     */
    allocptr = (char *) TYPEALIGN(ALIGNOF_XLOG_BUFFER, allocptr);
    XLogCtl->pages = allocptr;
    memset(XLogCtl->pages, 0, (Size) XLOG_BLCKSZ * XLOGbuffers);

    /*
     * Do basic initialization of XLogCtl shared data. (StartupXLOG will fill
     * in additional info.)
     */
    XLogCtl->XLogCacheBlck = XLOGbuffers - 1;
    XLogCtl->SharedRecoveryInProgress = true;
    XLogCtl->SharedHotStandbyActive = false;
    XLogCtl->WalWriterSleeping = false;
    XLogCtl->Insert.currpage = (XLogPageHeader) (XLogCtl->pages);
    SpinLockInit(&XLogCtl->info_lck);
    SpinLockInit(&XLogCtl->ulsn_lck);
    InitSharedLatch(&XLogCtl->recoveryWakeupLatch);

    /*
     * If we are not in bootstrap mode, pg_control should already exist. Read
     * and validate it immediately (see comments in ReadControlFile() for the
     * reasons why).
     */
    if (!IsBootstrapProcessingMode())
        ReadControlFile();
}

/*
 * This func must be called ONCE on system install.  It creates pg_control
 * and the initial XLOG segment.
 */
void
BootStrapXLOG(void)
{
    CheckPoint  checkPoint;
    char       *buffer;
    XLogPageHeader page;
    XLogLongPageHeader longpage;
    XLogRecord *record;
    bool        use_existent;
    uint64      sysidentifier;
    struct timeval tv;
    pg_crc32    crc;

    /*
     * Select a hopefully-unique system identifier code for this installation.
     * We use the result of gettimeofday(), including the fractional seconds
     * field, as being about as unique as we can easily get.  (Think not to
     * use random(), since it hasn't been seeded and there's no portable way
     * to seed it other than the system clock value...)  The upper half of the
     * uint64 value is just the tv_sec part, while the lower half is the XOR
     * of tv_sec and tv_usec.  This is to ensure that we don't lose uniqueness
     * unnecessarily if "uint64" is really only 32 bits wide.  A person
     * knowing this encoding can determine the initialization time of the
     * installation, which could perhaps be useful sometimes.
     */
    gettimeofday(&tv, NULL);
    sysidentifier = ((uint64) tv.tv_sec) << 32;
    sysidentifier |= (uint32) (tv.tv_sec | tv.tv_usec);

    /* First timeline ID is always 1 */
    ThisTimeLineID = 1;

    /* page buffer must be aligned suitably for O_DIRECT */
    buffer = (char *) palloc(XLOG_BLCKSZ + ALIGNOF_XLOG_BUFFER);
    page = (XLogPageHeader) TYPEALIGN(ALIGNOF_XLOG_BUFFER, buffer);
    memset(page, 0, XLOG_BLCKSZ);

    /*
     * Set up information for the initial checkpoint record
     *
     * The initial checkpoint record is written to the beginning of the WAL
     * segment with logid=0 logseg=1. The very first WAL segment, 0/0, is not
     * used, so that we can use 0/0 to mean "before any valid WAL segment".
     */
    checkPoint.redo = XLogSegSize + SizeOfXLogLongPHD;
    checkPoint.ThisTimeLineID = ThisTimeLineID;
    checkPoint.PrevTimeLineID = ThisTimeLineID;
    checkPoint.fullPageWrites = fullPageWrites;
    checkPoint.nextXidEpoch = 0;
    checkPoint.nextXid = FirstNormalTransactionId;
    checkPoint.nextOid = FirstBootstrapObjectId;
    checkPoint.nextMulti = FirstMultiXactId;
    checkPoint.nextMultiOffset = 0;
    checkPoint.oldestXid = FirstNormalTransactionId;
    checkPoint.oldestXidDB = TemplateDbOid;
    checkPoint.oldestMulti = FirstMultiXactId;
    checkPoint.oldestMultiDB = TemplateDbOid;
    checkPoint.time = (pg_time_t) time(NULL);
    checkPoint.oldestActiveXid = InvalidTransactionId;

    ShmemVariableCache->nextXid = checkPoint.nextXid;
    ShmemVariableCache->nextOid = checkPoint.nextOid;
    ShmemVariableCache->oidCount = 0;
    MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
    SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
    SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

    /* Set up the XLOG page header */
    page->xlp_magic = XLOG_PAGE_MAGIC;
    page->xlp_info = XLP_LONG_HEADER;
    page->xlp_tli = ThisTimeLineID;
    page->xlp_pageaddr = XLogSegSize;
    longpage = (XLogLongPageHeader) page;
    longpage->xlp_sysid = sysidentifier;
    longpage->xlp_seg_size = XLogSegSize;
    longpage->xlp_xlog_blcksz = XLOG_BLCKSZ;

    /* Insert the initial checkpoint record */
    record = (XLogRecord *) ((char *) page + SizeOfXLogLongPHD);
    record->xl_prev = 0;
    record->xl_xid = InvalidTransactionId;
    record->xl_tot_len = SizeOfXLogRecord + sizeof(checkPoint);
    record->xl_len = sizeof(checkPoint);
    record->xl_info = XLOG_CHECKPOINT_SHUTDOWN;
    record->xl_rmid = RM_XLOG_ID;
    memcpy(XLogRecGetData(record), &checkPoint, sizeof(checkPoint));

    INIT_CRC32(crc);
    COMP_CRC32(crc, &checkPoint, sizeof(checkPoint));
    COMP_CRC32(crc, (char *) record, offsetof(XLogRecord, xl_crc));
    FIN_CRC32(crc);
    record->xl_crc = crc;

    /* Create first XLOG segment file */
    use_existent = false;
    openLogFile = XLogFileInit(1, &use_existent, false);

    /* Write the first page with the initial record */
    errno = 0;
    if (write(openLogFile, page, XLOG_BLCKSZ) != XLOG_BLCKSZ)
    {
        /* if write didn't set errno, assume problem is no disk space */
        if (errno == 0)
            errno = ENOSPC;
        ereport(PANIC,
                (errcode_for_file_access(),
              errmsg("could not write bootstrap transaction log file: %m")));
    }

    if (pg_fsync(openLogFile) != 0)
        ereport(PANIC,
                (errcode_for_file_access(),
              errmsg("could not fsync bootstrap transaction log file: %m")));

    if (close(openLogFile))
        ereport(PANIC,
                (errcode_for_file_access(),
              errmsg("could not close bootstrap transaction log file: %m")));

    openLogFile = -1;

    /* Now create pg_control */

    memset(ControlFile, 0, sizeof(ControlFileData));
    /* Initialize pg_control status fields */
    ControlFile->system_identifier = sysidentifier;
    ControlFile->state = DB_SHUTDOWNED;
    ControlFile->time = checkPoint.time;
    ControlFile->checkPoint = checkPoint.redo;
    ControlFile->checkPointCopy = checkPoint;
    ControlFile->unloggedLSN = 1;

    /* Set important parameter values for use when replaying WAL */
    ControlFile->MaxConnections = MaxConnections;
    ControlFile->max_prepared_xacts = max_prepared_xacts;
    ControlFile->max_locks_per_xact = max_locks_per_xact;
    ControlFile->wal_level = wal_level;
    ControlFile->data_checksum_version = bootstrap_data_checksum_version;

    /* some additional ControlFile fields are set in WriteControlFile() */

    WriteControlFile();

    /* Bootstrap the commit log, too */
    BootStrapCLOG();
    BootStrapSUBTRANS();
    BootStrapMultiXact();

    pfree(buffer);
}

static char *
str_time(pg_time_t tnow)
{
    static char buf[128];

    pg_strftime(buf, sizeof(buf),
                "%Y-%m-%d %H:%M:%S %Z",
                pg_localtime(&tnow, log_timezone));

    return buf;
}

/*
 * See if there is a recovery command file (recovery.conf), and if so
 * read in parameters for archive recovery and XLOG streaming.
 *
 * The file is parsed using the main configuration parser.
 */
static void
readRecoveryCommandFile(void)
{
    FILE       *fd;
    TimeLineID  rtli = 0;
    bool        rtliGiven = false;
    ConfigVariable *item,
               *head = NULL,
               *tail = NULL;

    fd = AllocateFile(RECOVERY_COMMAND_FILE, "r");
    if (fd == NULL)
    {
        if (errno == ENOENT)
            return;             /* not there, so no archive recovery */
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not open recovery command file \"%s\": %m",
                        RECOVERY_COMMAND_FILE)));
    }

    /*
     * Since we're asking ParseConfigFp() to report errors as FATAL, there's
     * no need to check the return value.
     */
    (void) ParseConfigFp(fd, RECOVERY_COMMAND_FILE, 0, FATAL, &head, &tail);

    FreeFile(fd);

    for (item = head; item; item = item->next)
    {
        if (strcmp(item->name, "restore_command") == 0)
        {
            recoveryRestoreCommand = pstrdup(item->value);
            ereport(DEBUG2,
                    (errmsg_internal("restore_command = '%s'",
                                     recoveryRestoreCommand)));
        }
        else if (strcmp(item->name, "recovery_end_command") == 0)
        {
            recoveryEndCommand = pstrdup(item->value);
            ereport(DEBUG2,
                    (errmsg_internal("recovery_end_command = '%s'",
                                     recoveryEndCommand)));
        }
        else if (strcmp(item->name, "archive_cleanup_command") == 0)
        {
            archiveCleanupCommand = pstrdup(item->value);
            ereport(DEBUG2,
                    (errmsg_internal("archive_cleanup_command = '%s'",
                                     archiveCleanupCommand)));
        }
        else if (strcmp(item->name, "pause_at_recovery_target") == 0)
        {
            if (!parse_bool(item->value, &recoveryPauseAtTarget))
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("parameter \"%s\" requires a Boolean value", "pause_at_recovery_target")));
            ereport(DEBUG2,
                    (errmsg_internal("pause_at_recovery_target = '%s'",
                                     item->value)));
        }
        else if (strcmp(item->name, "recovery_target_timeline") == 0)
        {
            rtliGiven = true;
            if (strcmp(item->value, "latest") == 0)
                rtli = 0;
            else
            {
                errno = 0;
                rtli = (TimeLineID) strtoul(item->value, NULL, 0);
                if (errno == EINVAL || errno == ERANGE)
                    ereport(FATAL,
                            (errmsg("recovery_target_timeline is not a valid number: \"%s\"",
                                    item->value)));
            }
            if (rtli)
                ereport(DEBUG2,
                   (errmsg_internal("recovery_target_timeline = %u", rtli)));
            else
                ereport(DEBUG2,
                     (errmsg_internal("recovery_target_timeline = latest")));
        }
        else if (strcmp(item->name, "recovery_target_xid") == 0)
        {
            errno = 0;
            recoveryTargetXid = (TransactionId) strtoul(item->value, NULL, 0);
            if (errno == EINVAL || errno == ERANGE)
                ereport(FATAL,
                 (errmsg("recovery_target_xid is not a valid number: \"%s\"",
                         item->value)));
            ereport(DEBUG2,
                    (errmsg_internal("recovery_target_xid = %u",
                                     recoveryTargetXid)));
            recoveryTarget = RECOVERY_TARGET_XID;
        }
        else if (strcmp(item->name, "recovery_target_time") == 0)
        {
            /*
             * if recovery_target_xid or recovery_target_name specified, then
             * this overrides recovery_target_time
             */
            if (recoveryTarget == RECOVERY_TARGET_XID ||
                recoveryTarget == RECOVERY_TARGET_NAME)
                continue;
            recoveryTarget = RECOVERY_TARGET_TIME;

            /*
             * Convert the time string given by the user to TimestampTz form.
             */
            recoveryTargetTime =
                DatumGetTimestampTz(DirectFunctionCall3(timestamptz_in,
                                                CStringGetDatum(item->value),
                                                ObjectIdGetDatum(InvalidOid),
                                                        Int32GetDatum(-1)));
            ereport(DEBUG2,
                    (errmsg_internal("recovery_target_time = '%s'",
                                   timestamptz_to_str(recoveryTargetTime))));
        }
        else if (strcmp(item->name, "recovery_target_name") == 0)
        {
            /*
             * if recovery_target_xid specified, then this overrides
             * recovery_target_name
             */
            if (recoveryTarget == RECOVERY_TARGET_XID)
                continue;
            recoveryTarget = RECOVERY_TARGET_NAME;

            recoveryTargetName = pstrdup(item->value);
            if (strlen(recoveryTargetName) >= MAXFNAMELEN)
                ereport(FATAL,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("recovery_target_name is too long (maximum %d characters)",
                                MAXFNAMELEN - 1)));

            ereport(DEBUG2,
                    (errmsg_internal("recovery_target_name = '%s'",
                                     recoveryTargetName)));
        }
        else if (strcmp(item->name, "recovery_target_inclusive") == 0)
        {
            /*
             * does nothing if a recovery_target is not also set
             */
            if (!parse_bool(item->value, &recoveryTargetInclusive))
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("parameter \"%s\" requires a Boolean value",
                                "recovery_target_inclusive")));
            ereport(DEBUG2,
                    (errmsg_internal("recovery_target_inclusive = %s",
                                     item->value)));
        }
        else if (strcmp(item->name, "standby_mode") == 0)
        {
            if (!parse_bool(item->value, &StandbyModeRequested))
                ereport(ERROR,
                        (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                         errmsg("parameter \"%s\" requires a Boolean value",
                                "standby_mode")));
            ereport(DEBUG2,
                    (errmsg_internal("standby_mode = '%s'", item->value)));
        }
        else if (strcmp(item->name, "primary_conninfo") == 0)
        {
            PrimaryConnInfo = pstrdup(item->value);
            ereport(DEBUG2,
                    (errmsg_internal("primary_conninfo = '%s'",
                                     PrimaryConnInfo)));
        }
        else if (strcmp(item->name, "trigger_file") == 0)
        {
            TriggerFile = pstrdup(item->value);
            ereport(DEBUG2,
                    (errmsg_internal("trigger_file = '%s'",
                                     TriggerFile)));
        }
        else
            ereport(FATAL,
                    (errmsg("unrecognized recovery parameter \"%s\"",
                            item->name)));
    }

    /*
     * Check for compulsory parameters
     */
    if (StandbyModeRequested)
    {
        if (PrimaryConnInfo == NULL && recoveryRestoreCommand == NULL)
            ereport(WARNING,
                    (errmsg("recovery command file \"%s\" specified neither primary_conninfo nor restore_command",
                            RECOVERY_COMMAND_FILE),
                     errhint("The database server will regularly poll the pg_xlog subdirectory to check for files placed there.")));
    }
    else
    {
        if (recoveryRestoreCommand == NULL)
            ereport(FATAL,
                    (errmsg("recovery command file \"%s\" must specify restore_command when standby mode is not enabled",
                            RECOVERY_COMMAND_FILE)));
    }

    /* Enable fetching from archive recovery area */
    ArchiveRecoveryRequested = true;

    /*
     * If user specified recovery_target_timeline, validate it or compute the
     * "latest" value.  We can't do this until after we've gotten the restore
     * command and set InArchiveRecovery, because we need to fetch timeline
     * history files from the archive.
     */
    if (rtliGiven)
    {
        if (rtli)
        {
            /* Timeline 1 does not have a history file, all else should */
            if (rtli != 1 && !existsTimeLineHistory(rtli))
                ereport(FATAL,
                        (errmsg("recovery target timeline %u does not exist",
                                rtli)));
            recoveryTargetTLI = rtli;
            recoveryTargetIsLatest = false;
        }
        else
        {
            /* We start the "latest" search from pg_control's timeline */
            recoveryTargetTLI = findNewestTimeLine(recoveryTargetTLI);
            recoveryTargetIsLatest = true;
        }
    }

    FreeConfigVariables(head);
}

/*
 * Exit archive-recovery state
 */
static void
exitArchiveRecovery(TimeLineID endTLI, XLogSegNo endLogSegNo)
{
    char        recoveryPath[MAXPGPATH];
    char        xlogpath[MAXPGPATH];

    /*
     * We are no longer in archive recovery state.
     */
    InArchiveRecovery = false;

    /*
     * Update min recovery point one last time.
     */
    UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);

    /*
     * If the ending log segment is still open, close it (to avoid problems on
     * Windows with trying to rename or delete an open file).
     */
    if (readFile >= 0)
    {
        close(readFile);
        readFile = -1;
    }

    /*
     * If we are establishing a new timeline, we have to copy data from the
     * last WAL segment of the old timeline to create a starting WAL segment
     * for the new timeline.
     *
     * Notify the archiver that the last WAL segment of the old timeline is
     * ready to copy to archival storage. Otherwise, it is not archived for a
     * while.
     */
    if (endTLI != ThisTimeLineID)
    {
        XLogFileCopy(endLogSegNo, endTLI, endLogSegNo);

        if (XLogArchivingActive())
        {
            XLogFileName(xlogpath, endTLI, endLogSegNo);
            XLogArchiveNotify(xlogpath);
        }
    }

    /*
     * Let's just make real sure there are not .ready or .done flags posted
     * for the new segment.
     */
    XLogFileName(xlogpath, ThisTimeLineID, endLogSegNo);
    XLogArchiveCleanup(xlogpath);

    /*
     * Since there might be a partial WAL segment named RECOVERYXLOG, get rid
     * of it.
     */
    snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYXLOG");
    unlink(recoveryPath);       /* ignore any error */

    /* Get rid of any remaining recovered timeline-history file, too */
    snprintf(recoveryPath, MAXPGPATH, XLOGDIR "/RECOVERYHISTORY");
    unlink(recoveryPath);       /* ignore any error */

    /*
     * Rename the config file out of the way, so that we don't accidentally
     * re-enter archive recovery mode in a subsequent crash.
     */
    unlink(RECOVERY_COMMAND_DONE);
    if (rename(RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE) != 0)
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                        RECOVERY_COMMAND_FILE, RECOVERY_COMMAND_DONE)));

    ereport(LOG,
            (errmsg("archive recovery complete")));
}

/*
 * For point-in-time recovery, this function decides whether we want to
 * stop applying the XLOG at or after the current record.
 *
 * Returns TRUE if we are stopping, FALSE otherwise.  On TRUE return,
 * *includeThis is set TRUE if we should apply this record before stopping.
 *
 * We also track the timestamp of the latest applied COMMIT/ABORT
 * record in XLogCtl->recoveryLastXTime, for logging purposes.
 * Also, some information is saved in recoveryStopXid et al for use in
 * annotating the new timeline's history file.
 */
static bool
recoveryStopsHere(XLogRecord *record, bool *includeThis)
{
    bool        stopsHere;
    uint8       record_info;
    TimestampTz recordXtime;
    char        recordRPName[MAXFNAMELEN];

    /* We only consider stopping at COMMIT, ABORT or RESTORE POINT records */
    if (record->xl_rmid != RM_XACT_ID && record->xl_rmid != RM_XLOG_ID)
        return false;
    record_info = record->xl_info & ~XLR_INFO_MASK;
    if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT_COMPACT)
    {
        xl_xact_commit_compact *recordXactCommitData;

        recordXactCommitData = (xl_xact_commit_compact *) XLogRecGetData(record);
        recordXtime = recordXactCommitData->xact_time;
    }
    else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_COMMIT)
    {
        xl_xact_commit *recordXactCommitData;

        recordXactCommitData = (xl_xact_commit *) XLogRecGetData(record);
        recordXtime = recordXactCommitData->xact_time;
    }
    else if (record->xl_rmid == RM_XACT_ID && record_info == XLOG_XACT_ABORT)
    {
        xl_xact_abort *recordXactAbortData;

        recordXactAbortData = (xl_xact_abort *) XLogRecGetData(record);
        recordXtime = recordXactAbortData->xact_time;
    }
    else if (record->xl_rmid == RM_XLOG_ID && record_info == XLOG_RESTORE_POINT)
    {
        xl_restore_point *recordRestorePointData;

        recordRestorePointData = (xl_restore_point *) XLogRecGetData(record);
        recordXtime = recordRestorePointData->rp_time;
        strncpy(recordRPName, recordRestorePointData->rp_name, MAXFNAMELEN);
    }
    else
        return false;

    /* Do we have a PITR target at all? */
    if (recoveryTarget == RECOVERY_TARGET_UNSET)
    {
        /*
         * Save timestamp of latest transaction commit/abort if this is a
         * transaction record
         */
        if (record->xl_rmid == RM_XACT_ID)
            SetLatestXTime(recordXtime);
        return false;
    }

    if (recoveryTarget == RECOVERY_TARGET_XID)
    {
        /*
         * There can be only one transaction end record with this exact
         * transactionid
         *
         * when testing for an xid, we MUST test for equality only, since
         * transactions are numbered in the order they start, not the order
         * they complete. A higher numbered xid will complete before you about
         * 50% of the time...
         */
        stopsHere = (record->xl_xid == recoveryTargetXid);
        if (stopsHere)
            *includeThis = recoveryTargetInclusive;
    }
    else if (recoveryTarget == RECOVERY_TARGET_NAME)
    {
        /*
         * There can be many restore points that share the same name, so we
         * stop at the first one
         */
        stopsHere = (strcmp(recordRPName, recoveryTargetName) == 0);

        /*
         * Ignore recoveryTargetInclusive because this is not a transaction
         * record
         */
        *includeThis = false;
    }
    else
    {
        /*
         * There can be many transactions that share the same commit time, so
         * we stop after the last one, if we are inclusive, or stop at the
         * first one if we are exclusive
         */
        if (recoveryTargetInclusive)
            stopsHere = (recordXtime > recoveryTargetTime);
        else
            stopsHere = (recordXtime >= recoveryTargetTime);
        if (stopsHere)
            *includeThis = false;
    }

    if (stopsHere)
    {
        recoveryStopXid = record->xl_xid;
        recoveryStopTime = recordXtime;
        recoveryStopAfter = *includeThis;

        if (record_info == XLOG_XACT_COMMIT_COMPACT || record_info == XLOG_XACT_COMMIT)
        {
            if (recoveryStopAfter)
                ereport(LOG,
                        (errmsg("recovery stopping after commit of transaction %u, time %s",
                                recoveryStopXid,
                                timestamptz_to_str(recoveryStopTime))));
            else
                ereport(LOG,
                        (errmsg("recovery stopping before commit of transaction %u, time %s",
                                recoveryStopXid,
                                timestamptz_to_str(recoveryStopTime))));
        }
        else if (record_info == XLOG_XACT_ABORT)
        {
            if (recoveryStopAfter)
                ereport(LOG,
                        (errmsg("recovery stopping after abort of transaction %u, time %s",
                                recoveryStopXid,
                                timestamptz_to_str(recoveryStopTime))));
            else
                ereport(LOG,
                        (errmsg("recovery stopping before abort of transaction %u, time %s",
                                recoveryStopXid,
                                timestamptz_to_str(recoveryStopTime))));
        }
        else
        {
            strncpy(recoveryStopName, recordRPName, MAXFNAMELEN);

            ereport(LOG,
                (errmsg("recovery stopping at restore point \"%s\", time %s",
                        recoveryStopName,
                        timestamptz_to_str(recoveryStopTime))));
        }

        /*
         * Note that if we use a RECOVERY_TARGET_TIME then we can stop at a
         * restore point since they are timestamped, though the latest
         * transaction time is not updated.
         */
        if (record->xl_rmid == RM_XACT_ID && recoveryStopAfter)
            SetLatestXTime(recordXtime);
    }
    else if (record->xl_rmid == RM_XACT_ID)
        SetLatestXTime(recordXtime);

    return stopsHere;
}

/*
 * Wait until shared recoveryPause flag is cleared.
 *
 * XXX Could also be done with shared latch, avoiding the pg_usleep loop.
 * Probably not worth the trouble though.  This state shouldn't be one that
 * anyone cares about server power consumption in.
 */
static void
recoveryPausesHere(void)
{
    /* Don't pause unless users can connect! */
    if (!LocalHotStandbyActive)
        return;

    ereport(LOG,
            (errmsg("recovery has paused"),
             errhint("Execute pg_xlog_replay_resume() to continue.")));

    while (RecoveryIsPaused())
    {
        pg_usleep(1000000L);    /* 1000 ms */
        HandleStartupProcInterrupts();
    }
}

bool
RecoveryIsPaused(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    bool        recoveryPause;

    SpinLockAcquire(&xlogctl->info_lck);
    recoveryPause = xlogctl->recoveryPause;
    SpinLockRelease(&xlogctl->info_lck);

    return recoveryPause;
}

void
SetRecoveryPause(bool recoveryPause)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->recoveryPause = recoveryPause;
    SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Save timestamp of latest processed commit/abort record.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by processes other than the startup process.  Note in particular
 * that CreateRestartPoint is executed in the checkpointer.
 */
static void
SetLatestXTime(TimestampTz xtime)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->recoveryLastXTime = xtime;
    SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 */
TimestampTz
GetLatestXTime(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    TimestampTz xtime;

    SpinLockAcquire(&xlogctl->info_lck);
    xtime = xlogctl->recoveryLastXTime;
    SpinLockRelease(&xlogctl->info_lck);

    return xtime;
}

/*
 * Save timestamp of the next chunk of WAL records to apply.
 *
 * We keep this in XLogCtl, not a simple static variable, so that it can be
 * seen by all backends.
 */
static void
SetCurrentChunkStartTime(TimestampTz xtime)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->currentChunkStartTime = xtime;
    SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Fetch timestamp of latest processed commit/abort record.
 * Startup process maintains an accurate local copy in XLogReceiptTime
 */
TimestampTz
GetCurrentChunkReplayStartTime(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    TimestampTz xtime;

    SpinLockAcquire(&xlogctl->info_lck);
    xtime = xlogctl->currentChunkStartTime;
    SpinLockRelease(&xlogctl->info_lck);

    return xtime;
}

/*
 * Returns time of receipt of current chunk of XLOG data, as well as
 * whether it was received from streaming replication or from archives.
 */
void
GetXLogReceiptTime(TimestampTz *rtime, bool *fromStream)
{
    /*
     * This must be executed in the startup process, since we don't export the
     * relevant state to shared memory.
     */
    Assert(InRecovery);

    *rtime = XLogReceiptTime;
    *fromStream = (XLogReceiptSource == XLOG_FROM_STREAM);
}

/*
 * Note that text field supplied is a parameter name and does not require
 * translation
 */
#define RecoveryRequiresIntParameter(param_name, currValue, minValue) \
do { \
    if ((currValue) < (minValue)) \
        ereport(ERROR, \
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE), \
                 errmsg("hot standby is not possible because " \
                        "%s = %d is a lower setting than on the master server " \
                        "(its value was %d)", \
                        param_name, \
                        currValue, \
                        minValue))); \
} while(0)

/*
 * Check to see if required parameters are set high enough on this server
 * for various aspects of recovery operation.
 */
static void
CheckRequiredParameterValues(void)
{
    /*
     * For archive recovery, the WAL must be generated with at least 'archive'
     * wal_level.
     */
    if (InArchiveRecovery && ControlFile->wal_level == WAL_LEVEL_MINIMAL)
    {
        ereport(WARNING,
                (errmsg("WAL was generated with wal_level=minimal, data may be missing"),
                 errhint("This happens if you temporarily set wal_level=minimal without taking a new base backup.")));
    }

    /*
     * For Hot Standby, the WAL must be generated with 'hot_standby' mode, and
     * we must have at least as many backend slots as the primary.
     */
    if (InArchiveRecovery && EnableHotStandby)
    {
        if (ControlFile->wal_level < WAL_LEVEL_HOT_STANDBY)
            ereport(ERROR,
                    (errmsg("hot standby is not possible because wal_level was not set to \"hot_standby\" on the master server"),
                     errhint("Either set wal_level to \"hot_standby\" on the master, or turn off hot_standby here.")));

        /* We ignore autovacuum_max_workers when we make this test. */
        RecoveryRequiresIntParameter("max_connections",
                                     MaxConnections,
                                     ControlFile->MaxConnections);
        RecoveryRequiresIntParameter("max_prepared_transactions",
                                     max_prepared_xacts,
                                     ControlFile->max_prepared_xacts);
        RecoveryRequiresIntParameter("max_locks_per_transaction",
                                     max_locks_per_xact,
                                     ControlFile->max_locks_per_xact);
    }
}

/*
 * This must be called ONCE during postmaster or standalone-backend startup
 */
void
StartupXLOG(void)
{
    XLogCtlInsert *Insert;
    CheckPoint  checkPoint;
    bool        wasShutdown;
    bool        reachedStopPoint = false;
    bool        haveBackupLabel = false;
    XLogRecPtr  RecPtr,
                checkPointLoc,
                EndOfLog;
    XLogSegNo   endLogSegNo;
    TimeLineID  PrevTimeLineID;
    XLogRecord *record;
    uint32      freespace;
    TransactionId oldestActiveXID;
    bool        backupEndRequired = false;
    bool        backupFromStandby = false;
    DBState     dbstate_at_startup;
    XLogReaderState *xlogreader;
    XLogPageReadPrivate private;
    bool        fast_promoted = false;

    /*
     * Read control file and check XLOG status looks valid.
     *
     * Note: in most control paths, *ControlFile is already valid and we need
     * not do ReadControlFile() here, but might as well do it to be sure.
     */
    ReadControlFile();

    if (ControlFile->state < DB_SHUTDOWNED ||
        ControlFile->state > DB_IN_PRODUCTION ||
        !XRecOffIsValid(ControlFile->checkPoint))
        ereport(FATAL,
                (errmsg("control file contains invalid data")));

    if (ControlFile->state == DB_SHUTDOWNED)
        ereport(LOG,
                (errmsg("database system was shut down at %s",
                        str_time(ControlFile->time))));
    else if (ControlFile->state == DB_SHUTDOWNED_IN_RECOVERY)
        ereport(LOG,
                (errmsg("database system was shut down in recovery at %s",
                        str_time(ControlFile->time))));
    else if (ControlFile->state == DB_SHUTDOWNING)
        ereport(LOG,
                (errmsg("database system shutdown was interrupted; last known up at %s",
                        str_time(ControlFile->time))));
    else if (ControlFile->state == DB_IN_CRASH_RECOVERY)
        ereport(LOG,
           (errmsg("database system was interrupted while in recovery at %s",
                   str_time(ControlFile->time)),
            errhint("This probably means that some data is corrupted and"
                    " you will have to use the last backup for recovery.")));
    else if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY)
        ereport(LOG,
                (errmsg("database system was interrupted while in recovery at log time %s",
                        str_time(ControlFile->checkPointCopy.time)),
                 errhint("If this has occurred more than once some data might be corrupted"
              " and you might need to choose an earlier recovery target.")));
    else if (ControlFile->state == DB_IN_PRODUCTION)
        ereport(LOG,
              (errmsg("database system was interrupted; last known up at %s",
                      str_time(ControlFile->time))));

    /* This is just to allow attaching to startup process with a debugger */
#ifdef XLOG_REPLAY_DELAY
    if (ControlFile->state != DB_SHUTDOWNED)
        pg_usleep(60000000L);
#endif

    /*
     * Verify that pg_xlog and pg_xlog/archive_status exist.  In cases where
     * someone has performed a copy for PITR, these directories may have been
     * excluded and need to be re-created.
     */
    ValidateXLOGDirectoryStructure();

    /*
     * Clear out any old relcache cache files.  This is *necessary* if we do
     * any WAL replay, since that would probably result in the cache files
     * being out of sync with database reality.  In theory we could leave them
     * in place if the database had been cleanly shut down, but it seems
     * safest to just remove them always and let them be rebuilt during the
     * first backend startup.
     */
    RelationCacheInitFileRemove();

    /*
     * Initialize on the assumption we want to recover to the same timeline
     * that's active according to pg_control.
     */
    recoveryTargetTLI = ControlFile->checkPointCopy.ThisTimeLineID;

    /*
     * Check for recovery control file, and if so set up state for offline
     * recovery
     */
    readRecoveryCommandFile();

    /*
     * Save archive_cleanup_command in shared memory so that other processes
     * can see it.
     */
    strncpy(XLogCtl->archiveCleanupCommand,
            archiveCleanupCommand ? archiveCleanupCommand : "",
            sizeof(XLogCtl->archiveCleanupCommand));

    if (ArchiveRecoveryRequested)
    {
        if (StandbyModeRequested)
            ereport(LOG,
                    (errmsg("entering standby mode")));
        else if (recoveryTarget == RECOVERY_TARGET_XID)
            ereport(LOG,
                    (errmsg("starting point-in-time recovery to XID %u",
                            recoveryTargetXid)));
        else if (recoveryTarget == RECOVERY_TARGET_TIME)
            ereport(LOG,
                    (errmsg("starting point-in-time recovery to %s",
                            timestamptz_to_str(recoveryTargetTime))));
        else if (recoveryTarget == RECOVERY_TARGET_NAME)
            ereport(LOG,
                    (errmsg("starting point-in-time recovery to \"%s\"",
                            recoveryTargetName)));
        else
            ereport(LOG,
                    (errmsg("starting archive recovery")));
    }
    else if (ControlFile->minRecoveryPointTLI > 0)
    {
        /*
         * If the minRecoveryPointTLI is set when not in Archive Recovery
         * it means that we have crashed after ending recovery and
         * yet before we wrote a new checkpoint on the new timeline.
         * That means we are doing a crash recovery that needs to cross
         * timelines to get to our newly assigned timeline again.
         * The timeline we are headed for is exact and not 'latest'.
         * As soon as we hit a checkpoint, the minRecoveryPointTLI is
         * reset, so we will not enter crash recovery again.
         */
        Assert(ControlFile->minRecoveryPointTLI != 1);
        recoveryTargetTLI = ControlFile->minRecoveryPointTLI;
        recoveryTargetIsLatest = false;
    }

    /*
     * Take ownership of the wakeup latch if we're going to sleep during
     * recovery.
     */
    if (StandbyModeRequested)
        OwnLatch(&XLogCtl->recoveryWakeupLatch);

    /* Set up XLOG reader facility */
    MemSet(&private, 0, sizeof(XLogPageReadPrivate));
    xlogreader = XLogReaderAllocate(&XLogPageRead, &private);
    if (!xlogreader)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of memory"),
                 errdetail("Failed while allocating an XLog reading processor")));
    xlogreader->system_identifier = ControlFile->system_identifier;

    if (read_backup_label(&checkPointLoc, &backupEndRequired,
                          &backupFromStandby))
    {
        /*
         * Archive recovery was requested, and thanks to the backup label file,
         * we know how far we need to replay to reach consistency. Enter
         * archive recovery directly.
         */
        InArchiveRecovery = true;
        if (StandbyModeRequested)
            StandbyMode = true;

        /*
         * When a backup_label file is present, we want to roll forward from
         * the checkpoint it identifies, rather than using pg_control.
         */
        record = ReadCheckpointRecord(xlogreader, checkPointLoc, 0, true);
        if (record != NULL)
        {
            memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
            wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
            ereport(DEBUG1,
                    (errmsg("checkpoint record is at %X/%X",
                            (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
            InRecovery = true;  /* force recovery even if SHUTDOWNED */

            /*
             * Make sure that REDO location exists. This may not be the case
             * if there was a crash during an online backup, which left a
             * backup_label around that references a WAL segment that's
             * already been archived.
             */
            if (checkPoint.redo < checkPointLoc)
            {
                if (!ReadRecord(xlogreader, checkPoint.redo, LOG, false))
                    ereport(FATAL,
                            (errmsg("could not find redo location referenced by checkpoint record"),
                             errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
            }
        }
        else
        {
            ereport(FATAL,
                    (errmsg("could not locate required checkpoint record"),
                     errhint("If you are not restoring from a backup, try removing the file \"%s/backup_label\".", DataDir)));
            wasShutdown = false;    /* keep compiler quiet */
        }
        /* set flag to delete it later */
        haveBackupLabel = true;
    }
    else
    {
        /*
         * It's possible that archive recovery was requested, but we don't
         * know how far we need to replay the WAL before we reach consistency.
         * This can happen for example if a base backup is taken from a running
         * server using an atomic filesystem snapshot, without calling
         * pg_start/stop_backup. Or if you just kill a running master server
         * and put it into archive recovery by creating a recovery.conf file.
         *
         * Our strategy in that case is to perform crash recovery first,
         * replaying all the WAL present in pg_xlog, and only enter archive
         * recovery after that.
         *
         * But usually we already know how far we need to replay the WAL (up to
         * minRecoveryPoint, up to backupEndPoint, or until we see an
         * end-of-backup record), and we can enter archive recovery directly.
         */
        if (ArchiveRecoveryRequested &&
            (ControlFile->minRecoveryPoint != InvalidXLogRecPtr ||
             ControlFile->backupEndRequired ||
             ControlFile->backupEndPoint != InvalidXLogRecPtr ||
             ControlFile->state == DB_SHUTDOWNED))
        {
            InArchiveRecovery = true;
            if (StandbyModeRequested)
                StandbyMode = true;
        }

        /*
         * Get the last valid checkpoint record.  If the latest one according
         * to pg_control is broken, try the next-to-last one.
         */
        checkPointLoc = ControlFile->checkPoint;
        RedoStartLSN = ControlFile->checkPointCopy.redo;
        record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, true);
        if (record != NULL)
        {
            ereport(DEBUG1,
                    (errmsg("checkpoint record is at %X/%X",
                            (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
        }
        else if (StandbyMode)
        {
            /*
             * The last valid checkpoint record required for a streaming
             * recovery exists in neither standby nor the primary.
             */
            ereport(PANIC,
                    (errmsg("could not locate a valid checkpoint record")));
        }
        else
        {
            checkPointLoc = ControlFile->prevCheckPoint;
            record = ReadCheckpointRecord(xlogreader, checkPointLoc, 2, true);
            if (record != NULL)
            {
                ereport(LOG,
                        (errmsg("using previous checkpoint record at %X/%X",
                                (uint32) (checkPointLoc >> 32), (uint32) checkPointLoc)));
                InRecovery = true;      /* force recovery even if SHUTDOWNED */
            }
            else
                ereport(PANIC,
                     (errmsg("could not locate a valid checkpoint record")));
        }
        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
        wasShutdown = (record->xl_info == XLOG_CHECKPOINT_SHUTDOWN);
    }

    /*
     * If the location of the checkpoint record is not on the expected
     * timeline in the history of the requested timeline, we cannot proceed:
     * the backup is not part of the history of the requested timeline.
     */
    Assert(expectedTLEs); /* was initialized by reading checkpoint record */
    if (tliOfPointInHistory(checkPointLoc, expectedTLEs) !=
            checkPoint.ThisTimeLineID)
    {
        XLogRecPtr switchpoint;

        /*
         * tliSwitchPoint will throw an error if the checkpoint's timeline
         * is not in expectedTLEs at all.
         */
        switchpoint = tliSwitchPoint(ControlFile->checkPointCopy.ThisTimeLineID, expectedTLEs, NULL);
        ereport(FATAL,
                (errmsg("requested timeline %u is not a child of this server's history",
                        recoveryTargetTLI),
                 errdetail("Latest checkpoint is at %X/%X on timeline %u, but in the history of the requested timeline, the server forked off from that timeline at %X/%X",
                           (uint32) (ControlFile->checkPoint >> 32),
                           (uint32) ControlFile->checkPoint,
                           ControlFile->checkPointCopy.ThisTimeLineID,
                           (uint32) (switchpoint >> 32),
                           (uint32) switchpoint)));
    }

    /*
     * The min recovery point should be part of the requested timeline's
     * history, too.
     */
    if (!XLogRecPtrIsInvalid(ControlFile->minRecoveryPoint) &&
        tliOfPointInHistory(ControlFile->minRecoveryPoint - 1, expectedTLEs) !=
            ControlFile->minRecoveryPointTLI)
        ereport(FATAL,
                (errmsg("requested timeline %u does not contain minimum recovery point %X/%X on timeline %u",
                        recoveryTargetTLI,
                        (uint32) (ControlFile->minRecoveryPoint >> 32),
                        (uint32) ControlFile->minRecoveryPoint,
                        ControlFile->minRecoveryPointTLI)));

    LastRec = RecPtr = checkPointLoc;

    ereport(DEBUG1,
            (errmsg("redo record is at %X/%X; shutdown %s",
                    (uint32) (checkPoint.redo >> 32), (uint32) checkPoint.redo,
                    wasShutdown ? "TRUE" : "FALSE")));
    ereport(DEBUG1,
            (errmsg("next transaction ID: %u/%u; next OID: %u",
                    checkPoint.nextXidEpoch, checkPoint.nextXid,
                    checkPoint.nextOid)));
    ereport(DEBUG1,
            (errmsg("next MultiXactId: %u; next MultiXactOffset: %u",
                    checkPoint.nextMulti, checkPoint.nextMultiOffset)));
    ereport(DEBUG1,
            (errmsg("oldest unfrozen transaction ID: %u, in database %u",
                    checkPoint.oldestXid, checkPoint.oldestXidDB)));
    ereport(DEBUG1,
            (errmsg("oldest MultiXactId: %u, in database %u",
                    checkPoint.oldestMulti, checkPoint.oldestMultiDB)));
    if (!TransactionIdIsNormal(checkPoint.nextXid))
        ereport(PANIC,
                (errmsg("invalid next transaction ID")));

    /* initialize shared memory variables from the checkpoint record */
    ShmemVariableCache->nextXid = checkPoint.nextXid;
    ShmemVariableCache->nextOid = checkPoint.nextOid;
    ShmemVariableCache->oidCount = 0;
    MultiXactSetNextMXact(checkPoint.nextMulti, checkPoint.nextMultiOffset);
    SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
    SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);
    XLogCtl->ckptXidEpoch = checkPoint.nextXidEpoch;
    XLogCtl->ckptXid = checkPoint.nextXid;

    /*
     * Initialize unlogged LSN. On a clean shutdown, it's restored from the
     * control file. On recovery, all unlogged relations are blown away, so
     * the unlogged LSN counter can be reset too.
     */
    if (ControlFile->state == DB_SHUTDOWNED)
        XLogCtl->unloggedLSN = ControlFile->unloggedLSN;
    else
        XLogCtl->unloggedLSN = 1;

    /*
     * We must replay WAL entries using the same TimeLineID they were created
     * under, so temporarily adopt the TLI indicated by the checkpoint (see
     * also xlog_redo()).
     */
    ThisTimeLineID = checkPoint.ThisTimeLineID;

    /*
     * Copy any missing timeline history files between 'now' and the
     * recovery target timeline from archive to pg_xlog. While we don't need
     * those files ourselves - the history file of the recovery target
     * timeline covers all the previous timelines in the history too - a
     * cascading standby server might be interested in them. Or, if you
     * archive the WAL from this server to a different archive than the
     * master, it'd be good for all the history files to get archived there
     * after failover, so that you can use one of the old timelines as a
     * PITR target. Timeline history files are small, so it's better to copy
     * them unnecessarily than not copy them and regret later.
     */
    restoreTimeLineHistoryFiles(ThisTimeLineID, recoveryTargetTLI);

    lastFullPageWrites = checkPoint.fullPageWrites;

    RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

    if (RecPtr < checkPoint.redo)
        ereport(PANIC,
                (errmsg("invalid redo in checkpoint record")));

    /*
     * Check whether we need to force recovery from WAL.  If it appears to
     * have been a clean shutdown and we did not have a recovery.conf file,
     * then assume no recovery needed.
     */
    if (checkPoint.redo < RecPtr)
    {
        if (wasShutdown)
            ereport(PANIC,
                    (errmsg("invalid redo record in shutdown checkpoint")));
        InRecovery = true;
    }
    else if (ControlFile->state != DB_SHUTDOWNED)
        InRecovery = true;
    else if (ArchiveRecoveryRequested)
    {
        /* force recovery due to presence of recovery.conf */
        InRecovery = true;
    }

    /* REDO */
    if (InRecovery)
    {
        int         rmid;

        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /*
         * Update pg_control to show that we are recovering and to show the
         * selected checkpoint as the place we are starting from. We also mark
         * pg_control with any minimum recovery stop point obtained from a
         * backup history file.
         */
        dbstate_at_startup = ControlFile->state;
        if (InArchiveRecovery)
            ControlFile->state = DB_IN_ARCHIVE_RECOVERY;
        else
        {
            ereport(LOG,
                    (errmsg("database system was not properly shut down; "
                            "automatic recovery in progress")));
            if (recoveryTargetTLI > 0)
                ereport(LOG,
                    (errmsg("crash recovery starts in timeline %u "
                            "and has target timeline %u",
                            ControlFile->checkPointCopy.ThisTimeLineID,
                            recoveryTargetTLI)));
            ControlFile->state = DB_IN_CRASH_RECOVERY;
        }
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = checkPointLoc;
        ControlFile->checkPointCopy = checkPoint;
        if (InArchiveRecovery)
        {
            /* initialize minRecoveryPoint if not set yet */
            if (ControlFile->minRecoveryPoint < checkPoint.redo)
            {
                ControlFile->minRecoveryPoint = checkPoint.redo;
                ControlFile->minRecoveryPointTLI = checkPoint.ThisTimeLineID;
            }
        }

        /*
         * Set backupStartPoint if we're starting recovery from a base backup.
         *
         * Set backupEndPoint and use minRecoveryPoint as the backup end
         * location if we're starting recovery from a base backup which was
         * taken from the standby. In this case, the database system status in
         * pg_control must indicate DB_IN_ARCHIVE_RECOVERY. If not, which
         * means that backup is corrupted, so we cancel recovery.
         */
        if (haveBackupLabel)
        {
            ControlFile->backupStartPoint = checkPoint.redo;
            ControlFile->backupEndRequired = backupEndRequired;

            if (backupFromStandby)
            {
                if (dbstate_at_startup != DB_IN_ARCHIVE_RECOVERY)
                    ereport(FATAL,
                            (errmsg("backup_label contains data inconsistent with control file"),
                             errhint("This means that the backup is corrupted and you will "
                               "have to use another backup for recovery.")));
                ControlFile->backupEndPoint = ControlFile->minRecoveryPoint;
            }
        }
        ControlFile->time = (pg_time_t) time(NULL);
        /* No need to hold ControlFileLock yet, we aren't up far enough */
        UpdateControlFile();

        /* initialize our local copy of minRecoveryPoint */
        minRecoveryPoint = ControlFile->minRecoveryPoint;
        minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;

        /*
         * Reset pgstat data, because it may be invalid after recovery.
         */
        pgstat_reset_all();

        /*
         * If there was a backup label file, it's done its job and the info
         * has now been propagated into pg_control.  We must get rid of the
         * label file so that if we crash during recovery, we'll pick up at
         * the latest recovery restartpoint instead of going all the way back
         * to the backup start point.  It seems prudent though to just rename
         * the file out of the way rather than delete it completely.
         */
        if (haveBackupLabel)
        {
            unlink(BACKUP_LABEL_OLD);
            if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) != 0)
                ereport(FATAL,
                        (errcode_for_file_access(),
                         errmsg("could not rename file \"%s\" to \"%s\": %m",
                                BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
        }

        /* Check that the GUCs used to generate the WAL allow recovery */
        CheckRequiredParameterValues();

        /*
         * We're in recovery, so unlogged relations may be trashed and must be
         * reset.  This should be done BEFORE allowing Hot Standby
         * connections, so that read-only backends don't try to read whatever
         * garbage is left over from before.
         */
        ResetUnloggedRelations(UNLOGGED_RELATION_CLEANUP);

        /*
         * Likewise, delete any saved transaction snapshot files that got left
         * behind by crashed backends.
         */
        DeleteAllExportedSnapshotFiles();

        /*
         * Initialize for Hot Standby, if enabled. We won't let backends in
         * yet, not until we've reached the min recovery point specified in
         * control file and we've established a recovery snapshot from a
         * running-xacts WAL record.
         */
        if (ArchiveRecoveryRequested && EnableHotStandby)
        {
            TransactionId *xids;
            int         nxids;

            ereport(DEBUG1,
                    (errmsg("initializing for hot standby")));

            InitRecoveryTransactionEnvironment();

            if (wasShutdown)
                oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);
            else
                oldestActiveXID = checkPoint.oldestActiveXid;
            Assert(TransactionIdIsValid(oldestActiveXID));

            /*
             * Startup commit log and subtrans only. Other SLRUs are not
             * maintained during recovery and need not be started yet.
             */
            StartupCLOG();
            StartupSUBTRANS(oldestActiveXID);

            /*
             * If we're beginning at a shutdown checkpoint, we know that
             * nothing was running on the master at this point. So fake-up an
             * empty running-xacts record and use that here and now. Recover
             * additional standby state for prepared transactions.
             */
            if (wasShutdown)
            {
                RunningTransactionsData running;
                TransactionId latestCompletedXid;

                /*
                 * Construct a RunningTransactions snapshot representing a
                 * shut down server, with only prepared transactions still
                 * alive. We're never overflowed at this point because all
                 * subxids are listed with their parent prepared transactions.
                 */
                running.xcnt = nxids;
                running.subxcnt = 0;
                running.subxid_overflow = false;
                running.nextXid = checkPoint.nextXid;
                running.oldestRunningXid = oldestActiveXID;
                latestCompletedXid = checkPoint.nextXid;
                TransactionIdRetreat(latestCompletedXid);
                Assert(TransactionIdIsNormal(latestCompletedXid));
                running.latestCompletedXid = latestCompletedXid;
                running.xids = xids;

                ProcArrayApplyRecoveryInfo(&running);

                StandbyRecoverPreparedTransactions(false);
            }
        }

        /* Initialize resource managers */
        for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
        {
            if (RmgrTable[rmid].rm_startup != NULL)
                RmgrTable[rmid].rm_startup();
        }

        /*
         * Initialize shared replayEndRecPtr, lastReplayedEndRecPtr, and
         * recoveryLastXTime.
         *
         * This is slightly confusing if we're starting from an online
         * checkpoint; we've just read and replayed the chekpoint record, but
         * we're going to start replay from its redo pointer, which precedes
         * the location of the checkpoint record itself. So even though the
         * last record we've replayed is indeed ReadRecPtr, we haven't
         * replayed all the preceding records yet. That's OK for the current
         * use of these variables.
         */
        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->replayEndRecPtr = ReadRecPtr;
        xlogctl->replayEndTLI = ThisTimeLineID;
        xlogctl->lastReplayedEndRecPtr = EndRecPtr;
        xlogctl->lastReplayedTLI = ThisTimeLineID;
        xlogctl->recoveryLastXTime = 0;
        xlogctl->currentChunkStartTime = 0;
        xlogctl->recoveryPause = false;
        SpinLockRelease(&xlogctl->info_lck);

        /* Also ensure XLogReceiptTime has a sane value */
        XLogReceiptTime = GetCurrentTimestamp();

        /*
         * Let postmaster know we've started redo now, so that it can launch
         * checkpointer to perform restartpoints.  We don't bother during
         * crash recovery as restartpoints can only be performed during
         * archive recovery.  And we'd like to keep crash recovery simple, to
         * avoid introducing bugs that could affect you when recovering after
         * crash.
         *
         * After this point, we can no longer assume that we're the only
         * process in addition to postmaster!  Also, fsync requests are
         * subsequently to be handled by the checkpointer, not locally.
         */
        if (ArchiveRecoveryRequested && IsUnderPostmaster)
        {
            PublishStartupProcessInformation();
            SetForwardFsyncRequests();
            SendPostmasterSignal(PMSIGNAL_RECOVERY_STARTED);
            bgwriterLaunched = true;
        }

        /*
         * Allow read-only connections immediately if we're consistent
         * already.
         */
        CheckRecoveryConsistency();

        /*
         * Find the first record that logically follows the checkpoint --- it
         * might physically precede it, though.
         */
        if (checkPoint.redo < RecPtr)
        {
            /* back up to find the record */
            record = ReadRecord(xlogreader, checkPoint.redo, PANIC, false);
        }
        else
        {
            /* just have to read next record after CheckPoint */
            record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
        }

        if (record != NULL)
        {
            bool        recoveryContinue = true;
            bool        recoveryApply = true;
            ErrorContextCallback errcallback;
            TimestampTz xtime;

            InRedo = true;

            ereport(LOG,
                    (errmsg("redo starts at %X/%X",
                            (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));

            /*
             * main redo apply loop
             */
            do
            {
                bool switchedTLI = false;
#ifdef WAL_DEBUG
                if (XLOG_DEBUG ||
                 (rmid == RM_XACT_ID && trace_recovery_messages <= DEBUG2) ||
                    (rmid != RM_XACT_ID && trace_recovery_messages <= DEBUG3))
                {
                    StringInfoData buf;

                    initStringInfo(&buf);
                    appendStringInfo(&buf, "REDO @ %X/%X; LSN %X/%X: ",
                                     (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr,
                                     (uint32) (EndRecPtr >> 32), (uint32) EndRecPtr);
                    xlog_outrec(&buf, record);
                    appendStringInfo(&buf, " - ");
                    RmgrTable[record->xl_rmid].rm_desc(&buf,
                                                       record->xl_info,
                                                     XLogRecGetData(record));
                    elog(LOG, "%s", buf.data);
                    pfree(buf.data);
                }
#endif

                /* Handle interrupt signals of startup process */
                HandleStartupProcInterrupts();

                /*
                 * Pause WAL replay, if requested by a hot-standby session via
                 * SetRecoveryPause().
                 *
                 * Note that we intentionally don't take the info_lck spinlock
                 * here.  We might therefore read a slightly stale value of
                 * the recoveryPause flag, but it can't be very stale (no
                 * worse than the last spinlock we did acquire).  Since a
                 * pause request is a pretty asynchronous thing anyway,
                 * possibly responding to it one WAL record later than we
                 * otherwise would is a minor issue, so it doesn't seem worth
                 * adding another spinlock cycle to prevent that.
                 */
                if (xlogctl->recoveryPause)
                    recoveryPausesHere();

                /*
                 * Have we reached our recovery target?
                 */
                if (recoveryStopsHere(record, &recoveryApply))
                {
                    if (recoveryPauseAtTarget)
                    {
                        SetRecoveryPause(true);
                        recoveryPausesHere();
                    }
                    reachedStopPoint = true;    /* see below */
                    recoveryContinue = false;

                    /* Exit loop if we reached non-inclusive recovery target */
                    if (!recoveryApply)
                        break;
                }

                /* Setup error traceback support for ereport() */
                errcallback.callback = rm_redo_error_callback;
                errcallback.arg = (void *) record;
                errcallback.previous = error_context_stack;
                error_context_stack = &errcallback;

                /*
                 * ShmemVariableCache->nextXid must be beyond record's xid.
                 *
                 * We don't expect anyone else to modify nextXid, hence we
                 * don't need to hold a lock while examining it.  We still
                 * acquire the lock to modify it, though.
                 */
                if (TransactionIdFollowsOrEquals(record->xl_xid,
                                                 ShmemVariableCache->nextXid))
                {
                    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                    ShmemVariableCache->nextXid = record->xl_xid;
                    TransactionIdAdvance(ShmemVariableCache->nextXid);
                    LWLockRelease(XidGenLock);
                }

                /*
                 * Before replaying this record, check if this record
                 * causes the current timeline to change. The record is
                 * already considered to be part of the new timeline,
                 * so we update ThisTimeLineID before replaying it.
                 * That's important so that replayEndTLI, which is
                 * recorded as the minimum recovery point's TLI if
                 * recovery stops after this record, is set correctly.
                 */
                if (record->xl_rmid == RM_XLOG_ID)
                {
                    TimeLineID  newTLI = ThisTimeLineID;
                    TimeLineID  prevTLI = ThisTimeLineID;
                    uint8       info = record->xl_info & ~XLR_INFO_MASK;

                    if (info == XLOG_CHECKPOINT_SHUTDOWN)
                    {
                        CheckPoint  checkPoint;

                        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
                        newTLI = checkPoint.ThisTimeLineID;
                        prevTLI = checkPoint.PrevTimeLineID;
                    }
                    else if (info == XLOG_END_OF_RECOVERY)
                    {
                        xl_end_of_recovery  xlrec;

                        memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));
                        newTLI = xlrec.ThisTimeLineID;
                        prevTLI = xlrec.PrevTimeLineID;
                    }

                    if (newTLI != ThisTimeLineID)
                    {
                        /* Check that it's OK to switch to this TLI */
                        checkTimeLineSwitch(EndRecPtr, newTLI, prevTLI);

                        /* Following WAL records should be run with new TLI */
                        ThisTimeLineID = newTLI;
                        switchedTLI = true;
                    }
                }

                /*
                 * Update shared replayEndRecPtr before replaying this record,
                 * so that XLogFlush will update minRecoveryPoint correctly.
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->replayEndRecPtr = EndRecPtr;
                xlogctl->replayEndTLI = ThisTimeLineID;
                SpinLockRelease(&xlogctl->info_lck);

                /*
                 * If we are attempting to enter Hot Standby mode, process
                 * XIDs we see
                 */
                if (standbyState >= STANDBY_INITIALIZED &&
                    TransactionIdIsValid(record->xl_xid))
                    RecordKnownAssignedTransactionIds(record->xl_xid);

                /* Now apply the WAL record itself */
                RmgrTable[record->xl_rmid].rm_redo(EndRecPtr, record);

                /* Pop the error context stack */
                error_context_stack = errcallback.previous;

                /*
                 * Update lastReplayedEndRecPtr after this record has been
                 * successfully replayed.
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                xlogctl->lastReplayedEndRecPtr = EndRecPtr;
                xlogctl->lastReplayedTLI = ThisTimeLineID;
                SpinLockRelease(&xlogctl->info_lck);

                /* Remember this record as the last-applied one */
                LastRec = ReadRecPtr;

                /* Allow read-only connections if we're consistent now */
                CheckRecoveryConsistency();

                /*
                 * If this record was a timeline switch, wake up any
                 * walsenders to notice that we are on a new timeline.
                 */
                if (switchedTLI && AllowCascadeReplication())
                    WalSndWakeup();

                /* Exit loop if we reached inclusive recovery target */
                if (!recoveryContinue)
                    break;

                /* Else, try to fetch the next WAL record */
                record = ReadRecord(xlogreader, InvalidXLogRecPtr, LOG, false);
            } while (record != NULL);

            /*
             * end of main redo apply loop
             */

            ereport(LOG,
                    (errmsg("redo done at %X/%X",
                            (uint32) (ReadRecPtr >> 32), (uint32) ReadRecPtr)));
            xtime = GetLatestXTime();
            if (xtime)
                ereport(LOG,
                     (errmsg("last completed transaction was at log time %s",
                             timestamptz_to_str(xtime))));
            InRedo = false;
        }
        else
        {
            /* there are no WAL records following the checkpoint */
            ereport(LOG,
                    (errmsg("redo is not required")));
        }
    }

    /*
     * Kill WAL receiver, if it's still running, before we continue to write
     * the startup checkpoint record. It will trump over the checkpoint and
     * subsequent records if it's still alive when we start writing WAL.
     */
    ShutdownWalRcv();

    /*
     * We don't need the latch anymore. It's not strictly necessary to disown
     * it, but let's do it for the sake of tidiness.
     */
    if (StandbyModeRequested)
        DisownLatch(&XLogCtl->recoveryWakeupLatch);

    /*
     * We are now done reading the xlog from stream. Turn off streaming
     * recovery to force fetching the files (which would be required at end of
     * recovery, e.g., timeline history file) from archive or pg_xlog.
     */
    StandbyMode = false;

    /*
     * Re-fetch the last valid or last applied record, so we can identify the
     * exact endpoint of what we consider the valid portion of WAL.
     */
    record = ReadRecord(xlogreader, LastRec, PANIC, false);
    EndOfLog = EndRecPtr;
    XLByteToPrevSeg(EndOfLog, endLogSegNo);

    /*
     * Complain if we did not roll forward far enough to render the backup
     * dump consistent.  Note: it is indeed okay to look at the local variable
     * minRecoveryPoint here, even though ControlFile->minRecoveryPoint might
     * be further ahead --- ControlFile->minRecoveryPoint cannot have been
     * advanced beyond the WAL we processed.
     */
    if (InRecovery &&
        (EndOfLog < minRecoveryPoint ||
         !XLogRecPtrIsInvalid(ControlFile->backupStartPoint)))
    {
        if (reachedStopPoint)
        {
            /* stopped because of stop request */
            ereport(FATAL,
                    (errmsg("requested recovery stop point is before consistent recovery point")));
        }

        /*
         * Ran off end of WAL before reaching end-of-backup WAL record, or
         * minRecoveryPoint. That's usually a bad sign, indicating that you
         * tried to recover from an online backup but never called
         * pg_stop_backup(), or you didn't archive all the WAL up to that
         * point. However, this also happens in crash recovery, if the system
         * crashes while an online backup is in progress. We must not treat
         * that as an error, or the database will refuse to start up.
         */
        if (ArchiveRecoveryRequested || ControlFile->backupEndRequired)
        {
            if (ControlFile->backupEndRequired)
                ereport(FATAL,
                        (errmsg("WAL ends before end of online backup"),
                         errhint("All WAL generated while online backup was taken must be available at recovery.")));
            else if (!XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
                ereport(FATAL,
                        (errmsg("WAL ends before end of online backup"),
                         errhint("Online backup started with pg_start_backup() must be ended with pg_stop_backup(), and all WAL up to that point must be available at recovery.")));
            else
                ereport(FATAL,
                      (errmsg("WAL ends before consistent recovery point")));
        }
    }

    /*
     * Consider whether we need to assign a new timeline ID.
     *
     * If we are doing an archive recovery, we always assign a new ID.  This
     * handles a couple of issues.  If we stopped short of the end of WAL
     * during recovery, then we are clearly generating a new timeline and must
     * assign it a unique new ID.  Even if we ran to the end, modifying the
     * current last segment is problematic because it may result in trying to
     * overwrite an already-archived copy of that segment, and we encourage
     * DBAs to make their archive_commands reject that.  We can dodge the
     * problem by making the new active segment have a new timeline ID.
     *
     * In a normal crash recovery, we can just extend the timeline we were in.
     */
    PrevTimeLineID = ThisTimeLineID;
    if (ArchiveRecoveryRequested)
    {
        char    reason[200];

        Assert(InArchiveRecovery);

        ThisTimeLineID = findNewestTimeLine(recoveryTargetTLI) + 1;
        ereport(LOG,
                (errmsg("selected new timeline ID: %u", ThisTimeLineID)));

        /*
         * Create a comment for the history file to explain why and where
         * timeline changed.
         */
        if (recoveryTarget == RECOVERY_TARGET_XID)
            snprintf(reason, sizeof(reason),
                     "%s transaction %u",
                     recoveryStopAfter ? "after" : "before",
                     recoveryStopXid);
        else if (recoveryTarget == RECOVERY_TARGET_TIME)
            snprintf(reason, sizeof(reason),
                     "%s %s\n",
                     recoveryStopAfter ? "after" : "before",
                     timestamptz_to_str(recoveryStopTime));
        else if (recoveryTarget == RECOVERY_TARGET_NAME)
            snprintf(reason, sizeof(reason),
                     "at restore point \"%s\"",
                     recoveryStopName);
        else
            snprintf(reason, sizeof(reason), "no recovery target specified");

        writeTimeLineHistory(ThisTimeLineID, recoveryTargetTLI,
                             EndRecPtr, reason);
    }

    /* Save the selected TimeLineID in shared memory, too */
    XLogCtl->ThisTimeLineID = ThisTimeLineID;
    XLogCtl->PrevTimeLineID = PrevTimeLineID;

    /*
     * We are now done reading the old WAL.  Turn off archive fetching if it
     * was active, and make a writable copy of the last WAL segment. (Note
     * that we also have a copy of the last block of the old WAL in readBuf;
     * we will use that below.)
     */
    if (ArchiveRecoveryRequested)
        exitArchiveRecovery(xlogreader->readPageTLI, endLogSegNo);

    /*
     * Prepare to write WAL starting at EndOfLog position, and init xlog
     * buffer cache using the block containing the last record from the
     * previous incarnation.
     */
    openLogSegNo = endLogSegNo;
    openLogFile = XLogFileOpen(openLogSegNo);
    openLogOff = 0;
    Insert = &XLogCtl->Insert;
    Insert->PrevRecord = LastRec;
    XLogCtl->xlblocks[0] = ((EndOfLog - 1) / XLOG_BLCKSZ + 1) * XLOG_BLCKSZ;

    /*
     * Tricky point here: readBuf contains the *last* block that the LastRec
     * record spans, not the one it starts in.  The last block is indeed the
     * one we want to use.
     */
    if (EndOfLog % XLOG_BLCKSZ == 0)
    {
        memset(Insert->currpage, 0, XLOG_BLCKSZ);
    }
    else
    {
        Assert(readOff == (XLogCtl->xlblocks[0] - XLOG_BLCKSZ) % XLogSegSize);
        memcpy((char *) Insert->currpage, xlogreader->readBuf, XLOG_BLCKSZ);
    }
    Insert->currpos = (char *) Insert->currpage +
        (EndOfLog + XLOG_BLCKSZ - XLogCtl->xlblocks[0]);

    LogwrtResult.Write = LogwrtResult.Flush = EndOfLog;

    XLogCtl->LogwrtResult = LogwrtResult;

    XLogCtl->LogwrtRqst.Write = EndOfLog;
    XLogCtl->LogwrtRqst.Flush = EndOfLog;

    freespace = INSERT_FREESPACE(Insert);
    if (freespace > 0)
    {
        /* Make sure rest of page is zero */
        MemSet(Insert->currpos, 0, freespace);
        XLogCtl->Write.curridx = 0;
    }
    else
    {
        /*
         * Whenever LogwrtResult points to exactly the end of a page,
         * Write.curridx must point to the *next* page (see XLogWrite()).
         *
         * Note: it might seem we should do AdvanceXLInsertBuffer() here, but
         * this is sufficient.  The first actual attempt to insert a log
         * record will advance the insert state.
         */
        XLogCtl->Write.curridx = NextBufIdx(0);
    }

    /* Pre-scan prepared transactions to find out the range of XIDs present */
    oldestActiveXID = PrescanPreparedTransactions(NULL, NULL);

    /*
     * Update full_page_writes in shared memory and write an XLOG_FPW_CHANGE
     * record before resource manager writes cleanup WAL records or checkpoint
     * record is written.
     */
    Insert->fullPageWrites = lastFullPageWrites;
    LocalSetXLogInsertAllowed();
    UpdateFullPageWrites();
    LocalXLogInsertAllowed = -1;

    if (InRecovery)
    {
        int         rmid;

        /*
         * Resource managers might need to write WAL records, eg, to record
         * index cleanup actions.  So temporarily enable XLogInsertAllowed in
         * this process only.
         */
        LocalSetXLogInsertAllowed();

        /*
         * Allow resource managers to do any required cleanup.
         */
        for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
        {
            if (RmgrTable[rmid].rm_cleanup != NULL)
                RmgrTable[rmid].rm_cleanup();
        }

        /* Disallow XLogInsert again */
        LocalXLogInsertAllowed = -1;

        /*
         * Perform a checkpoint to update all our recovery activity to disk.
         *
         * Note that we write a shutdown checkpoint rather than an on-line
         * one. This is not particularly critical, but since we may be
         * assigning a new TLI, using a shutdown checkpoint allows us to have
         * the rule that TLI only changes in shutdown checkpoints, which
         * allows some extra error checking in xlog_redo.
         *
         * In fast promotion, only create a lightweight end-of-recovery record
         * instead of a full checkpoint. A checkpoint is requested later, after
         * we're fully out of recovery mode and already accepting queries.
         */
        if (bgwriterLaunched)
        {
            if (fast_promote)
            {
                checkPointLoc = ControlFile->prevCheckPoint;

                /*
                 * Confirm the last checkpoint is available for us to recover
                 * from if we fail. Note that we don't check for the secondary
                 * checkpoint since that isn't available in most base backups.
                 */
                record = ReadCheckpointRecord(xlogreader, checkPointLoc, 1, false);
                if (record != NULL)
                {
                    fast_promoted = true;
                    CreateEndOfRecoveryRecord();
                }
            }

            if (!fast_promoted)
                RequestCheckpoint(CHECKPOINT_END_OF_RECOVERY |
                                    CHECKPOINT_IMMEDIATE |
                                    CHECKPOINT_WAIT);
        }
        else
            CreateCheckPoint(CHECKPOINT_END_OF_RECOVERY | CHECKPOINT_IMMEDIATE);

        /*
         * And finally, execute the recovery_end_command, if any.
         */
        if (recoveryEndCommand)
            ExecuteRecoveryCommand(recoveryEndCommand,
                                   "recovery_end_command",
                                   true);
    }

    /*
     * Preallocate additional log files, if wanted.
     */
    PreallocXlogFiles(EndOfLog);

    /*
     * Reset initial contents of unlogged relations.  This has to be done
     * AFTER recovery is complete so that any unlogged relations created
     * during recovery also get picked up.
     */
    if (InRecovery)
        ResetUnloggedRelations(UNLOGGED_RELATION_INIT);

    /*
     * Okay, we're officially UP.
     */
    InRecovery = false;

    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    ControlFile->state = DB_IN_PRODUCTION;
    ControlFile->time = (pg_time_t) time(NULL);
    UpdateControlFile();
    LWLockRelease(ControlFileLock);

    /* start the archive_timeout timer running */
    XLogCtl->Write.lastSegSwitchTime = (pg_time_t) time(NULL);

    /* also initialize latestCompletedXid, to nextXid - 1 */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    ShmemVariableCache->latestCompletedXid = ShmemVariableCache->nextXid;
    TransactionIdRetreat(ShmemVariableCache->latestCompletedXid);
    LWLockRelease(ProcArrayLock);

    /*
     * Start up the commit log and subtrans, if not already done for hot
     * standby.
     */
    if (standbyState == STANDBY_DISABLED)
    {
        StartupCLOG();
        StartupSUBTRANS(oldestActiveXID);
    }

    /*
     * Perform end of recovery actions for any SLRUs that need it.
     */
    StartupMultiXact();
    TrimCLOG();

    /* Reload shared-memory state for prepared transactions */
    RecoverPreparedTransactions();

    /*
     * Shutdown the recovery environment. This must occur after
     * RecoverPreparedTransactions(), see notes for lock_twophase_recover()
     */
    if (standbyState != STANDBY_DISABLED)
        ShutdownRecoveryTransactionEnvironment();

    /* Shut down xlogreader */
    if (readFile >= 0)
    {
        close(readFile);
        readFile = -1;
    }
    XLogReaderFree(xlogreader);

    /*
     * If any of the critical GUCs have changed, log them before we allow
     * backends to write WAL.
     */
    LocalSetXLogInsertAllowed();
    XLogReportParameters();

    /*
     * All done.  Allow backends to write WAL.  (Although the bool flag is
     * probably atomic in itself, we use the info_lck here to ensure that
     * there are no race conditions concerning visibility of other recent
     * updates to shared memory.)
     */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->SharedRecoveryInProgress = false;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /*
     * If there were cascading standby servers connected to us, nudge any
     * wal sender processes to notice that we've been promoted.
     */
    WalSndWakeup();

    /*
     * If this was a fast promotion, request an (online) checkpoint now. This
     * isn't required for consistency, but the last restartpoint might be far
     * back, and in case of a crash, recovering from it might take a longer
     * than is appropriate now that we're not in standby mode anymore.
     */
    if (fast_promoted)
        RequestCheckpoint(0);
}

/*
 * Checks if recovery has reached a consistent state. When consistency is
 * reached and we have a valid starting standby snapshot, tell postmaster
 * that it can start accepting read-only connections.
 */
static void
CheckRecoveryConsistency(void)
{
    /*
     * During crash recovery, we don't reach a consistent state until we've
     * replayed all the WAL.
     */
    if (XLogRecPtrIsInvalid(minRecoveryPoint))
        return;

    /*
     * Have we reached the point where our base backup was completed?
     */
    if (!XLogRecPtrIsInvalid(ControlFile->backupEndPoint) &&
        ControlFile->backupEndPoint <= EndRecPtr)
    {
        /*
         * We have reached the end of base backup, as indicated by pg_control.
         * The data on disk is now consistent. Reset backupStartPoint and
         * backupEndPoint, and update minRecoveryPoint to make sure we don't
         * allow starting up at an earlier point even if recovery is stopped
         * and restarted soon after this.
         */
        elog(DEBUG1, "end of backup reached");

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

        if (ControlFile->minRecoveryPoint < EndRecPtr)
            ControlFile->minRecoveryPoint = EndRecPtr;

        ControlFile->backupStartPoint = InvalidXLogRecPtr;
        ControlFile->backupEndPoint = InvalidXLogRecPtr;
        ControlFile->backupEndRequired = false;
        UpdateControlFile();

        LWLockRelease(ControlFileLock);
    }

    /*
     * Have we passed our safe starting point? Note that minRecoveryPoint
     * is known to be incorrectly set if ControlFile->backupEndRequired,
     * until the XLOG_BACKUP_RECORD arrives to advise us of the correct
     * minRecoveryPoint. All we know prior to that is that we're not
     * consistent yet.
     */
    if (!reachedConsistency && !ControlFile->backupEndRequired &&
        minRecoveryPoint <= XLogCtl->lastReplayedEndRecPtr &&
        XLogRecPtrIsInvalid(ControlFile->backupStartPoint))
    {
        /*
         * Check to see if the XLOG sequence contained any unresolved
         * references to uninitialized pages.
         */
        XLogCheckInvalidPages();

        reachedConsistency = true;
        ereport(LOG,
                (errmsg("consistent recovery state reached at %X/%X",
                        (uint32) (XLogCtl->lastReplayedEndRecPtr >> 32),
                        (uint32) XLogCtl->lastReplayedEndRecPtr)));
    }

    /*
     * Have we got a valid starting snapshot that will allow queries to be
     * run? If so, we can tell postmaster that the database is consistent now,
     * enabling connections.
     */
    if (standbyState == STANDBY_SNAPSHOT_READY &&
        !LocalHotStandbyActive &&
        reachedConsistency &&
        IsUnderPostmaster)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->SharedHotStandbyActive = true;
        SpinLockRelease(&xlogctl->info_lck);

        LocalHotStandbyActive = true;

        SendPostmasterSignal(PMSIGNAL_BEGIN_HOT_STANDBY);
    }
}

/*
 * Is the system still in recovery?
 *
 * Unlike testing InRecovery, this works in any process that's connected to
 * shared memory.
 *
 * As a side-effect, we initialize the local TimeLineID and RedoRecPtr
 * variables the first time we see that recovery is finished.
 */
bool
RecoveryInProgress(void)
{
    /*
     * We check shared state each time only until we leave recovery mode. We
     * can't re-enter recovery, so there's no need to keep checking after the
     * shared variable has once been seen false.
     */
    if (!LocalRecoveryInProgress)
        return false;
    else
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /* spinlock is essential on machines with weak memory ordering! */
        SpinLockAcquire(&xlogctl->info_lck);
        LocalRecoveryInProgress = xlogctl->SharedRecoveryInProgress;
        SpinLockRelease(&xlogctl->info_lck);

        /*
         * Initialize TimeLineID and RedoRecPtr when we discover that recovery
         * is finished. InitPostgres() relies upon this behaviour to ensure
         * that InitXLOGAccess() is called at backend startup.  (If you change
         * this, see also LocalSetXLogInsertAllowed.)
         */
        if (!LocalRecoveryInProgress)
            InitXLOGAccess();

        return LocalRecoveryInProgress;
    }
}

/*
 * Is HotStandby active yet? This is only important in special backends
 * since normal backends won't ever be able to connect until this returns
 * true. Postmaster knows this by way of signal, not via shared memory.
 *
 * Unlike testing standbyState, this works in any process that's connected to
 * shared memory.
 */
bool
HotStandbyActive(void)
{
    /*
     * We check shared state each time only until Hot Standby is active. We
     * can't de-activate Hot Standby, so there's no need to keep checking
     * after the shared variable has once been seen true.
     */
    if (LocalHotStandbyActive)
        return true;
    else
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        /* spinlock is essential on machines with weak memory ordering! */
        SpinLockAcquire(&xlogctl->info_lck);
        LocalHotStandbyActive = xlogctl->SharedHotStandbyActive;
        SpinLockRelease(&xlogctl->info_lck);

        return LocalHotStandbyActive;
    }
}

/*
 * Is this process allowed to insert new WAL records?
 *
 * Ordinarily this is essentially equivalent to !RecoveryInProgress().
 * But we also have provisions for forcing the result "true" or "false"
 * within specific processes regardless of the global state.
 */
bool
XLogInsertAllowed(void)
{
    /*
     * If value is "unconditionally true" or "unconditionally false", just
     * return it.  This provides the normal fast path once recovery is known
     * done.
     */
    if (LocalXLogInsertAllowed >= 0)
        return (bool) LocalXLogInsertAllowed;

    /*
     * Else, must check to see if we're still in recovery.
     */
    if (RecoveryInProgress())
        return false;

    /*
     * On exit from recovery, reset to "unconditionally true", since there is
     * no need to keep checking.
     */
    LocalXLogInsertAllowed = 1;
    return true;
}

/*
 * Make XLogInsertAllowed() return true in the current process only.
 *
 * Note: it is allowed to switch LocalXLogInsertAllowed back to -1 later,
 * and even call LocalSetXLogInsertAllowed() again after that.
 */
static void
LocalSetXLogInsertAllowed(void)
{
    Assert(LocalXLogInsertAllowed == -1);
    LocalXLogInsertAllowed = 1;

    /* Initialize as RecoveryInProgress() would do when switching state */
    InitXLOGAccess();
}

/*
 * Subroutine to try to fetch and validate a prior checkpoint record.
 *
 * whichChkpt identifies the checkpoint (merely for reporting purposes).
 * 1 for "primary", 2 for "secondary", 0 for "other" (backup_label)
 */
static XLogRecord *
ReadCheckpointRecord(XLogReaderState *xlogreader, XLogRecPtr RecPtr,
                     int whichChkpt, bool report)
{
    XLogRecord *record;

    if (!XRecOffIsValid(RecPtr))
    {
        if (!report)
            return NULL;

        switch (whichChkpt)
        {
            case 1:
                ereport(LOG,
                (errmsg("invalid primary checkpoint link in control file")));
                break;
            case 2:
                ereport(LOG,
                        (errmsg("invalid secondary checkpoint link in control file")));
                break;
            default:
                ereport(LOG,
                   (errmsg("invalid checkpoint link in backup_label file")));
                break;
        }
        return NULL;
    }

    record = ReadRecord(xlogreader, RecPtr, LOG, true);

    if (record == NULL)
    {
        if (!report)
            return NULL;

        switch (whichChkpt)
        {
            case 1:
                ereport(LOG,
                        (errmsg("invalid primary checkpoint record")));
                break;
            case 2:
                ereport(LOG,
                        (errmsg("invalid secondary checkpoint record")));
                break;
            default:
                ereport(LOG,
                        (errmsg("invalid checkpoint record")));
                break;
        }
        return NULL;
    }
    if (record->xl_rmid != RM_XLOG_ID)
    {
        switch (whichChkpt)
        {
            case 1:
                ereport(LOG,
                        (errmsg("invalid resource manager ID in primary checkpoint record")));
                break;
            case 2:
                ereport(LOG,
                        (errmsg("invalid resource manager ID in secondary checkpoint record")));
                break;
            default:
                ereport(LOG,
                (errmsg("invalid resource manager ID in checkpoint record")));
                break;
        }
        return NULL;
    }
    if (record->xl_info != XLOG_CHECKPOINT_SHUTDOWN &&
        record->xl_info != XLOG_CHECKPOINT_ONLINE)
    {
        switch (whichChkpt)
        {
            case 1:
                ereport(LOG,
                   (errmsg("invalid xl_info in primary checkpoint record")));
                break;
            case 2:
                ereport(LOG,
                 (errmsg("invalid xl_info in secondary checkpoint record")));
                break;
            default:
                ereport(LOG,
                        (errmsg("invalid xl_info in checkpoint record")));
                break;
        }
        return NULL;
    }
    if (record->xl_len != sizeof(CheckPoint) ||
        record->xl_tot_len != SizeOfXLogRecord + sizeof(CheckPoint))
    {
        switch (whichChkpt)
        {
            case 1:
                ereport(LOG,
                    (errmsg("invalid length of primary checkpoint record")));
                break;
            case 2:
                ereport(LOG,
                  (errmsg("invalid length of secondary checkpoint record")));
                break;
            default:
                ereport(LOG,
                        (errmsg("invalid length of checkpoint record")));
                break;
        }
        return NULL;
    }
    return record;
}

/*
 * This must be called during startup of a backend process, except that
 * it need not be called in a standalone backend (which does StartupXLOG
 * instead).  We need to initialize the local copies of ThisTimeLineID and
 * RedoRecPtr.
 *
 * Note: before Postgres 8.0, we went to some effort to keep the postmaster
 * process's copies of ThisTimeLineID and RedoRecPtr valid too.  This was
 * unnecessary however, since the postmaster itself never touches XLOG anyway.
 */
void
InitXLOGAccess(void)
{
    /* ThisTimeLineID doesn't change so we need no lock to copy it */
    ThisTimeLineID = XLogCtl->ThisTimeLineID;
    Assert(ThisTimeLineID != 0 || IsBootstrapProcessingMode());

    /* Use GetRedoRecPtr to copy the RedoRecPtr safely */
    (void) GetRedoRecPtr();
}

/*
 * Once spawned, a backend may update its local RedoRecPtr from
 * XLogCtl->Insert.RedoRecPtr; it must hold the insert lock or info_lck
 * to do so.  This is done in XLogInsert() or GetRedoRecPtr().
 */
XLogRecPtr
GetRedoRecPtr(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    Assert(RedoRecPtr <= xlogctl->Insert.RedoRecPtr);
    RedoRecPtr = xlogctl->Insert.RedoRecPtr;
    SpinLockRelease(&xlogctl->info_lck);

    return RedoRecPtr;
}

/*
 * GetInsertRecPtr -- Returns the current insert position.
 *
 * NOTE: The value *actually* returned is the position of the last full
 * xlog page. It lags behind the real insert position by at most 1 page.
 * For that, we don't need to acquire WALInsertLock which can be quite
 * heavily contended, and an approximation is enough for the current
 * usage of this function.
 */
XLogRecPtr
GetInsertRecPtr(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    XLogRecPtr  recptr;

    SpinLockAcquire(&xlogctl->info_lck);
    recptr = xlogctl->LogwrtRqst.Write;
    SpinLockRelease(&xlogctl->info_lck);

    return recptr;
}

/*
 * GetFlushRecPtr -- Returns the current flush position, ie, the last WAL
 * position known to be fsync'd to disk.
 */
XLogRecPtr
GetFlushRecPtr(void)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    XLogRecPtr  recptr;

    SpinLockAcquire(&xlogctl->info_lck);
    recptr = xlogctl->LogwrtResult.Flush;
    SpinLockRelease(&xlogctl->info_lck);

    return recptr;
}

/*
 * Get the time of the last xlog segment switch
 */
pg_time_t
GetLastSegSwitchTime(void)
{
    pg_time_t   result;

    /* Need WALWriteLock, but shared lock is sufficient */
    LWLockAcquire(WALWriteLock, LW_SHARED);
    result = XLogCtl->Write.lastSegSwitchTime;
    LWLockRelease(WALWriteLock);

    return result;
}

/*
 * GetNextXidAndEpoch - get the current nextXid value and associated epoch
 *
 * This is exported for use by code that would like to have 64-bit XIDs.
 * We don't really support such things, but all XIDs within the system
 * can be presumed "close to" the result, and thus the epoch associated
 * with them can be determined.
 */
void
GetNextXidAndEpoch(TransactionId *xid, uint32 *epoch)
{
    uint32      ckptXidEpoch;
    TransactionId ckptXid;
    TransactionId nextXid;

    /* Must read checkpoint info first, else have race condition */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        ckptXidEpoch = xlogctl->ckptXidEpoch;
        ckptXid = xlogctl->ckptXid;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /* Now fetch current nextXid */
    nextXid = ReadNewTransactionId();

    /*
     * nextXid is certainly logically later than ckptXid.  So if it's
     * numerically less, it must have wrapped into the next epoch.
     */
    if (nextXid < ckptXid)
        ckptXidEpoch++;

    *xid = nextXid;
    *epoch = ckptXidEpoch;
}

/*
 * This must be called ONCE during postmaster or standalone-backend shutdown
 */
void
ShutdownXLOG(int code, Datum arg)
{
    ereport(LOG,
            (errmsg("shutting down")));

    if (RecoveryInProgress())
        CreateRestartPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
    else
    {
        /*
         * If archiving is enabled, rotate the last XLOG file so that all the
         * remaining records are archived (postmaster wakes up the archiver
         * process one more time at the end of shutdown). The checkpoint
         * record will go to the next XLOG file and won't be archived (yet).
         */
        if (XLogArchivingActive() && XLogArchiveCommandSet())
            RequestXLogSwitch();

        CreateCheckPoint(CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_IMMEDIATE);
    }
    ShutdownCLOG();
    ShutdownSUBTRANS();
    ShutdownMultiXact();

    ereport(LOG,
            (errmsg("database system is shut down")));
}

/*
 * Log start of a checkpoint.
 */
static void
LogCheckpointStart(int flags, bool restartpoint)
{
    const char *msg;

    /*
     * XXX: This is hopelessly untranslatable. We could call gettext_noop for
     * the main message, but what about all the flags?
     */
    if (restartpoint)
        msg = "restartpoint starting:%s%s%s%s%s%s%s";
    else
        msg = "checkpoint starting:%s%s%s%s%s%s%s";

    elog(LOG, msg,
         (flags & CHECKPOINT_IS_SHUTDOWN) ? " shutdown" : "",
         (flags & CHECKPOINT_END_OF_RECOVERY) ? " end-of-recovery" : "",
         (flags & CHECKPOINT_IMMEDIATE) ? " immediate" : "",
         (flags & CHECKPOINT_FORCE) ? " force" : "",
         (flags & CHECKPOINT_WAIT) ? " wait" : "",
         (flags & CHECKPOINT_CAUSE_XLOG) ? " xlog" : "",
         (flags & CHECKPOINT_CAUSE_TIME) ? " time" : "");
}

/*
 * Log end of a checkpoint.
 */
static void
LogCheckpointEnd(bool restartpoint)
{
    long        write_secs,
                sync_secs,
                total_secs,
                longest_secs,
                average_secs;
    int         write_usecs,
                sync_usecs,
                total_usecs,
                longest_usecs,
                average_usecs;
    uint64      average_sync_time;

    CheckpointStats.ckpt_end_t = GetCurrentTimestamp();

    TimestampDifference(CheckpointStats.ckpt_write_t,
                        CheckpointStats.ckpt_sync_t,
                        &write_secs, &write_usecs);

    TimestampDifference(CheckpointStats.ckpt_sync_t,
                        CheckpointStats.ckpt_sync_end_t,
                        &sync_secs, &sync_usecs);

    /* Accumulate checkpoint timing summary data, in milliseconds. */
    BgWriterStats.m_checkpoint_write_time +=
        write_secs * 1000 + write_usecs / 1000;
    BgWriterStats.m_checkpoint_sync_time +=
        sync_secs * 1000 + sync_usecs / 1000;

    /*
     * All of the published timing statistics are accounted for.  Only
     * continue if a log message is to be written.
     */
    if (!log_checkpoints)
        return;

    TimestampDifference(CheckpointStats.ckpt_start_t,
                        CheckpointStats.ckpt_end_t,
                        &total_secs, &total_usecs);

    /*
     * Timing values returned from CheckpointStats are in microseconds.
     * Convert to the second plus microsecond form that TimestampDifference
     * returns for homogeneous printing.
     */
    longest_secs = (long) (CheckpointStats.ckpt_longest_sync / 1000000);
    longest_usecs = CheckpointStats.ckpt_longest_sync -
        (uint64) longest_secs *1000000;

    average_sync_time = 0;
    if (CheckpointStats.ckpt_sync_rels > 0)
        average_sync_time = CheckpointStats.ckpt_agg_sync_time /
            CheckpointStats.ckpt_sync_rels;
    average_secs = (long) (average_sync_time / 1000000);
    average_usecs = average_sync_time - (uint64) average_secs *1000000;

    if (restartpoint)
        elog(LOG, "restartpoint complete: wrote %d buffers (%.1f%%); "
             "%d transaction log file(s) added, %d removed, %d recycled; "
             "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
             "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
             CheckpointStats.ckpt_bufs_written,
             (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
             CheckpointStats.ckpt_segs_added,
             CheckpointStats.ckpt_segs_removed,
             CheckpointStats.ckpt_segs_recycled,
             write_secs, write_usecs / 1000,
             sync_secs, sync_usecs / 1000,
             total_secs, total_usecs / 1000,
             CheckpointStats.ckpt_sync_rels,
             longest_secs, longest_usecs / 1000,
             average_secs, average_usecs / 1000);
    else
        elog(LOG, "checkpoint complete: wrote %d buffers (%.1f%%); "
             "%d transaction log file(s) added, %d removed, %d recycled; "
             "write=%ld.%03d s, sync=%ld.%03d s, total=%ld.%03d s; "
             "sync files=%d, longest=%ld.%03d s, average=%ld.%03d s",
             CheckpointStats.ckpt_bufs_written,
             (double) CheckpointStats.ckpt_bufs_written * 100 / NBuffers,
             CheckpointStats.ckpt_segs_added,
             CheckpointStats.ckpt_segs_removed,
             CheckpointStats.ckpt_segs_recycled,
             write_secs, write_usecs / 1000,
             sync_secs, sync_usecs / 1000,
             total_secs, total_usecs / 1000,
             CheckpointStats.ckpt_sync_rels,
             longest_secs, longest_usecs / 1000,
             average_secs, average_usecs / 1000);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * flags is a bitwise OR of the following:
 *  CHECKPOINT_IS_SHUTDOWN: checkpoint is for database shutdown.
 *  CHECKPOINT_END_OF_RECOVERY: checkpoint is for end of WAL recovery.
 *  CHECKPOINT_IMMEDIATE: finish the checkpoint ASAP,
 *      ignoring checkpoint_completion_target parameter.
 *  CHECKPOINT_FORCE: force a checkpoint even if no XLOG activity has occurred
 *      since the last one (implied by CHECKPOINT_IS_SHUTDOWN or
 *      CHECKPOINT_END_OF_RECOVERY).
 *
 * Note: flags contains other bits, of interest here only for logging purposes.
 * In particular note that this routine is synchronous and does not pay
 * attention to CHECKPOINT_WAIT.
 *
 * If !shutdown then we are writing an online checkpoint. This is a very special
 * kind of operation and WAL record because the checkpoint action occurs over
 * a period of time yet logically occurs at just a single LSN. The logical
 * position of the WAL record (redo ptr) is the same or earlier than the
 * physical position. When we replay WAL we locate the checkpoint via its
 * physical position then read the redo ptr and actually start replay at the
 * earlier logical position. Note that we don't write *anything* to WAL at
 * the logical position, so that location could be any other kind of WAL record.
 * All of this mechanism allows us to continue working while we checkpoint.
 * As a result, timing of actions is critical here and be careful to note that
 * this function will likely take minutes to execute on a busy system.
 */
void
CreateCheckPoint(int flags)
{
    bool        shutdown;
    CheckPoint  checkPoint;
    XLogRecPtr  recptr;
    XLogCtlInsert *Insert = &XLogCtl->Insert;
    XLogRecData rdata;
    uint32      freespace;
    XLogSegNo   _logSegNo;
    VirtualTransactionId *vxids;
    int nvxids;

    /*
     * An end-of-recovery checkpoint is really a shutdown checkpoint, just
     * issued at a different time.
     */
    if (flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY))
        shutdown = true;
    else
        shutdown = false;

    /* sanity check */
    if (RecoveryInProgress() && (flags & CHECKPOINT_END_OF_RECOVERY) == 0)
        elog(ERROR, "can't create a checkpoint during recovery");

    /*
     * Acquire CheckpointLock to ensure only one checkpoint happens at a time.
     * (This is just pro forma, since in the present system structure there is
     * only one process that is allowed to issue checkpoints at any given
     * time.)
     */
    LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

    /*
     * Prepare to accumulate statistics.
     *
     * Note: because it is possible for log_checkpoints to change while a
     * checkpoint proceeds, we always accumulate stats, even if
     * log_checkpoints is currently off.
     */
    MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
    CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

    /*
     * Use a critical section to force system panic if we have trouble.
     */
    START_CRIT_SECTION();

    if (shutdown)
    {
        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->state = DB_SHUTDOWNING;
        ControlFile->time = (pg_time_t) time(NULL);
        UpdateControlFile();
        LWLockRelease(ControlFileLock);
    }

    /*
     * Let smgr prepare for checkpoint; this has to happen before we determine
     * the REDO pointer.  Note that smgr must not do anything that'd have to
     * be undone if we decide no checkpoint is needed.
     */
    smgrpreckpt();

    /* Begin filling in the checkpoint WAL record */
    MemSet(&checkPoint, 0, sizeof(checkPoint));
    checkPoint.time = (pg_time_t) time(NULL);

    /*
     * For Hot Standby, derive the oldestActiveXid before we fix the redo
     * pointer. This allows us to begin accumulating changes to assemble our
     * starting snapshot of locks and transactions.
     */
    if (!shutdown && XLogStandbyInfoActive())
        checkPoint.oldestActiveXid = GetOldestActiveTransactionId();
    else
        checkPoint.oldestActiveXid = InvalidTransactionId;

    /*
     * We must hold WALInsertLock while examining insert state to determine
     * the checkpoint REDO pointer.
     */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);

    /*
     * If this isn't a shutdown or forced checkpoint, and we have not inserted
     * any XLOG records since the start of the last checkpoint, skip the
     * checkpoint.  The idea here is to avoid inserting duplicate checkpoints
     * when the system is idle. That wastes log space, and more importantly it
     * exposes us to possible loss of both current and previous checkpoint
     * records if the machine crashes just as we're writing the update.
     * (Perhaps it'd make even more sense to checkpoint only when the previous
     * checkpoint record is in a different xlog page?)
     *
     * We have to make two tests to determine that nothing has happened since
     * the start of the last checkpoint: current insertion point must match
     * the end of the last checkpoint record, and its redo pointer must point
     * to itself.
     */
    if ((flags & (CHECKPOINT_IS_SHUTDOWN | CHECKPOINT_END_OF_RECOVERY |
                  CHECKPOINT_FORCE)) == 0)
    {
        XLogRecPtr  curInsert;

        INSERT_RECPTR(curInsert, Insert, Insert->curridx);
        if (curInsert == ControlFile->checkPoint + 
            MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
            ControlFile->checkPoint == ControlFile->checkPointCopy.redo)
        {
            LWLockRelease(WALInsertLock);
            LWLockRelease(CheckpointLock);
            END_CRIT_SECTION();
            return;
        }
    }

    /*
     * An end-of-recovery checkpoint is created before anyone is allowed to
     * write WAL. To allow us to write the checkpoint record, temporarily
     * enable XLogInsertAllowed.  (This also ensures ThisTimeLineID is
     * initialized, which we need here and in AdvanceXLInsertBuffer.)
     */
    if (flags & CHECKPOINT_END_OF_RECOVERY)
        LocalSetXLogInsertAllowed();

    checkPoint.ThisTimeLineID = ThisTimeLineID;
    if (flags & CHECKPOINT_END_OF_RECOVERY)
        checkPoint.PrevTimeLineID = XLogCtl->PrevTimeLineID;
    else
        checkPoint.PrevTimeLineID = ThisTimeLineID;

    checkPoint.fullPageWrites = Insert->fullPageWrites;

    /*
     * Compute new REDO record ptr = location of next XLOG record.
     *
     * NB: this is NOT necessarily where the checkpoint record itself will be,
     * since other backends may insert more XLOG records while we're off doing
     * the buffer flush work.  Those XLOG records are logically after the
     * checkpoint, even though physically before it.  Got that?
     */
    freespace = INSERT_FREESPACE(Insert);
    if (freespace == 0)
    {
        (void) AdvanceXLInsertBuffer(false);
        /* OK to ignore update return flag, since we will do flush anyway */
        freespace = INSERT_FREESPACE(Insert);
    }
    INSERT_RECPTR(checkPoint.redo, Insert, Insert->curridx);

    /*
     * Here we update the shared RedoRecPtr for future XLogInsert calls; this
     * must be done while holding the insert lock AND the info_lck.
     *
     * Note: if we fail to complete the checkpoint, RedoRecPtr will be left
     * pointing past where it really needs to point.  This is okay; the only
     * consequence is that XLogInsert might back up whole buffers that it
     * didn't really need to.  We can't postpone advancing RedoRecPtr because
     * XLogInserts that happen while we are dumping buffers must assume that
     * their buffer changes are not included in the checkpoint.
     */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        RedoRecPtr = xlogctl->Insert.RedoRecPtr = checkPoint.redo;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /*
     * Now we can release WAL insert lock, allowing other xacts to proceed
     * while we are flushing disk buffers.
     */
    LWLockRelease(WALInsertLock);

    /*
     * If enabled, log checkpoint start.  We postpone this until now so as not
     * to log anything if we decided to skip the checkpoint.
     */
    if (log_checkpoints)
        LogCheckpointStart(flags, false);

    TRACE_POSTGRESQL_CHECKPOINT_START(flags);

    /*
     * In some cases there are groups of actions that must all occur on
     * one side or the other of a checkpoint record. Before flushing the
     * checkpoint record we must explicitly wait for any backend currently
     * performing those groups of actions.
     *
     * One example is end of transaction, so we must wait for any transactions
     * that are currently in commit critical sections.  If an xact inserted
     * its commit record into XLOG just before the REDO point, then a crash
     * restart from the REDO point would not replay that record, which means
     * that our flushing had better include the xact's update of pg_clog.  So
     * we wait till he's out of his commit critical section before proceeding.
     * See notes in RecordTransactionCommit().
     *
     * Because we've already released WALInsertLock, this test is a bit fuzzy:
     * it is possible that we will wait for xacts we didn't really need to
     * wait for.  But the delay should be short and it seems better to make
     * checkpoint take a bit longer than to hold locks longer than necessary.
     * (In fact, the whole reason we have this issue is that xact.c does
     * commit record XLOG insertion and clog update as two separate steps
     * protected by different locks, but again that seems best on grounds of
     * minimizing lock contention.)
     *
     * A transaction that has not yet set delayChkpt when we look cannot be at
     * risk, since he's not inserted his commit record yet; and one that's
     * already cleared it is not at risk either, since he's done fixing clog
     * and we will correctly flush the update below.  So we cannot miss any
     * xacts we need to wait for.
     */
    vxids = GetVirtualXIDsDelayingChkpt(&nvxids);
    if (nvxids > 0)
    {
        uint32  nwaits = 0;

        do
        {
            pg_usleep(10000L);  /* wait for 10 msec */
            nwaits++;
        } while (HaveVirtualXIDsDelayingChkpt(vxids, nvxids));
    }
    pfree(vxids);

    /*
     * Get the other info we need for the checkpoint record.
     */
    LWLockAcquire(XidGenLock, LW_SHARED);
    checkPoint.nextXid = ShmemVariableCache->nextXid;
    checkPoint.oldestXid = ShmemVariableCache->oldestXid;
    checkPoint.oldestXidDB = ShmemVariableCache->oldestXidDB;
    LWLockRelease(XidGenLock);

    /* Increase XID epoch if we've wrapped around since last checkpoint */
    checkPoint.nextXidEpoch = ControlFile->checkPointCopy.nextXidEpoch;
    if (checkPoint.nextXid < ControlFile->checkPointCopy.nextXid)
        checkPoint.nextXidEpoch++;

    LWLockAcquire(OidGenLock, LW_SHARED);
    checkPoint.nextOid = ShmemVariableCache->nextOid;
    if (!shutdown)
        checkPoint.nextOid += ShmemVariableCache->oidCount;
    LWLockRelease(OidGenLock);

    MultiXactGetCheckptMulti(shutdown,
                             &checkPoint.nextMulti,
                             &checkPoint.nextMultiOffset,
                             &checkPoint.oldestMulti,
                             &checkPoint.oldestMultiDB);

    /*
     * Having constructed the checkpoint record, ensure all shmem disk buffers
     * and commit-log buffers are flushed to disk.
     *
     * This I/O could fail for various reasons.  If so, we will fail to
     * complete the checkpoint, but there is no reason to force a system
     * panic. Accordingly, exit critical section while doing it.
     */
    END_CRIT_SECTION();

    CheckPointGuts(checkPoint.redo, flags);

    /*
     * Take a snapshot of running transactions and write this to WAL. This
     * allows us to reconstruct the state of running transactions during
     * archive recovery, if required. Skip, if this info disabled.
     *
     * If we are shutting down, or Startup process is completing crash
     * recovery we don't need to write running xact data.
     */
    if (!shutdown && XLogStandbyInfoActive())
        LogStandbySnapshot();

    START_CRIT_SECTION();

    /*
     * Now insert the checkpoint record into XLOG.
     */
    rdata.data = (char *) (&checkPoint);
    rdata.len = sizeof(checkPoint);
    rdata.buffer = InvalidBuffer;
    rdata.next = NULL;

    recptr = XLogInsert(RM_XLOG_ID,
                        shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
                        XLOG_CHECKPOINT_ONLINE,
                        &rdata);

    XLogFlush(recptr);

    /*
     * We mustn't write any new WAL after a shutdown checkpoint, or it will be
     * overwritten at next startup.  No-one should even try, this just allows
     * sanity-checking.  In the case of an end-of-recovery checkpoint, we want
     * to just temporarily disable writing until the system has exited
     * recovery.
     */
    if (shutdown)
    {
        if (flags & CHECKPOINT_END_OF_RECOVERY)
            LocalXLogInsertAllowed = -1;        /* return to "check" state */
        else
            LocalXLogInsertAllowed = 0; /* never again write WAL */
    }

    /*
     * We now have ProcLastRecPtr = start of actual checkpoint record, recptr
     * = end of actual checkpoint record.
     */
    if (shutdown && checkPoint.redo != ProcLastRecPtr)
        ereport(PANIC,
                (errmsg("concurrent transaction log activity while database system is shutting down")));

    /*
     * Select point at which we can truncate the log, which we base on the
     * prior checkpoint's earliest info.
     */
    XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

    /*
     * Update the control file.
     */
    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    if (shutdown)
        ControlFile->state = DB_SHUTDOWNED;
    ControlFile->prevCheckPoint = ControlFile->checkPoint;
    ControlFile->checkPoint = ProcLastRecPtr;
    ControlFile->checkPointCopy = checkPoint;
    ControlFile->time = (pg_time_t) time(NULL);
    /* crash recovery should always recover to the end of WAL */
    ControlFile->minRecoveryPoint = InvalidXLogRecPtr;
    ControlFile->minRecoveryPointTLI = 0;

    /*
     * Persist unloggedLSN value. It's reset on crash recovery, so this goes
     * unused on non-shutdown checkpoints, but seems useful to store it always
     * for debugging purposes.
     */
    SpinLockAcquire(&XLogCtl->ulsn_lck);
    ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
    SpinLockRelease(&XLogCtl->ulsn_lck);

    UpdateControlFile();
    LWLockRelease(ControlFileLock);

    /* Update shared-memory copy of checkpoint XID/epoch */
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
        xlogctl->ckptXid = checkPoint.nextXid;
        SpinLockRelease(&xlogctl->info_lck);
    }

    /*
     * We are now done with critical updates; no need for system panic if we
     * have trouble while fooling with old log segments.
     */
    END_CRIT_SECTION();

    /*
     * Let smgr do post-checkpoint cleanup (eg, deleting old files).
     */
    smgrpostckpt();

    /*
     * Delete old log files (those no longer needed even for previous
     * checkpoint or the standbys in XLOG streaming).
     */
    if (_logSegNo)
    {
        KeepLogSeg(recptr, &_logSegNo);
        _logSegNo--;
        RemoveOldXlogFiles(_logSegNo, recptr);
    }

    /*
     * Make more log segments if needed.  (Do this after recycling old log
     * segments, since that may supply some of the needed files.)
     */
    if (!shutdown)
        PreallocXlogFiles(recptr);

    /*
     * Truncate pg_subtrans if possible.  We can throw away all data before
     * the oldest XMIN of any running transaction.  No future transaction will
     * attempt to reference any pg_subtrans entry older than that (see Asserts
     * in subtrans.c).  During recovery, though, we mustn't do this because
     * StartupSUBTRANS hasn't been called yet.
     */
    if (!RecoveryInProgress())
        TruncateSUBTRANS(GetOldestXmin(true, false));

    /* Real work is done, but log and update stats before releasing lock. */
    LogCheckpointEnd(false);

    TRACE_POSTGRESQL_CHECKPOINT_DONE(CheckpointStats.ckpt_bufs_written,
                                     NBuffers,
                                     CheckpointStats.ckpt_segs_added,
                                     CheckpointStats.ckpt_segs_removed,
                                     CheckpointStats.ckpt_segs_recycled);

    LWLockRelease(CheckpointLock);
}

/*
 * Mark the end of recovery in WAL though without running a full checkpoint.
 * We can expect that a restartpoint is likely to be in progress as we
 * do this, though we are unwilling to wait for it to complete. So be
 * careful to avoid taking the CheckpointLock anywhere here.
 *
 * CreateRestartPoint() allows for the case where recovery may end before
 * the restartpoint completes so there is no concern of concurrent behaviour.
 */
void
CreateEndOfRecoveryRecord(void)
{
    xl_end_of_recovery  xlrec;
    XLogRecData         rdata;
    XLogRecPtr          recptr;

    /* sanity check */
    if (!RecoveryInProgress())
        elog(ERROR, "can only be used to end recovery");

    xlrec.end_time = time(NULL);

    LWLockAcquire(WALInsertLock, LW_SHARED);
    xlrec.ThisTimeLineID = ThisTimeLineID;
    xlrec.PrevTimeLineID = XLogCtl->PrevTimeLineID;
    LWLockRelease(WALInsertLock);

    LocalSetXLogInsertAllowed();

    START_CRIT_SECTION();

    rdata.data = (char *) &xlrec;
    rdata.len = sizeof(xl_end_of_recovery);
    rdata.buffer = InvalidBuffer;
    rdata.next = NULL;

    recptr = XLogInsert(RM_XLOG_ID, XLOG_END_OF_RECOVERY, &rdata);

    XLogFlush(recptr);

    /*
     * Update the control file so that crash recovery can follow
     * the timeline changes to this point.
     */
    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    ControlFile->time = (pg_time_t) xlrec.end_time;
    ControlFile->minRecoveryPoint = recptr;
    ControlFile->minRecoveryPointTLI = ThisTimeLineID;
    UpdateControlFile();
    LWLockRelease(ControlFileLock);

    END_CRIT_SECTION();

    LocalXLogInsertAllowed = -1;        /* return to "check" state */
}

/*
 * Flush all data in shared memory to disk, and fsync
 *
 * This is the common code shared between regular checkpoints and
 * recovery restartpoints.
 */
static void
CheckPointGuts(XLogRecPtr checkPointRedo, int flags)
{
    CheckPointCLOG();
    CheckPointSUBTRANS();
    CheckPointMultiXact();
    CheckPointPredicate();
    CheckPointRelationMap();
    CheckPointBuffers(flags);   /* performs all required fsyncs */
    /* We deliberately delay 2PC checkpointing as long as possible */
    CheckPointTwoPhase(checkPointRedo);
}

/*
 * Save a checkpoint for recovery restart if appropriate
 *
 * This function is called each time a checkpoint record is read from XLOG.
 * It must determine whether the checkpoint represents a safe restartpoint or
 * not.  If so, the checkpoint record is stashed in shared memory so that
 * CreateRestartPoint can consult it.  (Note that the latter function is
 * executed by the checkpointer, while this one will be executed by the
 * startup process.)
 */
static void
RecoveryRestartPoint(const CheckPoint *checkPoint)
{
    int         rmid;

    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    /*
     * Is it safe to restartpoint?  We must ask each of the resource managers
     * whether they have any partial state information that might prevent a
     * correct restart from this point.  If so, we skip this opportunity, but
     * return at the next checkpoint record for another try.
     */
    for (rmid = 0; rmid <= RM_MAX_ID; rmid++)
    {
        if (RmgrTable[rmid].rm_safe_restartpoint != NULL)
            if (!(RmgrTable[rmid].rm_safe_restartpoint()))
            {
                elog(trace_recovery(DEBUG2),
                     "RM %d not safe to record restart point at %X/%X",
                     rmid,
                     (uint32) (checkPoint->redo >> 32),
                     (uint32) checkPoint->redo);
                return;
            }
    }

    /*
     * Also refrain from creating a restartpoint if we have seen any
     * references to non-existent pages. Restarting recovery from the
     * restartpoint would not see the references, so we would lose the
     * cross-check that the pages belonged to a relation that was dropped
     * later.
     */
    if (XLogHaveInvalidPages())
    {
        elog(trace_recovery(DEBUG2),
             "could not record restart point at %X/%X because there "
             "are unresolved references to invalid pages",
             (uint32) (checkPoint->redo >> 32),
             (uint32) checkPoint->redo);
        return;
    }

    /*
     * Copy the checkpoint record to shared memory, so that checkpointer can
     * work out the next time it wants to perform a restartpoint.
     */
    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->lastCheckPointRecPtr = ReadRecPtr;
    xlogctl->lastCheckPoint = *checkPoint;
    SpinLockRelease(&xlogctl->info_lck);
}

/*
 * Establish a restartpoint if possible.
 *
 * This is similar to CreateCheckPoint, but is used during WAL recovery
 * to establish a point from which recovery can roll forward without
 * replaying the entire recovery log.
 *
 * Returns true if a new restartpoint was established. We can only establish
 * a restartpoint if we have replayed a safe checkpoint record since last
 * restartpoint.
 */
bool
CreateRestartPoint(int flags)
{
    XLogRecPtr  lastCheckPointRecPtr;
    CheckPoint  lastCheckPoint;
    XLogSegNo   _logSegNo;
    TimestampTz xtime;

    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    /*
     * Acquire CheckpointLock to ensure only one restartpoint or checkpoint
     * happens at a time.
     */
    LWLockAcquire(CheckpointLock, LW_EXCLUSIVE);

    /* Get a local copy of the last safe checkpoint record. */
    SpinLockAcquire(&xlogctl->info_lck);
    lastCheckPointRecPtr = xlogctl->lastCheckPointRecPtr;
    lastCheckPoint = xlogctl->lastCheckPoint;
    SpinLockRelease(&xlogctl->info_lck);

    /*
     * Check that we're still in recovery mode. It's ok if we exit recovery
     * mode after this check, the restart point is valid anyway.
     */
    if (!RecoveryInProgress())
    {
        ereport(DEBUG2,
              (errmsg("skipping restartpoint, recovery has already ended")));
        LWLockRelease(CheckpointLock);
        return false;
    }

    /*
     * If the last checkpoint record we've replayed is already our last
     * restartpoint, we can't perform a new restart point. We still update
     * minRecoveryPoint in that case, so that if this is a shutdown restart
     * point, we won't start up earlier than before. That's not strictly
     * necessary, but when hot standby is enabled, it would be rather weird if
     * the database opened up for read-only connections at a point-in-time
     * before the last shutdown. Such time travel is still possible in case of
     * immediate shutdown, though.
     *
     * We don't explicitly advance minRecoveryPoint when we do create a
     * restartpoint. It's assumed that flushing the buffers will do that as a
     * side-effect.
     */
    if (XLogRecPtrIsInvalid(lastCheckPointRecPtr) ||
        lastCheckPoint.redo <= ControlFile->checkPointCopy.redo)
    {
        ereport(DEBUG2,
                (errmsg("skipping restartpoint, already performed at %X/%X",
                        (uint32) (lastCheckPoint.redo >> 32), (uint32) lastCheckPoint.redo)));

        UpdateMinRecoveryPoint(InvalidXLogRecPtr, true);
        if (flags & CHECKPOINT_IS_SHUTDOWN)
        {
            LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
            ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
            ControlFile->time = (pg_time_t) time(NULL);
            UpdateControlFile();
            LWLockRelease(ControlFileLock);
        }
        LWLockRelease(CheckpointLock);
        return false;
    }

    /*
     * Update the shared RedoRecPtr so that the startup process can calculate
     * the number of segments replayed since last restartpoint, and request a
     * restartpoint if it exceeds checkpoint_segments.
     *
     * You need to hold WALInsertLock and info_lck to update it, although
     * during recovery acquiring WALInsertLock is just pro forma, because
     * there is no other processes updating Insert.RedoRecPtr.
     */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->Insert.RedoRecPtr = lastCheckPoint.redo;
    SpinLockRelease(&xlogctl->info_lck);
    LWLockRelease(WALInsertLock);

    /*
     * Prepare to accumulate statistics.
     *
     * Note: because it is possible for log_checkpoints to change while a
     * checkpoint proceeds, we always accumulate stats, even if
     * log_checkpoints is currently off.
     */
    MemSet(&CheckpointStats, 0, sizeof(CheckpointStats));
    CheckpointStats.ckpt_start_t = GetCurrentTimestamp();

    if (log_checkpoints)
        LogCheckpointStart(flags, true);

    CheckPointGuts(lastCheckPoint.redo, flags);

    /*
     * Select point at which we can truncate the xlog, which we base on the
     * prior checkpoint's earliest info.
     */
    XLByteToSeg(ControlFile->checkPointCopy.redo, _logSegNo);

    /*
     * Update pg_control, using current time.  Check that it still shows
     * IN_ARCHIVE_RECOVERY state and an older checkpoint, else do nothing;
     * this is a quick hack to make sure nothing really bad happens if somehow
     * we get here after the end-of-recovery checkpoint.
     */
    LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
    if (ControlFile->state == DB_IN_ARCHIVE_RECOVERY &&
        ControlFile->checkPointCopy.redo < lastCheckPoint.redo)
    {
        ControlFile->prevCheckPoint = ControlFile->checkPoint;
        ControlFile->checkPoint = lastCheckPointRecPtr;
        ControlFile->checkPointCopy = lastCheckPoint;
        ControlFile->time = (pg_time_t) time(NULL);
        if (flags & CHECKPOINT_IS_SHUTDOWN)
            ControlFile->state = DB_SHUTDOWNED_IN_RECOVERY;
        UpdateControlFile();
    }
    LWLockRelease(ControlFileLock);

    /*
     * Delete old log files (those no longer needed even for previous
     * checkpoint/restartpoint) to prevent the disk holding the xlog from
     * growing full.
     */
    if (_logSegNo)
    {
        XLogRecPtr  receivePtr;
        XLogRecPtr  replayPtr;
        XLogRecPtr  endptr;

        /*
         * Get the current end of xlog replayed or received, whichever is later.
         */
        receivePtr = GetWalRcvWriteRecPtr(NULL, NULL);
        replayPtr = GetXLogReplayRecPtr(NULL);
        endptr = (receivePtr < replayPtr) ? replayPtr : receivePtr;

        KeepLogSeg(endptr, &_logSegNo);
        _logSegNo--;

        /*
         * Update ThisTimeLineID to the timeline we're currently replaying,
         * so that we install any recycled segments on that timeline.
         *
         * There is no guarantee that the WAL segments will be useful on the
         * current timeline; if recovery proceeds to a new timeline right
         * after this, the pre-allocated WAL segments on this timeline will
         * not be used, and will go wasted until recycled on the next
         * restartpoint. We'll live with that.
         */
        (void) GetXLogReplayRecPtr(&ThisTimeLineID);

        RemoveOldXlogFiles(_logSegNo, endptr);

        /*
         * Make more log segments if needed.  (Do this after recycling old log
         * segments, since that may supply some of the needed files.)
         */
        PreallocXlogFiles(endptr);
    }

    /*
     * Truncate pg_subtrans if possible.  We can throw away all data before
     * the oldest XMIN of any running transaction.  No future transaction will
     * attempt to reference any pg_subtrans entry older than that (see Asserts
     * in subtrans.c).  When hot standby is disabled, though, we mustn't do
     * this because StartupSUBTRANS hasn't been called yet.
     */
    if (EnableHotStandby)
        TruncateSUBTRANS(GetOldestXmin(true, false));

    /* Real work is done, but log and update before releasing lock. */
    LogCheckpointEnd(true);

    xtime = GetLatestXTime();
    ereport((log_checkpoints ? LOG : DEBUG2),
            (errmsg("recovery restart point at %X/%X",
                    (uint32) (lastCheckPoint.redo >> 32), (uint32) lastCheckPoint.redo),
           xtime ? errdetail("last completed transaction was at log time %s",
                             timestamptz_to_str(xtime)) : 0));

    LWLockRelease(CheckpointLock);

    /*
     * Finally, execute archive_cleanup_command, if any.
     */
    if (XLogCtl->archiveCleanupCommand[0])
        ExecuteRecoveryCommand(XLogCtl->archiveCleanupCommand,
                               "archive_cleanup_command",
                               false);

    return true;
}

/*
 * Retreat *logSegNo to the last segment that we need to retain because of
 * wal_keep_segments. This is calculated by subtracting wal_keep_segments
 * from the given xlog location, recptr.
 */
static void
KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
    XLogSegNo   segno;

    if (wal_keep_segments == 0)
        return;

    XLByteToSeg(recptr, segno);

    /* avoid underflow, don't go below 1 */
    if (segno <= wal_keep_segments)
        segno = 1;
    else
        segno = segno - wal_keep_segments;

    /* don't delete WAL segments newer than the calculated segment */
    if (segno < *logSegNo)
        *logSegNo = segno;
}

/*
 * Write a NEXTOID log record
 */
void
XLogPutNextOid(Oid nextOid)
{
    XLogRecData rdata;

    rdata.data = (char *) (&nextOid);
    rdata.len = sizeof(Oid);
    rdata.buffer = InvalidBuffer;
    rdata.next = NULL;
    (void) XLogInsert(RM_XLOG_ID, XLOG_NEXTOID, &rdata);

    /*
     * We need not flush the NEXTOID record immediately, because any of the
     * just-allocated OIDs could only reach disk as part of a tuple insert or
     * update that would have its own XLOG record that must follow the NEXTOID
     * record.  Therefore, the standard buffer LSN interlock applied to those
     * records will ensure no such OID reaches disk before the NEXTOID record
     * does.
     *
     * Note, however, that the above statement only covers state "within" the
     * database.  When we use a generated OID as a file or directory name, we
     * are in a sense violating the basic WAL rule, because that filesystem
     * change may reach disk before the NEXTOID WAL record does.  The impact
     * of this is that if a database crash occurs immediately afterward, we
     * might after restart re-generate the same OID and find that it conflicts
     * with the leftover file or directory.  But since for safety's sake we
     * always loop until finding a nonconflicting filename, this poses no real
     * problem in practice. See pgsql-hackers discussion 27-Sep-2006.
     */
}

/*
 * Write an XLOG SWITCH record.
 *
 * Here we just blindly issue an XLogInsert request for the record.
 * All the magic happens inside XLogInsert.
 *
 * The return value is either the end+1 address of the switch record,
 * or the end+1 address of the prior segment if we did not need to
 * write a switch record because we are already at segment start.
 */
XLogRecPtr
RequestXLogSwitch(void)
{
    XLogRecPtr  RecPtr;
    XLogRecData rdata;

    /* XLOG SWITCH, alone among xlog record types, has no data */
    rdata.buffer = InvalidBuffer;
    rdata.data = NULL;
    rdata.len = 0;
    rdata.next = NULL;

    RecPtr = XLogInsert(RM_XLOG_ID, XLOG_SWITCH, &rdata);

    return RecPtr;
}

/*
 * Write a RESTORE POINT record
 */
XLogRecPtr
XLogRestorePoint(const char *rpName)
{
    XLogRecPtr  RecPtr;
    XLogRecData rdata;
    xl_restore_point xlrec;

    xlrec.rp_time = GetCurrentTimestamp();
    strncpy(xlrec.rp_name, rpName, MAXFNAMELEN);

    rdata.buffer = InvalidBuffer;
    rdata.data = (char *) &xlrec;
    rdata.len = sizeof(xl_restore_point);
    rdata.next = NULL;

    RecPtr = XLogInsert(RM_XLOG_ID, XLOG_RESTORE_POINT, &rdata);

    ereport(LOG,
            (errmsg("restore point \"%s\" created at %X/%X",
                    rpName, (uint32) (RecPtr >> 32), (uint32) RecPtr)));

    return RecPtr;
}

/*
 * Write a backup block if needed when we are setting a hint. Note that
 * this may be called for a variety of page types, not just heaps.
 *
 * Callable while holding just share lock on the buffer content.
 *
 * We can't use the plain backup block mechanism since that relies on the
 * Buffer being exclusively locked. Since some modifications (setting LSN, hint
 * bits) are allowed in a sharelocked buffer that can lead to wal checksum
 * failures. So instead we copy the page and insert the copied data as normal
 * record data.
 *
 * We only need to do something if page has not yet been full page written in
 * this checkpoint round. The LSN of the inserted wal record is returned if we
 * had to write, InvalidXLogRecPtr otherwise.
 *
 * It is possible that multiple concurrent backends could attempt to write WAL
 * records. In that case, multiple copies of the same block would be recorded
 * in separate WAL records by different backends, though that is still OK from
 * a correctness perspective.
 *
 * Note that this only works for buffers that fit the standard page model,
 * i.e. those for which buffer_std == true
 */
XLogRecPtr
XLogSaveBufferForHint(Buffer buffer)
{
    XLogRecPtr recptr = InvalidXLogRecPtr;
    XLogRecPtr lsn;
    XLogRecData rdata[2];
    BkpBlock bkpb;

    /*
     * Ensure no checkpoint can change our view of RedoRecPtr.
     */
    Assert(MyPgXact->delayChkpt);

    /*
     * Update RedoRecPtr so XLogCheckBuffer can make the right decision
     */
    GetRedoRecPtr();

    /*
     * Setup phony rdata element for use within XLogCheckBuffer only.
     * We reuse and reset rdata for any actual WAL record insert.
     */
    rdata[0].buffer = buffer;
    rdata[0].buffer_std = true;

    /*
     * Check buffer while not holding an exclusive lock.
     */
    if (XLogCheckBuffer(rdata, false, &lsn, &bkpb))
    {
        char copied_buffer[BLCKSZ];
        char *origdata = (char *) BufferGetBlock(buffer);

        /*
         * Copy buffer so we don't have to worry about concurrent hint bit or
         * lsn updates. We assume pd_lower/upper cannot be changed without an
         * exclusive lock, so the contents bkp are not racy.
         */
        memcpy(copied_buffer, origdata, bkpb.hole_offset);
        memcpy(copied_buffer + bkpb.hole_offset,
                origdata + bkpb.hole_offset + bkpb.hole_length,
                BLCKSZ - bkpb.hole_offset - bkpb.hole_length);

        /*
         * Header for backup block.
         */
        rdata[0].data = (char *) &bkpb;
        rdata[0].len = sizeof(BkpBlock);
        rdata[0].buffer = InvalidBuffer;
        rdata[0].next = &(rdata[1]);

        /*
         * Save copy of the buffer.
         */
        rdata[1].data = copied_buffer;
        rdata[1].len = BLCKSZ - bkpb.hole_length;
        rdata[1].buffer = InvalidBuffer;
        rdata[1].next = NULL;

        recptr = XLogInsert(RM_XLOG_ID, XLOG_HINT, rdata);
    }

    return recptr;
}

/*
 * Check if any of the GUC parameters that are critical for hot standby
 * have changed, and update the value in pg_control file if necessary.
 */
static void
XLogReportParameters(void)
{
    if (wal_level != ControlFile->wal_level ||
        MaxConnections != ControlFile->MaxConnections ||
        max_prepared_xacts != ControlFile->max_prepared_xacts ||
        max_locks_per_xact != ControlFile->max_locks_per_xact)
    {
        /*
         * The change in number of backend slots doesn't need to be WAL-logged
         * if archiving is not enabled, as you can't start archive recovery
         * with wal_level=minimal anyway. We don't really care about the
         * values in pg_control either if wal_level=minimal, but seems better
         * to keep them up-to-date to avoid confusion.
         */
        if (wal_level != ControlFile->wal_level || XLogIsNeeded())
        {
            XLogRecData rdata;
            xl_parameter_change xlrec;

            xlrec.MaxConnections = MaxConnections;
            xlrec.max_prepared_xacts = max_prepared_xacts;
            xlrec.max_locks_per_xact = max_locks_per_xact;
            xlrec.wal_level = wal_level;

            rdata.buffer = InvalidBuffer;
            rdata.data = (char *) &xlrec;
            rdata.len = sizeof(xlrec);
            rdata.next = NULL;

            XLogInsert(RM_XLOG_ID, XLOG_PARAMETER_CHANGE, &rdata);
        }

        ControlFile->MaxConnections = MaxConnections;
        ControlFile->max_prepared_xacts = max_prepared_xacts;
        ControlFile->max_locks_per_xact = max_locks_per_xact;
        ControlFile->wal_level = wal_level;
        UpdateControlFile();
    }
}

/*
 * Update full_page_writes in shared memory, and write an
 * XLOG_FPW_CHANGE record if necessary.
 *
 * Note: this function assumes there is no other process running
 * concurrently that could update it.
 */
void
UpdateFullPageWrites(void)
{
    XLogCtlInsert *Insert = &XLogCtl->Insert;

    /*
     * Do nothing if full_page_writes has not been changed.
     *
     * It's safe to check the shared full_page_writes without the lock,
     * because we assume that there is no concurrently running process which
     * can update it.
     */
    if (fullPageWrites == Insert->fullPageWrites)
        return;

    START_CRIT_SECTION();

    /*
     * It's always safe to take full page images, even when not strictly
     * required, but not the other round. So if we're setting full_page_writes
     * to true, first set it true and then write the WAL record. If we're
     * setting it to false, first write the WAL record and then set the global
     * flag.
     */
    if (fullPageWrites)
    {
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
        Insert->fullPageWrites = true;
        LWLockRelease(WALInsertLock);
    }

    /*
     * Write an XLOG_FPW_CHANGE record. This allows us to keep track of
     * full_page_writes during archive recovery, if required.
     */
    if (XLogStandbyInfoActive() && !RecoveryInProgress())
    {
        XLogRecData rdata;

        rdata.data = (char *) (&fullPageWrites);
        rdata.len = sizeof(bool);
        rdata.buffer = InvalidBuffer;
        rdata.next = NULL;

        XLogInsert(RM_XLOG_ID, XLOG_FPW_CHANGE, &rdata);
    }

    if (!fullPageWrites)
    {
        LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
        Insert->fullPageWrites = false;
        LWLockRelease(WALInsertLock);
    }
    END_CRIT_SECTION();
}

/*
 * Check that it's OK to switch to new timeline during recovery.
 *
 * 'lsn' is the address of the shutdown checkpoint record we're about to
 * replay. (Currently, timeline can only change at a shutdown checkpoint).
 */
static void
checkTimeLineSwitch(XLogRecPtr lsn, TimeLineID newTLI, TimeLineID prevTLI)
{
    /* Check that the record agrees on what the current (old) timeline is */
    if (prevTLI != ThisTimeLineID)
        ereport(PANIC,
                (errmsg("unexpected prev timeline ID %u (current timeline ID %u) in checkpoint record",
                        prevTLI, ThisTimeLineID)));
    /*
     * The new timeline better be in the list of timelines we expect
     * to see, according to the timeline history. It should also not
     * decrease.
     */
    if (newTLI < ThisTimeLineID || !tliInHistory(newTLI, expectedTLEs))
        ereport(PANIC,
                (errmsg("unexpected timeline ID %u (after %u) in checkpoint record",
                        newTLI, ThisTimeLineID)));

    /*
     * If we have not yet reached min recovery point, and we're about
     * to switch to a timeline greater than the timeline of the min
     * recovery point: trouble. After switching to the new timeline,
     * we could not possibly visit the min recovery point on the
     * correct timeline anymore. This can happen if there is a newer
     * timeline in the archive that branched before the timeline the
     * min recovery point is on, and you attempt to do PITR to the
     * new timeline.
     */
    if (!XLogRecPtrIsInvalid(minRecoveryPoint) &&
        lsn < minRecoveryPoint &&
        newTLI > minRecoveryPointTLI)
        ereport(PANIC,
                (errmsg("unexpected timeline ID %u in checkpoint record, before reaching minimum recovery point %X/%X on timeline %u",
                        newTLI,
                        (uint32) (minRecoveryPoint >> 32),
                        (uint32) minRecoveryPoint,
                        minRecoveryPointTLI)));

    /* Looks good */
}

/*
 * XLOG resource manager's routines
 *
 * Definitions of info values are in include/catalog/pg_control.h, though
 * not all record types are related to control file updates.
 */
void
xlog_redo(XLogRecPtr lsn, XLogRecord *record)
{
    uint8       info = record->xl_info & ~XLR_INFO_MASK;

    /* Backup blocks are not used by XLOG rmgr */
    Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

    if (info == XLOG_NEXTOID)
    {
        Oid         nextOid;

        /*
         * We used to try to take the maximum of ShmemVariableCache->nextOid
         * and the recorded nextOid, but that fails if the OID counter wraps
         * around.  Since no OID allocation should be happening during replay
         * anyway, better to just believe the record exactly.  We still take
         * OidGenLock while setting the variable, just in case.
         */
        memcpy(&nextOid, XLogRecGetData(record), sizeof(Oid));
        LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
        ShmemVariableCache->nextOid = nextOid;
        ShmemVariableCache->oidCount = 0;
        LWLockRelease(OidGenLock);
    }
    else if (info == XLOG_CHECKPOINT_SHUTDOWN)
    {
        CheckPoint  checkPoint;

        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
        /* In a SHUTDOWN checkpoint, believe the counters exactly */
        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
        ShmemVariableCache->nextXid = checkPoint.nextXid;
        LWLockRelease(XidGenLock);
        LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        LWLockRelease(OidGenLock);
        MultiXactSetNextMXact(checkPoint.nextMulti,
                              checkPoint.nextMultiOffset);
        SetTransactionIdLimit(checkPoint.oldestXid, checkPoint.oldestXidDB);
        SetMultiXactIdLimit(checkPoint.oldestMulti, checkPoint.oldestMultiDB);

        /*
         * If we see a shutdown checkpoint while waiting for an end-of-backup
         * record, the backup was canceled and the end-of-backup record will
         * never arrive.
         */
        if (ArchiveRecoveryRequested &&
            !XLogRecPtrIsInvalid(ControlFile->backupStartPoint) &&
            XLogRecPtrIsInvalid(ControlFile->backupEndPoint))
            ereport(PANIC,
            (errmsg("online backup was canceled, recovery cannot continue")));

        /*
         * If we see a shutdown checkpoint, we know that nothing was running
         * on the master at this point. So fake-up an empty running-xacts
         * record and use that here and now. Recover additional standby state
         * for prepared transactions.
         */
        if (standbyState >= STANDBY_INITIALIZED)
        {
            TransactionId *xids;
            int         nxids;
            TransactionId oldestActiveXID;
            TransactionId latestCompletedXid;
            RunningTransactionsData running;

            oldestActiveXID = PrescanPreparedTransactions(&xids, &nxids);

            /*
             * Construct a RunningTransactions snapshot representing a shut
             * down server, with only prepared transactions still alive. We're
             * never overflowed at this point because all subxids are listed
             * with their parent prepared transactions.
             */
            running.xcnt = nxids;
            running.subxcnt = 0;
            running.subxid_overflow = false;
            running.nextXid = checkPoint.nextXid;
            running.oldestRunningXid = oldestActiveXID;
            latestCompletedXid = checkPoint.nextXid;
            TransactionIdRetreat(latestCompletedXid);
            Assert(TransactionIdIsNormal(latestCompletedXid));
            running.latestCompletedXid = latestCompletedXid;
            running.xids = xids;

            ProcArrayApplyRecoveryInfo(&running);

            StandbyRecoverPreparedTransactions(true);
        }

        /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
        ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
        ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

        /* Update shared-memory copy of checkpoint XID/epoch */
        {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
            xlogctl->ckptXid = checkPoint.nextXid;
            SpinLockRelease(&xlogctl->info_lck);
        }

        /*
         * We should've already switched to the new TLI before replaying this
         * record.
         */
        if (checkPoint.ThisTimeLineID != ThisTimeLineID)
            ereport(PANIC,
                    (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                            checkPoint.ThisTimeLineID, ThisTimeLineID)));

        RecoveryRestartPoint(&checkPoint);
    }
    else if (info == XLOG_CHECKPOINT_ONLINE)
    {
        CheckPoint  checkPoint;

        memcpy(&checkPoint, XLogRecGetData(record), sizeof(CheckPoint));
        /* In an ONLINE checkpoint, treat the XID counter as a minimum */
        LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
        if (TransactionIdPrecedes(ShmemVariableCache->nextXid,
                                  checkPoint.nextXid))
            ShmemVariableCache->nextXid = checkPoint.nextXid;
        LWLockRelease(XidGenLock);
        /* ... but still treat OID counter as exact */
        LWLockAcquire(OidGenLock, LW_EXCLUSIVE);
        ShmemVariableCache->nextOid = checkPoint.nextOid;
        ShmemVariableCache->oidCount = 0;
        LWLockRelease(OidGenLock);
        MultiXactAdvanceNextMXact(checkPoint.nextMulti,
                                  checkPoint.nextMultiOffset);
        if (TransactionIdPrecedes(ShmemVariableCache->oldestXid,
                                  checkPoint.oldestXid))
            SetTransactionIdLimit(checkPoint.oldestXid,
                                  checkPoint.oldestXidDB);
        MultiXactAdvanceOldest(checkPoint.oldestMulti,
                               checkPoint.oldestMultiDB);

        /* ControlFile->checkPointCopy always tracks the latest ckpt XID */
        ControlFile->checkPointCopy.nextXidEpoch = checkPoint.nextXidEpoch;
        ControlFile->checkPointCopy.nextXid = checkPoint.nextXid;

        /* Update shared-memory copy of checkpoint XID/epoch */
        {
            /* use volatile pointer to prevent code rearrangement */
            volatile XLogCtlData *xlogctl = XLogCtl;

            SpinLockAcquire(&xlogctl->info_lck);
            xlogctl->ckptXidEpoch = checkPoint.nextXidEpoch;
            xlogctl->ckptXid = checkPoint.nextXid;
            SpinLockRelease(&xlogctl->info_lck);
        }

        /* TLI should not change in an on-line checkpoint */
        if (checkPoint.ThisTimeLineID != ThisTimeLineID)
            ereport(PANIC,
                    (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                            checkPoint.ThisTimeLineID, ThisTimeLineID)));

        RecoveryRestartPoint(&checkPoint);
    }
    else if (info == XLOG_END_OF_RECOVERY)
    {
        xl_end_of_recovery xlrec;

        memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_end_of_recovery));

        /*
         * For Hot Standby, we could treat this like a Shutdown Checkpoint,
         * but this case is rarer and harder to test, so the benefit doesn't
         * outweigh the potential extra cost of maintenance.
         */

        /*
         * We should've already switched to the new TLI before replaying this
         * record.
         */
        if (xlrec.ThisTimeLineID != ThisTimeLineID)
            ereport(PANIC,
                    (errmsg("unexpected timeline ID %u (should be %u) in checkpoint record",
                            xlrec.ThisTimeLineID, ThisTimeLineID)));
    }
    else if (info == XLOG_NOOP)
    {
        /* nothing to do here */
    }
    else if (info == XLOG_SWITCH)
    {
        /* nothing to do here */
    }
    else if (info == XLOG_RESTORE_POINT)
    {
        /* nothing to do here */
    }
    else if (info == XLOG_HINT)
    {
        char *data;
        BkpBlock bkpb;

        /*
         * Hint bit records contain a backup block stored "inline" in the normal
         * data since the locking when writing hint records isn't sufficient to
         * use the normal backup block mechanism, which assumes exclusive lock
         * on the buffer supplied.
         *
         * Since the only change in these backup block are hint bits, there are
         * no recovery conflicts generated.
         *
         * This also means there is no corresponding API call for this,
         * so an smgr implementation has no need to implement anything.
         * Which means nothing is needed in md.c etc
         */
        data = XLogRecGetData(record);
        memcpy(&bkpb, data, sizeof(BkpBlock));
        data += sizeof(BkpBlock);

        RestoreBackupBlockContents(lsn, bkpb, data, false, false);
    }
    else if (info == XLOG_BACKUP_END)
    {
        XLogRecPtr  startpoint;

        memcpy(&startpoint, XLogRecGetData(record), sizeof(startpoint));

        if (ControlFile->backupStartPoint == startpoint)
        {
            /*
             * We have reached the end of base backup, the point where
             * pg_stop_backup() was done. The data on disk is now consistent.
             * Reset backupStartPoint, and update minRecoveryPoint to make
             * sure we don't allow starting up at an earlier point even if
             * recovery is stopped and restarted soon after this.
             */
            elog(DEBUG1, "end of backup reached");

            LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);

            if (ControlFile->minRecoveryPoint < lsn)
            {
                ControlFile->minRecoveryPoint = lsn;
                ControlFile->minRecoveryPointTLI = ThisTimeLineID;
            }
            ControlFile->backupStartPoint = InvalidXLogRecPtr;
            ControlFile->backupEndRequired = false;
            UpdateControlFile();

            LWLockRelease(ControlFileLock);
        }
    }
    else if (info == XLOG_PARAMETER_CHANGE)
    {
        xl_parameter_change xlrec;

        /* Update our copy of the parameters in pg_control */
        memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_parameter_change));

        LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
        ControlFile->MaxConnections = xlrec.MaxConnections;
        ControlFile->max_prepared_xacts = xlrec.max_prepared_xacts;
        ControlFile->max_locks_per_xact = xlrec.max_locks_per_xact;
        ControlFile->wal_level = xlrec.wal_level;

        /*
         * Update minRecoveryPoint to ensure that if recovery is aborted, we
         * recover back up to this point before allowing hot standby again.
         * This is particularly important if wal_level was set to 'archive'
         * before, and is now 'hot_standby', to ensure you don't run queries
         * against the WAL preceding the wal_level change. Same applies to
         * decreasing max_* settings.
         */
        minRecoveryPoint = ControlFile->minRecoveryPoint;
        minRecoveryPointTLI = ControlFile->minRecoveryPointTLI;
        if (minRecoveryPoint != 0 && minRecoveryPoint < lsn)
        {
            ControlFile->minRecoveryPoint = lsn;
            ControlFile->minRecoveryPointTLI = ThisTimeLineID;
        }

        UpdateControlFile();
        LWLockRelease(ControlFileLock);

        /* Check to see if any changes to max_connections give problems */
        CheckRequiredParameterValues();
    }
    else if (info == XLOG_FPW_CHANGE)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        bool        fpw;

        memcpy(&fpw, XLogRecGetData(record), sizeof(bool));

        /*
         * Update the LSN of the last replayed XLOG_FPW_CHANGE record so that
         * do_pg_start_backup() and do_pg_stop_backup() can check whether
         * full_page_writes has been disabled during online backup.
         */
        if (!fpw)
        {
            SpinLockAcquire(&xlogctl->info_lck);
            if (xlogctl->lastFpwDisableRecPtr < ReadRecPtr)
                xlogctl->lastFpwDisableRecPtr = ReadRecPtr;
            SpinLockRelease(&xlogctl->info_lck);
        }

        /* Keep track of full_page_writes */
        lastFullPageWrites = fpw;
    }
}

#ifdef WAL_DEBUG

static void
xlog_outrec(StringInfo buf, XLogRecord *record)
{
    int         i;

    appendStringInfo(buf, "prev %X/%X; xid %u",
                     (uint32) (record->xl_prev >> 32),
                     (uint32) record->xl_prev,
                     record->xl_xid);

    appendStringInfo(buf, "; len %u",
                     record->xl_len);

    for (i = 0; i < XLR_MAX_BKP_BLOCKS; i++)
    {
        if (record->xl_info & XLR_BKP_BLOCK(i))
            appendStringInfo(buf, "; bkpb%d", i);
    }

    appendStringInfo(buf, ": %s", RmgrTable[record->xl_rmid].rm_name);
}
#endif   /* WAL_DEBUG */


/*
 * Return the (possible) sync flag used for opening a file, depending on the
 * value of the GUC wal_sync_method.
 */
static int
get_sync_bit(int method)
{
    int         o_direct_flag = 0;

    /* If fsync is disabled, never open in sync mode */
    if (!enableFsync)
        return 0;

    /*
     * Optimize writes by bypassing kernel cache with O_DIRECT when using
     * O_SYNC/O_FSYNC and O_DSYNC.  But only if archiving and streaming are
     * disabled, otherwise the archive command or walsender process will read
     * the WAL soon after writing it, which is guaranteed to cause a physical
     * read if we bypassed the kernel cache. We also skip the
     * posix_fadvise(POSIX_FADV_DONTNEED) call in XLogFileClose() for the same
     * reason.
     *
     * Never use O_DIRECT in walreceiver process for similar reasons; the WAL
     * written by walreceiver is normally read by the startup process soon
     * after its written. Also, walreceiver performs unaligned writes, which
     * don't work with O_DIRECT, so it is required for correctness too.
     */
    if (!XLogIsNeeded() && !AmWalReceiverProcess())
        o_direct_flag = PG_O_DIRECT;

    switch (method)
    {
            /*
             * enum values for all sync options are defined even if they are
             * not supported on the current platform.  But if not, they are
             * not included in the enum option array, and therefore will never
             * be seen here.
             */
        case SYNC_METHOD_FSYNC:
        case SYNC_METHOD_FSYNC_WRITETHROUGH:
        case SYNC_METHOD_FDATASYNC:
            return 0;
#ifdef OPEN_SYNC_FLAG
        case SYNC_METHOD_OPEN:
            return OPEN_SYNC_FLAG | o_direct_flag;
#endif
#ifdef OPEN_DATASYNC_FLAG
        case SYNC_METHOD_OPEN_DSYNC:
            return OPEN_DATASYNC_FLAG | o_direct_flag;
#endif
        default:
            /* can't happen (unless we are out of sync with option array) */
            elog(ERROR, "unrecognized wal_sync_method: %d", method);
            return 0;           /* silence warning */
    }
}

/*
 * GUC support
 */
void
assign_xlog_sync_method(int new_sync_method, void *extra)
{
    if (sync_method != new_sync_method)
    {
        /*
         * To ensure that no blocks escape unsynced, force an fsync on the
         * currently open log segment (if any).  Also, if the open flag is
         * changing, close the log file so it will be reopened (with new flag
         * bit) at next use.
         */
        if (openLogFile >= 0)
        {
            if (pg_fsync(openLogFile) != 0)
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fsync log segment %s: %m",
                                XLogFileNameP(ThisTimeLineID, openLogSegNo))));
            if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
                XLogFileClose();
        }
    }
}


/*
 * Issue appropriate kind of fsync (if any) for an XLOG output file.
 *
 * 'fd' is a file descriptor for the XLOG file to be fsync'd.
 * 'log' and 'seg' are for error reporting purposes.
 */
void
issue_xlog_fsync(int fd, XLogSegNo segno)
{
    switch (sync_method)
    {
        case SYNC_METHOD_FSYNC:
            if (pg_fsync_no_writethrough(fd) != 0)
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fsync log file %s: %m",
                                XLogFileNameP(ThisTimeLineID, segno))));
            break;
#ifdef HAVE_FSYNC_WRITETHROUGH
        case SYNC_METHOD_FSYNC_WRITETHROUGH:
            if (pg_fsync_writethrough(fd) != 0)
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fsync write-through log file %s: %m",
                                XLogFileNameP(ThisTimeLineID, segno))));
            break;
#endif
#ifdef HAVE_FDATASYNC
        case SYNC_METHOD_FDATASYNC:
            if (pg_fdatasync(fd) != 0)
                ereport(PANIC,
                        (errcode_for_file_access(),
                         errmsg("could not fdatasync log file %s: %m",
                                XLogFileNameP(ThisTimeLineID, segno))));
            break;
#endif
        case SYNC_METHOD_OPEN:
        case SYNC_METHOD_OPEN_DSYNC:
            /* write synced it already */
            break;
        default:
            elog(PANIC, "unrecognized wal_sync_method: %d", sync_method);
            break;
    }
}

/*
 * Return the filename of given log segment, as a palloc'd string.
 */
char *
XLogFileNameP(TimeLineID tli, XLogSegNo segno)
{
    char       *result = palloc(MAXFNAMELEN);
    XLogFileName(result, tli, segno);
    return result;
}

/*
 * do_pg_start_backup is the workhorse of the user-visible pg_start_backup()
 * function. It creates the necessary starting checkpoint and constructs the
 * backup label file.
 *
 * There are two kind of backups: exclusive and non-exclusive. An exclusive
 * backup is started with pg_start_backup(), and there can be only one active
 * at a time. The backup label file of an exclusive backup is written to
 * $PGDATA/backup_label, and it is removed by pg_stop_backup().
 *
 * A non-exclusive backup is used for the streaming base backups (see
 * src/backend/replication/basebackup.c). The difference to exclusive backups
 * is that the backup label file is not written to disk. Instead, its would-be
 * contents are returned in *labelfile, and the caller is responsible for
 * including it in the backup archive as 'backup_label'. There can be many
 * non-exclusive backups active at the same time, and they don't conflict
 * with an exclusive backup either.
 *
 * Returns the minimum WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *starttli_p.
 *
 * Every successfully started non-exclusive backup must be stopped by calling
 * do_pg_stop_backup() or do_pg_abort_backup().
 */
XLogRecPtr
do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
                   char **labelfile)
{
    bool        exclusive = (labelfile == NULL);
    bool        backup_started_in_recovery = false;
    XLogRecPtr  checkpointloc;
    XLogRecPtr  startpoint;
    TimeLineID  starttli;
    pg_time_t   stamp_time;
    char        strfbuf[128];
    char        xlogfilename[MAXFNAMELEN];
    XLogSegNo   _logSegNo;
    struct stat stat_buf;
    FILE       *fp;
    StringInfoData labelfbuf;

    backup_started_in_recovery = RecoveryInProgress();

    if (!superuser() && !has_rolreplication(GetUserId()))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
           errmsg("must be superuser or replication role to run a backup")));

    /*
     * Currently only non-exclusive backup can be taken during recovery.
     */
    if (backup_started_in_recovery && exclusive)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("recovery is in progress"),
                 errhint("WAL control functions cannot be executed during recovery.")));

    /*
     * During recovery, we don't need to check WAL level. Because, if WAL
     * level is not sufficient, it's impossible to get here during recovery.
     */
    if (!backup_started_in_recovery && !XLogIsNeeded())
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
              errmsg("WAL level not sufficient for making an online backup"),
                 errhint("wal_level must be set to \"archive\" or \"hot_standby\" at server start.")));

    if (strlen(backupidstr) > MAXPGPATH)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("backup label too long (max %d bytes)",
                        MAXPGPATH)));

    /*
     * Mark backup active in shared memory.  We must do full-page WAL writes
     * during an on-line backup even if not doing so at other times, because
     * it's quite possible for the backup dump to obtain a "torn" (partially
     * written) copy of a database page if it reads the page concurrently with
     * our write to the same page.  This can be fixed as long as the first
     * write to the page in the WAL sequence is a full-page write. Hence, we
     * turn on forcePageWrites and then force a CHECKPOINT, to ensure there
     * are no dirty pages in shared memory that might get dumped while the
     * backup is in progress without having a corresponding WAL record.  (Once
     * the backup is complete, we need not force full-page writes anymore,
     * since we expect that any pages not modified during the backup interval
     * must have been correctly captured by the backup.)
     *
     * Note that forcePageWrites has no effect during an online backup from
     * the standby.
     *
     * We must hold WALInsertLock to change the value of forcePageWrites, to
     * ensure adequate interlocking against XLogInsert().
     */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
    if (exclusive)
    {
        if (XLogCtl->Insert.exclusiveBackup)
        {
            LWLockRelease(WALInsertLock);
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("a backup is already in progress"),
                     errhint("Run pg_stop_backup() and try again.")));
        }
        XLogCtl->Insert.exclusiveBackup = true;
    }
    else
        XLogCtl->Insert.nonExclusiveBackups++;
    XLogCtl->Insert.forcePageWrites = true;
    LWLockRelease(WALInsertLock);

    /* Ensure we release forcePageWrites if fail below */
    PG_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
    {
        bool        gotUniqueStartpoint = false;

        /*
         * Force an XLOG file switch before the checkpoint, to ensure that the
         * WAL segment the checkpoint is written to doesn't contain pages with
         * old timeline IDs.  That would otherwise happen if you called
         * pg_start_backup() right after restoring from a PITR archive: the
         * first WAL segment containing the startup checkpoint has pages in
         * the beginning with the old timeline ID.  That can cause trouble at
         * recovery: we won't have a history file covering the old timeline if
         * pg_xlog directory was not included in the base backup and the WAL
         * archive was cleared too before starting the backup.
         *
         * This also ensures that we have emitted a WAL page header that has
         * XLP_BKP_REMOVABLE off before we emit the checkpoint record.
         * Therefore, if a WAL archiver (such as pglesslog) is trying to
         * compress out removable backup blocks, it won't remove any that
         * occur after this point.
         *
         * During recovery, we skip forcing XLOG file switch, which means that
         * the backup taken during recovery is not available for the special
         * recovery case described above.
         */
        if (!backup_started_in_recovery)
            RequestXLogSwitch();

        do
        {
            bool        checkpointfpw;

            /*
             * Force a CHECKPOINT.  Aside from being necessary to prevent torn
             * page problems, this guarantees that two successive backup runs
             * will have different checkpoint positions and hence different
             * history file names, even if nothing happened in between.
             *
             * During recovery, establish a restartpoint if possible. We use
             * the last restartpoint as the backup starting checkpoint. This
             * means that two successive backup runs can have same checkpoint
             * positions.
             *
             * Since the fact that we are executing do_pg_start_backup()
             * during recovery means that checkpointer is running, we can use
             * RequestCheckpoint() to establish a restartpoint.
             *
             * We use CHECKPOINT_IMMEDIATE only if requested by user (via
             * passing fast = true).  Otherwise this can take awhile.
             */
            RequestCheckpoint(CHECKPOINT_FORCE | CHECKPOINT_WAIT |
                              (fast ? CHECKPOINT_IMMEDIATE : 0));

            /*
             * Now we need to fetch the checkpoint record location, and also
             * its REDO pointer.  The oldest point in WAL that would be needed
             * to restore starting from the checkpoint is precisely the REDO
             * pointer.
             */
            LWLockAcquire(ControlFileLock, LW_SHARED);
            checkpointloc = ControlFile->checkPoint;
            startpoint = ControlFile->checkPointCopy.redo;
            starttli = ControlFile->checkPointCopy.ThisTimeLineID;
            checkpointfpw = ControlFile->checkPointCopy.fullPageWrites;
            LWLockRelease(ControlFileLock);

            if (backup_started_in_recovery)
            {
                /* use volatile pointer to prevent code rearrangement */
                volatile XLogCtlData *xlogctl = XLogCtl;
                XLogRecPtr  recptr;

                /*
                 * Check to see if all WAL replayed during online backup
                 * (i.e., since last restartpoint used as backup starting
                 * checkpoint) contain full-page writes.
                 */
                SpinLockAcquire(&xlogctl->info_lck);
                recptr = xlogctl->lastFpwDisableRecPtr;
                SpinLockRelease(&xlogctl->info_lck);

                if (!checkpointfpw || startpoint <= recptr)
                    ereport(ERROR,
                          (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                           errmsg("WAL generated with full_page_writes=off was replayed "
                                  "since last restartpoint"),
                           errhint("This means that the backup being taken on the standby "
                                   "is corrupt and should not be used. "
                                   "Enable full_page_writes and run CHECKPOINT on the master, "
                                   "and then try an online backup again.")));

                /*
                 * During recovery, since we don't use the end-of-backup WAL
                 * record and don't write the backup history file, the
                 * starting WAL location doesn't need to be unique. This means
                 * that two base backups started at the same time might use
                 * the same checkpoint as starting locations.
                 */
                gotUniqueStartpoint = true;
            }

            /*
             * If two base backups are started at the same time (in WAL sender
             * processes), we need to make sure that they use different
             * checkpoints as starting locations, because we use the starting
             * WAL location as a unique identifier for the base backup in the
             * end-of-backup WAL record and when we write the backup history
             * file. Perhaps it would be better generate a separate unique ID
             * for each backup instead of forcing another checkpoint, but
             * taking a checkpoint right after another is not that expensive
             * either because only few buffers have been dirtied yet.
             */
            LWLockAcquire(WALInsertLock, LW_SHARED);
            if (XLogCtl->Insert.lastBackupStart < startpoint)
            {
                XLogCtl->Insert.lastBackupStart = startpoint;
                gotUniqueStartpoint = true;
            }
            LWLockRelease(WALInsertLock);
        } while (!gotUniqueStartpoint);

        XLByteToSeg(startpoint, _logSegNo);
        XLogFileName(xlogfilename, ThisTimeLineID, _logSegNo);

        /*
         * Construct backup label file
         */
        initStringInfo(&labelfbuf);

        /* Use the log timezone here, not the session timezone */
        stamp_time = (pg_time_t) time(NULL);
        pg_strftime(strfbuf, sizeof(strfbuf),
                    "%Y-%m-%d %H:%M:%S %Z",
                    pg_localtime(&stamp_time, log_timezone));
        appendStringInfo(&labelfbuf, "START WAL LOCATION: %X/%X (file %s)\n",
                         (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename);
        appendStringInfo(&labelfbuf, "CHECKPOINT LOCATION: %X/%X\n",
                         (uint32) (checkpointloc >> 32), (uint32) checkpointloc);
        appendStringInfo(&labelfbuf, "BACKUP METHOD: %s\n",
                         exclusive ? "pg_start_backup" : "streamed");
        appendStringInfo(&labelfbuf, "BACKUP FROM: %s\n",
                         backup_started_in_recovery ? "standby" : "master");
        appendStringInfo(&labelfbuf, "START TIME: %s\n", strfbuf);
        appendStringInfo(&labelfbuf, "LABEL: %s\n", backupidstr);

        /*
         * Okay, write the file, or return its contents to caller.
         */
        if (exclusive)
        {
            /*
             * Check for existing backup label --- implies a backup is already
             * running.  (XXX given that we checked exclusiveBackup above,
             * maybe it would be OK to just unlink any such label file?)
             */
            if (stat(BACKUP_LABEL_FILE, &stat_buf) != 0)
            {
                if (errno != ENOENT)
                    ereport(ERROR,
                            (errcode_for_file_access(),
                             errmsg("could not stat file \"%s\": %m",
                                    BACKUP_LABEL_FILE)));
            }
            else
                ereport(ERROR,
                        (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                         errmsg("a backup is already in progress"),
                         errhint("If you're sure there is no backup in progress, remove file \"%s\" and try again.",
                                 BACKUP_LABEL_FILE)));

            fp = AllocateFile(BACKUP_LABEL_FILE, "w");

            if (!fp)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not create file \"%s\": %m",
                                BACKUP_LABEL_FILE)));
            if (fwrite(labelfbuf.data, labelfbuf.len, 1, fp) != 1 ||
                fflush(fp) != 0 ||
                pg_fsync(fileno(fp)) != 0 ||
                ferror(fp) ||
                FreeFile(fp))
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not write file \"%s\": %m",
                                BACKUP_LABEL_FILE)));
            pfree(labelfbuf.data);
        }
        else
            *labelfile = labelfbuf.data;
    }
    PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));

    /*
     * We're done.  As a convenience, return the starting WAL location.
     */
    if (starttli_p)
        *starttli_p = starttli;
    return startpoint;
}

/* Error cleanup callback for pg_start_backup */
static void
pg_start_backup_callback(int code, Datum arg)
{
    bool        exclusive = DatumGetBool(arg);

    /* Update backup counters and forcePageWrites on failure */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
    if (exclusive)
    {
        Assert(XLogCtl->Insert.exclusiveBackup);
        XLogCtl->Insert.exclusiveBackup = false;
    }
    else
    {
        Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
        XLogCtl->Insert.nonExclusiveBackups--;
    }

    if (!XLogCtl->Insert.exclusiveBackup &&
        XLogCtl->Insert.nonExclusiveBackups == 0)
    {
        XLogCtl->Insert.forcePageWrites = false;
    }
    LWLockRelease(WALInsertLock);
}

/*
 * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
 * function.

 * If labelfile is NULL, this stops an exclusive backup. Otherwise this stops
 * the non-exclusive backup specified by 'labelfile'.
 *
 * Returns the last WAL position that must be present to restore from this
 * backup, and the corresponding timeline ID in *stoptli_p.
 */
XLogRecPtr
do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
{
    bool        exclusive = (labelfile == NULL);
    bool        backup_started_in_recovery = false;
    XLogRecPtr  startpoint;
    XLogRecPtr  stoppoint;
    TimeLineID  stoptli;
    XLogRecData rdata;
    pg_time_t   stamp_time;
    char        strfbuf[128];
    char        histfilepath[MAXPGPATH];
    char        startxlogfilename[MAXFNAMELEN];
    char        stopxlogfilename[MAXFNAMELEN];
    char        lastxlogfilename[MAXFNAMELEN];
    char        histfilename[MAXFNAMELEN];
    char        backupfrom[20];
    XLogSegNo   _logSegNo;
    FILE       *lfp;
    FILE       *fp;
    char        ch;
    int         seconds_before_warning;
    int         waits = 0;
    bool        reported_waiting = false;
    char       *remaining;
    char       *ptr;
    uint32      hi,
                lo;

    backup_started_in_recovery = RecoveryInProgress();

    if (!superuser() && !has_rolreplication(GetUserId()))
        ereport(ERROR,
                (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
         (errmsg("must be superuser or replication role to run a backup"))));

    /*
     * Currently only non-exclusive backup can be taken during recovery.
     */
    if (backup_started_in_recovery && exclusive)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("recovery is in progress"),
                 errhint("WAL control functions cannot be executed during recovery.")));

    /*
     * During recovery, we don't need to check WAL level. Because, if WAL
     * level is not sufficient, it's impossible to get here during recovery.
     */
    if (!backup_started_in_recovery && !XLogIsNeeded())
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
              errmsg("WAL level not sufficient for making an online backup"),
                 errhint("wal_level must be set to \"archive\" or \"hot_standby\" at server start.")));

    /*
     * OK to update backup counters and forcePageWrites
     */
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
    if (exclusive)
        XLogCtl->Insert.exclusiveBackup = false;
    else
    {
        /*
         * The user-visible pg_start/stop_backup() functions that operate on
         * exclusive backups can be called at any time, but for non-exclusive
         * backups, it is expected that each do_pg_start_backup() call is
         * matched by exactly one do_pg_stop_backup() call.
         */
        Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
        XLogCtl->Insert.nonExclusiveBackups--;
    }

    if (!XLogCtl->Insert.exclusiveBackup &&
        XLogCtl->Insert.nonExclusiveBackups == 0)
    {
        XLogCtl->Insert.forcePageWrites = false;
    }
    LWLockRelease(WALInsertLock);

    if (exclusive)
    {
        /*
         * Read the existing label file into memory.
         */
        struct stat statbuf;
        int         r;

        if (stat(BACKUP_LABEL_FILE, &statbuf))
        {
            if (errno != ENOENT)
                ereport(ERROR,
                        (errcode_for_file_access(),
                         errmsg("could not stat file \"%s\": %m",
                                BACKUP_LABEL_FILE)));
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("a backup is not in progress")));
        }

        lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
        if (!lfp)
        {
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m",
                            BACKUP_LABEL_FILE)));
        }
        labelfile = palloc(statbuf.st_size + 1);
        r = fread(labelfile, statbuf.st_size, 1, lfp);
        labelfile[statbuf.st_size] = '\0';

        /*
         * Close and remove the backup label file
         */
        if (r != 1 || ferror(lfp) || FreeFile(lfp))
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m",
                            BACKUP_LABEL_FILE)));
        if (unlink(BACKUP_LABEL_FILE) != 0)
            ereport(ERROR,
                    (errcode_for_file_access(),
                     errmsg("could not remove file \"%s\": %m",
                            BACKUP_LABEL_FILE)));
    }

    /*
     * Read and parse the START WAL LOCATION line (this code is pretty crude,
     * but we are not expecting any variability in the file format).
     */
    if (sscanf(labelfile, "START WAL LOCATION: %X/%X (file %24s)%c",
               &hi, &lo, startxlogfilename,
               &ch) != 4 || ch != '\n')
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
    startpoint = ((uint64) hi) << 32 | lo;
    remaining = strchr(labelfile, '\n') + 1;    /* %n is not portable enough */

    /*
     * Parse the BACKUP FROM line. If we are taking an online backup from the
     * standby, we confirm that the standby has not been promoted during the
     * backup.
     */
    ptr = strstr(remaining, "BACKUP FROM:");
    if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
    if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("the standby was promoted during online backup"),
                 errhint("This means that the backup being taken is corrupt "
                         "and should not be used. "
                         "Try taking another online backup.")));

    /*
     * During recovery, we don't write an end-of-backup record. We assume that
     * pg_control was backed up last and its minimum recovery point can be
     * available as the backup end location. Since we don't have an
     * end-of-backup record, we use the pg_control value to check whether
     * we've reached the end of backup when starting recovery from this
     * backup. We have no way of checking if pg_control wasn't backed up last
     * however.
     *
     * We don't force a switch to new WAL file and wait for all the required
     * files to be archived. This is okay if we use the backup to start the
     * standby. But, if it's for an archive recovery, to ensure all the
     * required files are available, a user should wait for them to be
     * archived, or include them into the backup.
     *
     * We return the current minimum recovery point as the backup end
     * location. Note that it can be greater than the exact backup end
     * location if the minimum recovery point is updated after the backup of
     * pg_control. This is harmless for current uses.
     *
     * XXX currently a backup history file is for informational and debug
     * purposes only. It's not essential for an online backup. Furthermore,
     * even if it's created, it will not be archived during recovery because
     * an archiver is not invoked. So it doesn't seem worthwhile to write a
     * backup history file during recovery.
     */
    if (backup_started_in_recovery)
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;
        XLogRecPtr  recptr;

        /*
         * Check to see if all WAL replayed during online backup contain
         * full-page writes.
         */
        SpinLockAcquire(&xlogctl->info_lck);
        recptr = xlogctl->lastFpwDisableRecPtr;
        SpinLockRelease(&xlogctl->info_lck);

        if (startpoint <= recptr)
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
               errmsg("WAL generated with full_page_writes=off was replayed "
                      "during online backup"),
                 errhint("This means that the backup being taken on the standby "
                         "is corrupt and should not be used. "
                 "Enable full_page_writes and run CHECKPOINT on the master, "
                         "and then try an online backup again.")));


        LWLockAcquire(ControlFileLock, LW_SHARED);
        stoppoint = ControlFile->minRecoveryPoint;
        stoptli = ControlFile->minRecoveryPointTLI;
        LWLockRelease(ControlFileLock);

        if (stoptli_p)
            *stoptli_p = stoptli;
        return stoppoint;
    }

    /*
     * Write the backup-end xlog record
     */
    rdata.data = (char *) (&startpoint);
    rdata.len = sizeof(startpoint);
    rdata.buffer = InvalidBuffer;
    rdata.next = NULL;
    stoppoint = XLogInsert(RM_XLOG_ID, XLOG_BACKUP_END, &rdata);
    stoptli = ThisTimeLineID;

    /*
     * Force a switch to a new xlog segment file, so that the backup is valid
     * as soon as archiver moves out the current segment file.
     */
    RequestXLogSwitch();

    XLByteToPrevSeg(stoppoint, _logSegNo);
    XLogFileName(stopxlogfilename, ThisTimeLineID, _logSegNo);

    /* Use the log timezone here, not the session timezone */
    stamp_time = (pg_time_t) time(NULL);
    pg_strftime(strfbuf, sizeof(strfbuf),
                "%Y-%m-%d %H:%M:%S %Z",
                pg_localtime(&stamp_time, log_timezone));

    /*
     * Write the backup history file
     */
    XLByteToSeg(startpoint, _logSegNo);
    BackupHistoryFilePath(histfilepath, ThisTimeLineID, _logSegNo,
                          (uint32) (startpoint % XLogSegSize));
    fp = AllocateFile(histfilepath, "w");
    if (!fp)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create file \"%s\": %m",
                        histfilepath)));
    fprintf(fp, "START WAL LOCATION: %X/%X (file %s)\n",
            (uint32) (startpoint >> 32), (uint32) startpoint, startxlogfilename);
    fprintf(fp, "STOP WAL LOCATION: %X/%X (file %s)\n",
            (uint32) (stoppoint >> 32), (uint32) stoppoint, stopxlogfilename);
    /* transfer remaining lines from label to history file */
    fprintf(fp, "%s", remaining);
    fprintf(fp, "STOP TIME: %s\n", strfbuf);
    if (fflush(fp) || ferror(fp) || FreeFile(fp))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write file \"%s\": %m",
                        histfilepath)));

    /*
     * Clean out any no-longer-needed history files.  As a side effect, this
     * will post a .ready file for the newly created history file, notifying
     * the archiver that history file may be archived immediately.
     */
    CleanupBackupHistory();

    /*
     * If archiving is enabled, wait for all the required WAL files to be
     * archived before returning. If archiving isn't enabled, the required WAL
     * needs to be transported via streaming replication (hopefully with
     * wal_keep_segments set high enough), or some more exotic mechanism like
     * polling and copying files from pg_xlog with script. We have no
     * knowledge of those mechanisms, so it's up to the user to ensure that he
     * gets all the required WAL.
     *
     * We wait until both the last WAL file filled during backup and the
     * history file have been archived, and assume that the alphabetic sorting
     * property of the WAL files ensures any earlier WAL files are safely
     * archived as well.
     *
     * We wait forever, since archive_command is supposed to work and we
     * assume the admin wanted his backup to work completely. If you don't
     * wish to wait, you can set statement_timeout.  Also, some notices are
     * issued to clue in anyone who might be doing this interactively.
     */
    if (waitforarchive && XLogArchivingActive())
    {
        XLByteToPrevSeg(stoppoint, _logSegNo);
        XLogFileName(lastxlogfilename, ThisTimeLineID, _logSegNo);

        XLByteToSeg(startpoint, _logSegNo);
        BackupHistoryFileName(histfilename, ThisTimeLineID, _logSegNo,
                              (uint32) (startpoint % XLogSegSize));

        seconds_before_warning = 60;
        waits = 0;

        while (XLogArchiveIsBusy(lastxlogfilename) ||
               XLogArchiveIsBusy(histfilename))
        {
            CHECK_FOR_INTERRUPTS();

            if (!reported_waiting && waits > 5)
            {
                ereport(NOTICE,
                        (errmsg("pg_stop_backup cleanup done, waiting for required WAL segments to be archived")));
                reported_waiting = true;
            }

            pg_usleep(1000000L);

            if (++waits >= seconds_before_warning)
            {
                seconds_before_warning *= 2;    /* This wraps in >10 years... */
                ereport(WARNING,
                        (errmsg("pg_stop_backup still waiting for all required WAL segments to be archived (%d seconds elapsed)",
                                waits),
                         errhint("Check that your archive_command is executing properly.  "
                                 "pg_stop_backup can be canceled safely, "
                                 "but the database backup will not be usable without all the WAL segments.")));
            }
        }

        ereport(NOTICE,
                (errmsg("pg_stop_backup complete, all required WAL segments have been archived")));
    }
    else if (waitforarchive)
        ereport(NOTICE,
                (errmsg("WAL archiving is not enabled; you must ensure that all required WAL segments are copied through other means to complete the backup")));

    /*
     * We're done.  As a convenience, return the ending WAL location.
     */
    if (stoptli_p)
        *stoptli_p = stoptli;
    return stoppoint;
}


/*
 * do_pg_abort_backup: abort a running backup
 *
 * This does just the most basic steps of do_pg_stop_backup(), by taking the
 * system out of backup mode, thus making it a lot more safe to call from
 * an error handler.
 *
 * NB: This is only for aborting a non-exclusive backup that doesn't write
 * backup_label. A backup started with pg_stop_backup() needs to be finished
 * with pg_stop_backup().
 */
void
do_pg_abort_backup(void)
{
    LWLockAcquire(WALInsertLock, LW_EXCLUSIVE);
    Assert(XLogCtl->Insert.nonExclusiveBackups > 0);
    XLogCtl->Insert.nonExclusiveBackups--;

    if (!XLogCtl->Insert.exclusiveBackup &&
        XLogCtl->Insert.nonExclusiveBackups == 0)
    {
        XLogCtl->Insert.forcePageWrites = false;
    }
    LWLockRelease(WALInsertLock);
}

/*
 * Get latest redo apply position.
 *
 * Exported to allow WALReceiver to read the pointer directly.
 */
XLogRecPtr
GetXLogReplayRecPtr(TimeLineID *replayTLI)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;
    XLogRecPtr  recptr;
    TimeLineID  tli;

    SpinLockAcquire(&xlogctl->info_lck);
    recptr = xlogctl->lastReplayedEndRecPtr;
    tli = xlogctl->lastReplayedTLI;
    SpinLockRelease(&xlogctl->info_lck);

    if (replayTLI)
        *replayTLI = tli;
    return recptr;
}

/*
 * Get latest WAL insert pointer
 */
XLogRecPtr
GetXLogInsertRecPtr(void)
{
    XLogCtlInsert *Insert = &XLogCtl->Insert;
    XLogRecPtr  current_recptr;

    LWLockAcquire(WALInsertLock, LW_SHARED);
    INSERT_RECPTR(current_recptr, Insert, Insert->curridx);
    LWLockRelease(WALInsertLock);

    return current_recptr;
}

/*
 * Get latest WAL write pointer
 */
XLogRecPtr
GetXLogWriteRecPtr(void)
{
    {
        /* use volatile pointer to prevent code rearrangement */
        volatile XLogCtlData *xlogctl = XLogCtl;

        SpinLockAcquire(&xlogctl->info_lck);
        LogwrtResult = xlogctl->LogwrtResult;
        SpinLockRelease(&xlogctl->info_lck);
    }

    return LogwrtResult.Write;
}

/*
 * Returns the redo pointer of the last checkpoint or restartpoint. This is
 * the oldest point in WAL that we still need, if we have to restart recovery.
 */
void
GetOldestRestartPoint(XLogRecPtr *oldrecptr, TimeLineID *oldtli)
{
    LWLockAcquire(ControlFileLock, LW_SHARED);
    *oldrecptr = ControlFile->checkPointCopy.redo;
    *oldtli = ControlFile->checkPointCopy.ThisTimeLineID;
    LWLockRelease(ControlFileLock);
}

/*
 * read_backup_label: check to see if a backup_label file is present
 *
 * If we see a backup_label during recovery, we assume that we are recovering
 * from a backup dump file, and we therefore roll forward from the checkpoint
 * identified by the label file, NOT what pg_control says.  This avoids the
 * problem that pg_control might have been archived one or more checkpoints
 * later than the start of the dump, and so if we rely on it as the start
 * point, we will fail to restore a consistent database state.
 *
 * Returns TRUE if a backup_label was found (and fills the checkpoint
 * location and its REDO location into *checkPointLoc and RedoStartLSN,
 * respectively); returns FALSE if not. If this backup_label came from a
 * streamed backup, *backupEndRequired is set to TRUE. If this backup_label
 * was created during recovery, *backupFromStandby is set to TRUE.
 */
static bool
read_backup_label(XLogRecPtr *checkPointLoc, bool *backupEndRequired,
                  bool *backupFromStandby)
{
    char        startxlogfilename[MAXFNAMELEN];
    TimeLineID  tli;
    FILE       *lfp;
    char        ch;
    char        backuptype[20];
    char        backupfrom[20];
    uint32      hi,
                lo;

    *backupEndRequired = false;
    *backupFromStandby = false;

    /*
     * See if label file is present
     */
    lfp = AllocateFile(BACKUP_LABEL_FILE, "r");
    if (!lfp)
    {
        if (errno != ENOENT)
            ereport(FATAL,
                    (errcode_for_file_access(),
                     errmsg("could not read file \"%s\": %m",
                            BACKUP_LABEL_FILE)));
        return false;           /* it's not there, all is fine */
    }

    /*
     * Read and parse the START WAL LOCATION and CHECKPOINT lines (this code
     * is pretty crude, but we are not expecting any variability in the file
     * format).
     */
    if (fscanf(lfp, "START WAL LOCATION: %X/%X (file %08X%16s)%c",
               &hi, &lo, &tli, startxlogfilename, &ch) != 5 || ch != '\n')
        ereport(FATAL,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
    RedoStartLSN = ((uint64) hi) << 32 | lo;
    if (fscanf(lfp, "CHECKPOINT LOCATION: %X/%X%c",
               &hi, &lo, &ch) != 3 || ch != '\n')
        ereport(FATAL,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("invalid data in file \"%s\"", BACKUP_LABEL_FILE)));
    *checkPointLoc = ((uint64) hi) << 32 | lo;

    /*
     * BACKUP METHOD and BACKUP FROM lines are new in 9.2. We can't restore
     * from an older backup anyway, but since the information on it is not
     * strictly required, don't error out if it's missing for some reason.
     */
    if (fscanf(lfp, "BACKUP METHOD: %19s\n", backuptype) == 1)
    {
        if (strcmp(backuptype, "streamed") == 0)
            *backupEndRequired = true;
    }

    if (fscanf(lfp, "BACKUP FROM: %19s\n", backupfrom) == 1)
    {
        if (strcmp(backupfrom, "standby") == 0)
            *backupFromStandby = true;
    }

    if (ferror(lfp) || FreeFile(lfp))
        ereport(FATAL,
                (errcode_for_file_access(),
                 errmsg("could not read file \"%s\": %m",
                        BACKUP_LABEL_FILE)));

    return true;
}

/*
 * Error context callback for errors occurring during rm_redo().
 */
static void
rm_redo_error_callback(void *arg)
{
    XLogRecord *record = (XLogRecord *) arg;
    StringInfoData buf;

    initStringInfo(&buf);
    RmgrTable[record->xl_rmid].rm_desc(&buf,
                                       record->xl_info,
                                       XLogRecGetData(record));

    /* don't bother emitting empty description */
    if (buf.len > 0)
        errcontext("xlog redo %s", buf.data);

    pfree(buf.data);
}

/*
 * BackupInProgress: check if online backup mode is active
 *
 * This is done by checking for existence of the "backup_label" file.
 */
bool
BackupInProgress(void)
{
    struct stat stat_buf;

    return (stat(BACKUP_LABEL_FILE, &stat_buf) == 0);
}

/*
 * CancelBackup: rename the "backup_label" file to cancel backup mode
 *
 * If the "backup_label" file exists, it will be renamed to "backup_label.old".
 * Note that this will render an online backup in progress useless.
 * To correctly finish an online backup, pg_stop_backup must be called.
 */
void
CancelBackup(void)
{
    struct stat stat_buf;

    /* if the file is not there, return */
    if (stat(BACKUP_LABEL_FILE, &stat_buf) < 0)
        return;

    /* remove leftover file from previously canceled backup if it exists */
    unlink(BACKUP_LABEL_OLD);

    if (rename(BACKUP_LABEL_FILE, BACKUP_LABEL_OLD) == 0)
    {
        ereport(LOG,
                (errmsg("online backup mode canceled"),
                 errdetail("\"%s\" was renamed to \"%s\".",
                           BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
    }
    else
    {
        ereport(WARNING,
                (errcode_for_file_access(),
                 errmsg("online backup mode was not canceled"),
                 errdetail("Could not rename \"%s\" to \"%s\": %m.",
                           BACKUP_LABEL_FILE, BACKUP_LABEL_OLD)));
    }
}

/*
 * Read the XLOG page containing RecPtr into readBuf (if not read already).
 * Returns number of bytes read, if the page is read successfully, or -1
 * in case of errors.  When errors occur, they are ereport'ed, but only
 * if they have not been previously reported.
 *
 * This is responsible for restoring files from archive as needed, as well
 * as for waiting for the requested WAL record to arrive in standby mode.
 *
 * 'emode' specifies the log level used for reporting "file not found" or
 * "end of WAL" situations in archive recovery, or in standby mode when a
 * trigger file is found. If set to WARNING or below, XLogPageRead() returns
 * false in those situations, on higher log levels the ereport() won't
 * return.
 *
 * In standby mode, if after a successful return of XLogPageRead() the
 * caller finds the record it's interested in to be broken, it should
 * ereport the error with the level determined by
 * emode_for_corrupt_record(), and then set lastSourceFailed
 * and call XLogPageRead() again with the same arguments. This lets
 * XLogPageRead() to try fetching the record from another source, or to
 * sleep and retry.
 */
static int
XLogPageRead(XLogReaderState *xlogreader, XLogRecPtr targetPagePtr, int reqLen,
             XLogRecPtr targetRecPtr, char *readBuf, TimeLineID *readTLI)
{
    XLogPageReadPrivate *private =
        (XLogPageReadPrivate *) xlogreader->private_data;
    int         emode = private->emode;
    uint32      targetPageOff;
    XLogSegNo   targetSegNo PG_USED_FOR_ASSERTS_ONLY;

    XLByteToSeg(targetPagePtr, targetSegNo);
    targetPageOff = targetPagePtr % XLogSegSize;

    /*
     * See if we need to switch to a new segment because the requested record
     * is not in the currently open one.
     */
    if (readFile >= 0 && !XLByteInSeg(targetPagePtr, readSegNo))
    {
        /*
         * Request a restartpoint if we've replayed too much xlog since the
         * last one.
         */
        if (StandbyModeRequested && bgwriterLaunched)
        {
            if (XLogCheckpointNeeded(readSegNo))
            {
                (void) GetRedoRecPtr();
                if (XLogCheckpointNeeded(readSegNo))
                    RequestCheckpoint(CHECKPOINT_CAUSE_XLOG);
            }
        }

        close(readFile);
        readFile = -1;
        readSource = 0;
    }

    XLByteToSeg(targetPagePtr, readSegNo);

retry:
    /* See if we need to retrieve more data */
    if (readFile < 0 ||
        (readSource == XLOG_FROM_STREAM &&
         receivedUpto < targetPagePtr + reqLen))
    {
        if (!WaitForWALToBecomeAvailable(targetPagePtr + reqLen,
                                         private->randAccess,
                                         private->fetching_ckpt,
                                         targetRecPtr))
        {
            if (readFile >= 0)
                close(readFile);
            readFile = -1;
            readLen = 0;
            readSource = 0;

            return -1;
        }
    }

    /*
     * At this point, we have the right segment open and if we're streaming we
     * know the requested record is in it.
     */
    Assert(readFile != -1);

    /*
     * If the current segment is being streamed from master, calculate how
     * much of the current page we have received already. We know the
     * requested record has been received, but this is for the benefit of
     * future calls, to allow quick exit at the top of this function.
     */
    if (readSource == XLOG_FROM_STREAM)
    {
        if (((targetPagePtr) / XLOG_BLCKSZ) != (receivedUpto / XLOG_BLCKSZ))
            readLen = XLOG_BLCKSZ;
        else
            readLen = receivedUpto % XLogSegSize - targetPageOff;
    }
    else
        readLen = XLOG_BLCKSZ;

    /* Read the requested page */
    readOff = targetPageOff;
    if (lseek(readFile, (off_t) readOff, SEEK_SET) < 0)
    {
        char fname[MAXFNAMELEN];

        XLogFileName(fname, curFileTLI, readSegNo);
        ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
                (errcode_for_file_access(),
         errmsg("could not seek in log segment %s to offset %u: %m",
                        fname, readOff)));
        goto next_record_is_invalid;
    }

    if (read(readFile, readBuf, XLOG_BLCKSZ) != XLOG_BLCKSZ)
    {
        char fname[MAXFNAMELEN];

        XLogFileName(fname, curFileTLI, readSegNo);
        ereport(emode_for_corrupt_record(emode, targetPagePtr + reqLen),
                (errcode_for_file_access(),
         errmsg("could not read from log segment %s, offset %u: %m",
                        fname, readOff)));
        goto next_record_is_invalid;
    }

    Assert(targetSegNo == readSegNo);
    Assert(targetPageOff == readOff);
    Assert(reqLen <= readLen);

    *readTLI = curFileTLI;
    return readLen;

next_record_is_invalid:
    lastSourceFailed = true;

    if (readFile >= 0)
        close(readFile);
    readFile = -1;
    readLen = 0;
    readSource = 0;

    /* In standby-mode, keep trying */
    if (StandbyMode)
        goto retry;
    else
        return -1;
}

/*
 * Open the WAL segment containing WAL position 'RecPtr'.
 *
 * The segment can be fetched via restore_command, or via walreceiver having
 * streamed the record, or it can already be present in pg_xlog. Checking
 * pg_xlog is mainly for crash recovery, but it will be polled in standby mode
 * too, in case someone copies a new segment directly to pg_xlog. That is not
 * documented or recommended, though.
 *
 * If 'fetching_ckpt' is true, we're fetching a checkpoint record, and should
 * prepare to read WAL starting from RedoStartLSN after this.
 *
 * 'RecPtr' might not point to the beginning of the record we're interested
 * in, it might also point to the page or segment header. In that case,
 * 'tliRecPtr' is the position of the WAL record we're interested in. It is
 * used to decide which timeline to stream the requested WAL from.
 *
 * If the the record is not immediately available, the function returns false
 * if we're not in standby mode. In standby mode, waits for it to become
 * available.
 *
 * When the requested record becomes available, the function opens the file
 * containing it (if not open already), and returns true. When end of standby
 * mode is triggered by the user, and there is no more WAL available, returns
 * false.
 */
static bool
WaitForWALToBecomeAvailable(XLogRecPtr RecPtr, bool randAccess,
                            bool fetching_ckpt, XLogRecPtr tliRecPtr)
{
    static pg_time_t last_fail_time = 0;
    pg_time_t now;

    /*-------
     * Standby mode is implemented by a state machine:
     *
     * 1. Read from archive (XLOG_FROM_ARCHIVE)
     * 2. Read from pg_xlog (XLOG_FROM_PG_XLOG)
     * 3. Check trigger file
     * 4. Read from primary server via walreceiver (XLOG_FROM_STREAM)
     * 5. Rescan timelines
     * 6. Sleep 5 seconds, and loop back to 1.
     *
     * Failure to read from the current source advances the state machine to
     * the next state. In addition, successfully reading a file from pg_xlog
     * moves the state machine from state 2 back to state 1 (we always prefer
     * files in the archive over files in pg_xlog).
     *
     * 'currentSource' indicates the current state. There are no currentSource
     * values for "check trigger", "rescan timelines", and "sleep" states,
     * those actions are taken when reading from the previous source fails, as
     * part of advancing to the next state.
     *-------
     */
    if (!InArchiveRecovery)
        currentSource = XLOG_FROM_PG_XLOG;
    else if (currentSource == 0)
        currentSource = XLOG_FROM_ARCHIVE;

    for (;;)
    {
        int     oldSource = currentSource;

        /*
         * First check if we failed to read from the current source, and
         * advance the state machine if so. The failure to read might've
         * happened outside this function, e.g when a CRC check fails on a
         * record, or within this loop.
         */
        if (lastSourceFailed)
        {
            switch (currentSource)
            {
                case XLOG_FROM_ARCHIVE:
                    currentSource = XLOG_FROM_PG_XLOG;
                    break;

                case XLOG_FROM_PG_XLOG:
                    /*
                     * Check to see if the trigger file exists. Note that we do
                     * this only after failure, so when you create the trigger
                     * file, we still finish replaying as much as we can from
                     * archive and pg_xlog before failover.
                     */
                    if (StandbyMode && CheckForStandbyTrigger())
                    {
                        ShutdownWalRcv();
                        return false;
                    }

                    /*
                     * Not in standby mode, and we've now tried the archive and
                     * pg_xlog.
                     */
                    if (!StandbyMode)
                        return false;

                    /*
                     * If primary_conninfo is set, launch walreceiver to try to
                     * stream the missing WAL.
                     *
                     * If fetching_ckpt is TRUE, RecPtr points to the initial
                     * checkpoint location. In that case, we use RedoStartLSN
                     * as the streaming start position instead of RecPtr, so
                     * that when we later jump backwards to start redo at
                     * RedoStartLSN, we will have the logs streamed already.
                     */
                    if (PrimaryConnInfo)
                    {
                        XLogRecPtr ptr;
                        TimeLineID tli;

                        if (fetching_ckpt)
                        {
                            ptr = RedoStartLSN;
                            tli = ControlFile->checkPointCopy.ThisTimeLineID;
                        }
                        else
                        {
                            ptr = RecPtr;
                            tli = tliOfPointInHistory(tliRecPtr, expectedTLEs);

                            if (curFileTLI > 0 && tli < curFileTLI)
                                elog(ERROR, "according to history file, WAL location %X/%X belongs to timeline %u, but previous recovered WAL file came from timeline %u",
                                     (uint32) (ptr >> 32), (uint32) ptr,
                                     tli, curFileTLI);
                        }
                        curFileTLI = tli;
                        RequestXLogStreaming(curFileTLI, ptr, PrimaryConnInfo);
                    }
                    /*
                     * Move to XLOG_FROM_STREAM state in either case. We'll get
                     * immediate failure if we didn't launch walreceiver, and
                     * move on to the next state.
                     */
                    currentSource = XLOG_FROM_STREAM;
                    break;

                case XLOG_FROM_STREAM:
                    /*
                     * Failure while streaming. Most likely, we got here because
                     * streaming replication was terminated, or promotion was
                     * triggered. But we also get here if we find an invalid
                     * record in the WAL streamed from master, in which case
                     * something is seriously wrong. There's little chance that
                     * the problem will just go away, but PANIC is not good for
                     * availability either, especially in hot standby mode. So,
                     * we treat that the same as disconnection, and retry from
                     * archive/pg_xlog again. The WAL in the archive should be
                     * identical to what was streamed, so it's unlikely that it
                     * helps, but one can hope...
                     */
                    /*
                     * Before we leave XLOG_FROM_STREAM state, make sure that
                     * walreceiver is not active, so that it won't overwrite
                     * WAL that we restore from archive.
                     */
                    if (WalRcvStreaming())
                        ShutdownWalRcv();

                    /*
                     * Before we sleep, re-scan for possible new timelines if
                     * we were requested to recover to the latest timeline.
                     */
                    if (recoveryTargetIsLatest)
                    {
                        if (rescanLatestTimeLine())
                        {
                            currentSource = XLOG_FROM_ARCHIVE;
                            break;
                        }
                    }

                    /*
                     * XLOG_FROM_STREAM is the last state in our state machine,
                     * so we've exhausted all the options for obtaining the
                     * requested WAL. We're going to loop back and retry from
                     * the archive, but if it hasn't been long since last
                     * attempt, sleep 5 seconds to avoid busy-waiting.
                     */
                    now = (pg_time_t) time(NULL);
                    if ((now - last_fail_time) < 5)
                    {
                        pg_usleep(1000000L * (5 - (now - last_fail_time)));
                        now = (pg_time_t) time(NULL);
                    }
                    last_fail_time = now;
                    currentSource = XLOG_FROM_ARCHIVE;
                    break;

                default:
                    elog(ERROR, "unexpected WAL source %d", currentSource);
            }
        }
        else if (currentSource == XLOG_FROM_PG_XLOG)
        {
            /*
             * We just successfully read a file in pg_xlog. We prefer files
             * in the archive over ones in pg_xlog, so try the next file
             * again from the archive first.
             */
            if (InArchiveRecovery)
                currentSource = XLOG_FROM_ARCHIVE;
        }

        if (currentSource != oldSource)
            elog(DEBUG2, "switched WAL source from %s to %s after %s",
                 xlogSourceNames[oldSource], xlogSourceNames[currentSource],
                 lastSourceFailed ? "failure" : "success");

        /*
         * We've now handled possible failure. Try to read from the chosen
         * source.
         */
        lastSourceFailed = false;

        switch (currentSource)
        {
            case XLOG_FROM_ARCHIVE:
            case XLOG_FROM_PG_XLOG:
                /* Close any old file we might have open. */
                if (readFile >= 0)
                {
                    close(readFile);
                    readFile = -1;
                }
                /* Reset curFileTLI if random fetch. */
                if (randAccess)
                    curFileTLI = 0;

                /*
                 * Try to restore the file from archive, or read an existing
                 * file from pg_xlog.
                 */
                readFile = XLogFileReadAnyTLI(readSegNo, DEBUG2, currentSource);
                if (readFile >= 0)
                    return true;    /* success! */

                /*
                 * Nope, not found in archive or pg_xlog.
                 */
                lastSourceFailed = true;
                break;

            case XLOG_FROM_STREAM:
            {
                bool        havedata;

                /*
                 * Check if WAL receiver is still active.
                 */
                if (!WalRcvStreaming())
                {
                    lastSourceFailed = true;
                    break;
                }

                /*
                 * Walreceiver is active, so see if new data has arrived.
                 *
                 * We only advance XLogReceiptTime when we obtain fresh WAL
                 * from walreceiver and observe that we had already processed
                 * everything before the most recent "chunk" that it flushed to
                 * disk.  In steady state where we are keeping up with the
                 * incoming data, XLogReceiptTime will be updated on each cycle.
                 * When we are behind, XLogReceiptTime will not advance, so the
                 * grace time allotted to conflicting queries will decrease.
                 */
                if (RecPtr < receivedUpto)
                    havedata = true;
                else
                {
                    XLogRecPtr  latestChunkStart;

                    receivedUpto = GetWalRcvWriteRecPtr(&latestChunkStart, &receiveTLI);
                    if (RecPtr < receivedUpto && receiveTLI == curFileTLI)
                    {
                        havedata = true;
                        if (latestChunkStart <= RecPtr)
                        {
                            XLogReceiptTime = GetCurrentTimestamp();
                            SetCurrentChunkStartTime(XLogReceiptTime);
                        }
                    }
                    else
                        havedata = false;
                }
                if (havedata)
                {
                    /*
                     * Great, streamed far enough.  Open the file if it's not
                     * open already.  Also read the timeline history file if
                     * we haven't initialized timeline history yet; it should
                     * be streamed over and present in pg_xlog by now.  Use
                     * XLOG_FROM_STREAM so that source info is set correctly
                     * and XLogReceiptTime isn't changed.
                     */
                    if (readFile < 0)
                    {
                        if (!expectedTLEs)
                            expectedTLEs = readTimeLineHistory(receiveTLI);
                        readFile = XLogFileRead(readSegNo, PANIC,
                                                receiveTLI,
                                                XLOG_FROM_STREAM, false);
                        Assert(readFile >= 0);
                    }
                    else
                    {
                        /* just make sure source info is correct... */
                        readSource = XLOG_FROM_STREAM;
                        XLogReceiptSource = XLOG_FROM_STREAM;
                        return true;
                    }
                    break;
                }

                /*
                 * Data not here yet. Check for trigger, then wait for
                 * walreceiver to wake us up when new WAL arrives.
                 */
                if (CheckForStandbyTrigger())
                {
                    /*
                     * Note that we don't "return false" immediately here.
                     * After being triggered, we still want to replay all the
                     * WAL that was already streamed. It's in pg_xlog now, so
                     * we just treat this as a failure, and the state machine
                     * will move on to replay the streamed WAL from pg_xlog,
                     * and then recheck the trigger and exit replay.
                     */
                    lastSourceFailed = true;
                    break;
                }

                /*
                 * Wait for more WAL to arrive. Time out after 5 seconds, like
                 * when polling the archive, to react to a trigger file
                 * promptly.
                 */
                WaitLatch(&XLogCtl->recoveryWakeupLatch,
                          WL_LATCH_SET | WL_TIMEOUT,
                          5000L);
                ResetLatch(&XLogCtl->recoveryWakeupLatch);
                break;
            }

            default:
                elog(ERROR, "unexpected WAL source %d", currentSource);
        }

        /*
         * This possibly-long loop needs to handle interrupts of startup
         * process.
         */
        HandleStartupProcInterrupts();
    } while (StandbyMode);

    return false;
}

/*
 * Determine what log level should be used to report a corrupt WAL record
 * in the current WAL page, previously read by XLogPageRead().
 *
 * 'emode' is the error mode that would be used to report a file-not-found
 * or legitimate end-of-WAL situation.   Generally, we use it as-is, but if
 * we're retrying the exact same record that we've tried previously, only
 * complain the first time to keep the noise down.  However, we only do when
 * reading from pg_xlog, because we don't expect any invalid records in archive
 * or in records streamed from master. Files in the archive should be complete,
 * and we should never hit the end of WAL because we stop and wait for more WAL
 * to arrive before replaying it.
 *
 * NOTE: This function remembers the RecPtr value it was last called with,
 * to suppress repeated messages about the same record. Only call this when
 * you are about to ereport(), or you might cause a later message to be
 * erroneously suppressed.
 */
static int
emode_for_corrupt_record(int emode, XLogRecPtr RecPtr)
{
    static XLogRecPtr lastComplaint = 0;

    if (readSource == XLOG_FROM_PG_XLOG && emode == LOG)
    {
        if (RecPtr == lastComplaint)
            emode = DEBUG1;
        else
            lastComplaint = RecPtr;
    }
    return emode;
}

/*
 * Check to see whether the user-specified trigger file exists and whether a
 * promote request has arrived.  If either condition holds, return true.
 */
static bool
CheckForStandbyTrigger(void)
{
    struct stat stat_buf;
    static bool triggered = false;

    if (triggered)
        return true;

    if (IsPromoteTriggered())
    {
        /*
         * In 9.1 and 9.2 the postmaster unlinked the promote file
         * inside the signal handler. We now leave the file in place
         * and let the Startup process do the unlink. This allows
         * Startup to know whether we're doing fast or normal
         * promotion. Fast promotion takes precedence.
         */
        if (stat(FAST_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
        {
            unlink(FAST_PROMOTE_SIGNAL_FILE);
            unlink(PROMOTE_SIGNAL_FILE);
            fast_promote = true;
        }
        else if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
        {
            unlink(PROMOTE_SIGNAL_FILE);
            fast_promote = false;
        }

        ereport(LOG, (errmsg("received promote request")));

        ResetPromoteTriggered();
        triggered = true;
        return true;
    }

    if (TriggerFile == NULL)
        return false;

    if (stat(TriggerFile, &stat_buf) == 0)
    {
        ereport(LOG,
                (errmsg("trigger file found: %s", TriggerFile)));
        unlink(TriggerFile);
        triggered = true;
        fast_promote = true;
        return true;
    }
    return false;
}

/*
 * Check to see if a promote request has arrived. Should be
 * called by postmaster after receiving SIGUSR1.
 */
bool
CheckPromoteSignal(void)
{
    struct stat stat_buf;

    if (stat(PROMOTE_SIGNAL_FILE, &stat_buf) == 0 ||
        stat(FAST_PROMOTE_SIGNAL_FILE, &stat_buf) == 0)
        return true;

    return false;
}

/*
 * Wake up startup process to replay newly arrived WAL, or to notice that
 * failover has been requested.
 */
void
WakeupRecovery(void)
{
    SetLatch(&XLogCtl->recoveryWakeupLatch);
}

/*
 * Update the WalWriterSleeping flag.
 */
void
SetWalWriterSleeping(bool sleeping)
{
    /* use volatile pointer to prevent code rearrangement */
    volatile XLogCtlData *xlogctl = XLogCtl;

    SpinLockAcquire(&xlogctl->info_lck);
    xlogctl->WalWriterSleeping = sleeping;
    SpinLockRelease(&xlogctl->info_lck);
}