standby.c

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/*-------------------------------------------------------------------------
 *
 * standby.c
 *    Misc functions used in Hot Standby mode.
 *
 *  All functions for handling RM_STANDBY_ID, which relate to
 *  AccessExclusiveLocks and starting snapshots for Hot Standby mode.
 *  Plus conflict recovery processing.
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/storage/ipc/standby.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/sinvaladt.h"
#include "storage/standby.h"
#include "utils/ps_status.h"
#include "utils/timeout.h"
#include "utils/timestamp.h"

/* User-settable GUC parameters */
int         vacuum_defer_cleanup_age;
int         max_standby_archive_delay = 30 * 1000;
int         max_standby_streaming_delay = 30 * 1000;

static List *RecoveryLockList;

static void ResolveRecoveryConflictWithVirtualXIDs(VirtualTransactionId *waitlist,
                                       ProcSignalReason reason);
static void ResolveRecoveryConflictWithLock(Oid dbOid, Oid relOid);
static void SendRecoveryConflictWithBufferPin(ProcSignalReason reason);
static void LogCurrentRunningXacts(RunningTransactions CurrRunningXacts);
static void LogAccessExclusiveLocks(int nlocks, xl_standby_lock *locks);


/*
 * InitRecoveryTransactionEnvironment
 *      Initialize tracking of in-progress transactions in master
 *
 * We need to issue shared invalidations and hold locks. Holding locks
 * means others may want to wait on us, so we need to make a lock table
 * vxact entry like a real transaction. We could create and delete
 * lock table entries for each transaction but its simpler just to create
 * one permanent entry and leave it there all the time. Locks are then
 * acquired and released as needed. Yes, this means you can see the
 * Startup process in pg_locks once we have run this.
 */
void
InitRecoveryTransactionEnvironment(void)
{
    VirtualTransactionId vxid;

    /*
     * Initialize shared invalidation management for Startup process, being
     * careful to register ourselves as a sendOnly process so we don't need to
     * read messages, nor will we get signalled when the queue starts filling
     * up.
     */
    SharedInvalBackendInit(true);

    /*
     * Lock a virtual transaction id for Startup process.
     *
     * We need to do GetNextLocalTransactionId() because
     * SharedInvalBackendInit() leaves localTransactionid invalid and the lock
     * manager doesn't like that at all.
     *
     * Note that we don't need to run XactLockTableInsert() because nobody
     * needs to wait on xids. That sounds a little strange, but table locks
     * are held by vxids and row level locks are held by xids. All queries
     * hold AccessShareLocks so never block while we write or lock new rows.
     */
    vxid.backendId = MyBackendId;
    vxid.localTransactionId = GetNextLocalTransactionId();
    VirtualXactLockTableInsert(vxid);

    standbyState = STANDBY_INITIALIZED;
}

/*
 * ShutdownRecoveryTransactionEnvironment
 *      Shut down transaction tracking
 *
 * Prepare to switch from hot standby mode to normal operation. Shut down
 * recovery-time transaction tracking.
 */
void
ShutdownRecoveryTransactionEnvironment(void)
{
    /* Mark all tracked in-progress transactions as finished. */
    ExpireAllKnownAssignedTransactionIds();

    /* Release all locks the tracked transactions were holding */
    StandbyReleaseAllLocks();

    /* Cleanup our VirtualTransaction */
    VirtualXactLockTableCleanup();
}


/*
 * -----------------------------------------------------
 *      Standby wait timers and backend cancel logic
 * -----------------------------------------------------
 */

/*
 * Determine the cutoff time at which we want to start canceling conflicting
 * transactions.  Returns zero (a time safely in the past) if we are willing
 * to wait forever.
 */
static TimestampTz
GetStandbyLimitTime(void)
{
    TimestampTz rtime;
    bool        fromStream;

    /*
     * The cutoff time is the last WAL data receipt time plus the appropriate
     * delay variable.  Delay of -1 means wait forever.
     */
    GetXLogReceiptTime(&rtime, &fromStream);
    if (fromStream)
    {
        if (max_standby_streaming_delay < 0)
            return 0;           /* wait forever */
        return TimestampTzPlusMilliseconds(rtime, max_standby_streaming_delay);
    }
    else
    {
        if (max_standby_archive_delay < 0)
            return 0;           /* wait forever */
        return TimestampTzPlusMilliseconds(rtime, max_standby_archive_delay);
    }
}

#define STANDBY_INITIAL_WAIT_US  1000
static int  standbyWait_us = STANDBY_INITIAL_WAIT_US;

/*
 * Standby wait logic for ResolveRecoveryConflictWithVirtualXIDs.
 * We wait here for a while then return. If we decide we can't wait any
 * more then we return true, if we can wait some more return false.
 */
static bool
WaitExceedsMaxStandbyDelay(void)
{
    TimestampTz ltime;

    /* Are we past the limit time? */
    ltime = GetStandbyLimitTime();
    if (ltime && GetCurrentTimestamp() >= ltime)
        return true;

    /*
     * Sleep a bit (this is essential to avoid busy-waiting).
     */
    pg_usleep(standbyWait_us);

    /*
     * Progressively increase the sleep times, but not to more than 1s, since
     * pg_usleep isn't interruptable on some platforms.
     */
    standbyWait_us *= 2;
    if (standbyWait_us > 1000000)
        standbyWait_us = 1000000;

    return false;
}

/*
 * This is the main executioner for any query backend that conflicts with
 * recovery processing. Judgement has already been passed on it within
 * a specific rmgr. Here we just issue the orders to the procs. The procs
 * then throw the required error as instructed.
 */
static void
ResolveRecoveryConflictWithVirtualXIDs(VirtualTransactionId *waitlist,
                                       ProcSignalReason reason)
{
    TimestampTz waitStart;
    char       *new_status;

    /* Fast exit, to avoid a kernel call if there's no work to be done. */
    if (!VirtualTransactionIdIsValid(*waitlist))
        return;

    waitStart = GetCurrentTimestamp();
    new_status = NULL;          /* we haven't changed the ps display */

    while (VirtualTransactionIdIsValid(*waitlist))
    {
        /* reset standbyWait_us for each xact we wait for */
        standbyWait_us = STANDBY_INITIAL_WAIT_US;

        /* wait until the virtual xid is gone */
        while (!VirtualXactLock(*waitlist, false))
        {
            /*
             * Report via ps if we have been waiting for more than 500 msec
             * (should that be configurable?)
             */
            if (update_process_title && new_status == NULL &&
                TimestampDifferenceExceeds(waitStart, GetCurrentTimestamp(),
                                           500))
            {
                const char *old_status;
                int         len;

                old_status = get_ps_display(&len);
                new_status = (char *) palloc(len + 8 + 1);
                memcpy(new_status, old_status, len);
                strcpy(new_status + len, " waiting");
                set_ps_display(new_status, false);
                new_status[len] = '\0'; /* truncate off " waiting" */
            }

            /* Is it time to kill it? */
            if (WaitExceedsMaxStandbyDelay())
            {
                pid_t       pid;

                /*
                 * Now find out who to throw out of the balloon.
                 */
                Assert(VirtualTransactionIdIsValid(*waitlist));
                pid = CancelVirtualTransaction(*waitlist, reason);

                /*
                 * Wait a little bit for it to die so that we avoid flooding
                 * an unresponsive backend when system is heavily loaded.
                 */
                if (pid != 0)
                    pg_usleep(5000L);
            }
        }

        /* The virtual transaction is gone now, wait for the next one */
        waitlist++;
    }

    /* Reset ps display if we changed it */
    if (new_status)
    {
        set_ps_display(new_status, false);
        pfree(new_status);
    }
}

void
ResolveRecoveryConflictWithSnapshot(TransactionId latestRemovedXid, RelFileNode node)
{
    VirtualTransactionId *backends;

    /*
     * If we get passed InvalidTransactionId then we are a little surprised,
     * but it is theoretically possible in normal running. It also happens
     * when replaying already applied WAL records after a standby crash or
     * restart. If latestRemovedXid is invalid then there is no conflict. That
     * rule applies across all record types that suffer from this conflict.
     */
    if (!TransactionIdIsValid(latestRemovedXid))
        return;

    backends = GetConflictingVirtualXIDs(latestRemovedXid,
                                         node.dbNode);

    ResolveRecoveryConflictWithVirtualXIDs(backends,
                                         PROCSIG_RECOVERY_CONFLICT_SNAPSHOT);
}

void
ResolveRecoveryConflictWithTablespace(Oid tsid)
{
    VirtualTransactionId *temp_file_users;

    /*
     * Standby users may be currently using this tablespace for their
     * temporary files. We only care about current users because
     * temp_tablespace parameter will just ignore tablespaces that no longer
     * exist.
     *
     * Ask everybody to cancel their queries immediately so we can ensure no
     * temp files remain and we can remove the tablespace. Nuke the entire
     * site from orbit, it's the only way to be sure.
     *
     * XXX: We could work out the pids of active backends using this
     * tablespace by examining the temp filenames in the directory. We would
     * then convert the pids into VirtualXIDs before attempting to cancel
     * them.
     *
     * We don't wait for commit because drop tablespace is non-transactional.
     */
    temp_file_users = GetConflictingVirtualXIDs(InvalidTransactionId,
                                                InvalidOid);
    ResolveRecoveryConflictWithVirtualXIDs(temp_file_users,
                                       PROCSIG_RECOVERY_CONFLICT_TABLESPACE);
}

void
ResolveRecoveryConflictWithDatabase(Oid dbid)
{
    /*
     * We don't do ResolveRecoveryConflictWithVirtualXIDs() here since that
     * only waits for transactions and completely idle sessions would block
     * us. This is rare enough that we do this as simply as possible: no wait,
     * just force them off immediately.
     *
     * No locking is required here because we already acquired
     * AccessExclusiveLock. Anybody trying to connect while we do this will
     * block during InitPostgres() and then disconnect when they see the
     * database has been removed.
     */
    while (CountDBBackends(dbid) > 0)
    {
        CancelDBBackends(dbid, PROCSIG_RECOVERY_CONFLICT_DATABASE, true);

        /*
         * Wait awhile for them to die so that we avoid flooding an
         * unresponsive backend when system is heavily loaded.
         */
        pg_usleep(10000);
    }
}

static void
ResolveRecoveryConflictWithLock(Oid dbOid, Oid relOid)
{
    VirtualTransactionId *backends;
    bool        lock_acquired = false;
    int         num_attempts = 0;
    LOCKTAG     locktag;

    SET_LOCKTAG_RELATION(locktag, dbOid, relOid);

    /*
     * If blowing away everybody with conflicting locks doesn't work, after
     * the first two attempts then we just start blowing everybody away until
     * it does work. We do this because its likely that we either have too
     * many locks and we just can't get one at all, or that there are many
     * people crowding for the same table. Recovery must win; the end
     * justifies the means.
     */
    while (!lock_acquired)
    {
        if (++num_attempts < 3)
            backends = GetLockConflicts(&locktag, AccessExclusiveLock);
        else
            backends = GetConflictingVirtualXIDs(InvalidTransactionId,
                                                 InvalidOid);

        ResolveRecoveryConflictWithVirtualXIDs(backends,
                                             PROCSIG_RECOVERY_CONFLICT_LOCK);

        if (LockAcquireExtended(&locktag, AccessExclusiveLock, true, true, false)
            != LOCKACQUIRE_NOT_AVAIL)
            lock_acquired = true;
    }
}

/*
 * ResolveRecoveryConflictWithBufferPin is called from LockBufferForCleanup()
 * to resolve conflicts with other backends holding buffer pins.
 *
 * The ProcWaitForSignal() sleep normally done in LockBufferForCleanup()
 * (when not InHotStandby) is performed here, for code clarity.
 *
 * We either resolve conflicts immediately or set a timeout to wake us at
 * the limit of our patience.
 *
 * Resolve conflicts by sending a PROCSIG signal to all backends to check if
 * they hold one of the buffer pins that is blocking Startup process. If so,
 * those backends will take an appropriate error action, ERROR or FATAL.
 *
 * We also must check for deadlocks.  Deadlocks occur because if queries
 * wait on a lock, that must be behind an AccessExclusiveLock, which can only
 * be cleared if the Startup process replays a transaction completion record.
 * If Startup process is also waiting then that is a deadlock. The deadlock
 * can occur if the query is waiting and then the Startup sleeps, or if
 * Startup is sleeping and the query waits on a lock. We protect against
 * only the former sequence here, the latter sequence is checked prior to
 * the query sleeping, in CheckRecoveryConflictDeadlock().
 *
 * Deadlocks are extremely rare, and relatively expensive to check for,
 * so we don't do a deadlock check right away ... only if we have had to wait
 * at least deadlock_timeout.
 */
void
ResolveRecoveryConflictWithBufferPin(void)
{
    TimestampTz ltime;

    Assert(InHotStandby);

    ltime = GetStandbyLimitTime();

    if (ltime == 0)
    {
        /*
         * We're willing to wait forever for conflicts, so set timeout for
         * deadlock check only
         */
        enable_timeout_after(STANDBY_DEADLOCK_TIMEOUT, DeadlockTimeout);
    }
    else if (GetCurrentTimestamp() >= ltime)
    {
        /*
         * We're already behind, so clear a path as quickly as possible.
         */
        SendRecoveryConflictWithBufferPin(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
    }
    else
    {
        /*
         * Wake up at ltime, and check for deadlocks as well if we will be
         * waiting longer than deadlock_timeout
         */
        EnableTimeoutParams timeouts[2];

        timeouts[0].id = STANDBY_TIMEOUT;
        timeouts[0].type = TMPARAM_AT;
        timeouts[0].fin_time = ltime;
        timeouts[1].id = STANDBY_DEADLOCK_TIMEOUT;
        timeouts[1].type = TMPARAM_AFTER;
        timeouts[1].delay_ms = DeadlockTimeout;
        enable_timeouts(timeouts, 2);
    }

    /* Wait to be signaled by UnpinBuffer() */
    ProcWaitForSignal();

    /*
     * Clear any timeout requests established above.  We assume here that
     * the Startup process doesn't have any other timeouts than what this
     * function uses.  If that stops being true, we could cancel the
     * timeouts individually, but that'd be slower.
     */
    disable_all_timeouts(false);
}

static void
SendRecoveryConflictWithBufferPin(ProcSignalReason reason)
{
    Assert(reason == PROCSIG_RECOVERY_CONFLICT_BUFFERPIN ||
           reason == PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK);

    /*
     * We send signal to all backends to ask them if they are holding the
     * buffer pin which is delaying the Startup process. We must not set the
     * conflict flag yet, since most backends will be innocent. Let the
     * SIGUSR1 handling in each backend decide their own fate.
     */
    CancelDBBackends(InvalidOid, reason, false);
}

/*
 * In Hot Standby perform early deadlock detection.  We abort the lock
 * wait if we are about to sleep while holding the buffer pin that Startup
 * process is waiting for.
 *
 * Note: this code is pessimistic, because there is no way for it to
 * determine whether an actual deadlock condition is present: the lock we
 * need to wait for might be unrelated to any held by the Startup process.
 * Sooner or later, this mechanism should get ripped out in favor of somehow
 * accounting for buffer locks in DeadLockCheck().  However, errors here
 * seem to be very low-probability in practice, so for now it's not worth
 * the trouble.
 */
void
CheckRecoveryConflictDeadlock(void)
{
    Assert(!InRecovery);        /* do not call in Startup process */

    if (!HoldingBufferPinThatDelaysRecovery())
        return;

    /*
     * Error message should match ProcessInterrupts() but we avoid calling
     * that because we aren't handling an interrupt at this point. Note that
     * we only cancel the current transaction here, so if we are in a
     * subtransaction and the pin is held by a parent, then the Startup
     * process will continue to wait even though we have avoided deadlock.
     */
    ereport(ERROR,
            (errcode(ERRCODE_T_R_DEADLOCK_DETECTED),
             errmsg("canceling statement due to conflict with recovery"),
       errdetail("User transaction caused buffer deadlock with recovery.")));
}


/* --------------------------------
 *      timeout handler routines
 * --------------------------------
 */

/*
 * StandbyDeadLockHandler() will be called if STANDBY_DEADLOCK_TIMEOUT
 * occurs before STANDBY_TIMEOUT.  Send out a request for hot-standby
 * backends to check themselves for deadlocks.
 */
void
StandbyDeadLockHandler(void)
{
    SendRecoveryConflictWithBufferPin(PROCSIG_RECOVERY_CONFLICT_STARTUP_DEADLOCK);
}

/*
 * StandbyTimeoutHandler() will be called if STANDBY_TIMEOUT is exceeded.
 * Send out a request to release conflicting buffer pins unconditionally,
 * so we can press ahead with applying changes in recovery.
 */
void
StandbyTimeoutHandler(void)
{
    /* forget any pending STANDBY_DEADLOCK_TIMEOUT request */
    disable_timeout(STANDBY_DEADLOCK_TIMEOUT, false);

    SendRecoveryConflictWithBufferPin(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN);
}


/*
 * -----------------------------------------------------
 * Locking in Recovery Mode
 * -----------------------------------------------------
 *
 * All locks are held by the Startup process using a single virtual
 * transaction. This implementation is both simpler and in some senses,
 * more correct. The locks held mean "some original transaction held
 * this lock, so query access is not allowed at this time". So the Startup
 * process is the proxy by which the original locks are implemented.
 *
 * We only keep track of AccessExclusiveLocks, which are only ever held by
 * one transaction on one relation, and don't worry about lock queuing.
 *
 * We keep a single dynamically expandible list of locks in local memory,
 * RelationLockList, so we can keep track of the various entries made by
 * the Startup process's virtual xid in the shared lock table.
 *
 * We record the lock against the top-level xid, rather than individual
 * subtransaction xids. This means AccessExclusiveLocks held by aborted
 * subtransactions are not released as early as possible on standbys.
 *
 * List elements use type xl_rel_lock, since the WAL record type exactly
 * matches the information that we need to keep track of.
 *
 * We use session locks rather than normal locks so we don't need
 * ResourceOwners.
 */


void
StandbyAcquireAccessExclusiveLock(TransactionId xid, Oid dbOid, Oid relOid)
{
    xl_standby_lock *newlock;
    LOCKTAG     locktag;

    /* Already processed? */
    if (!TransactionIdIsValid(xid) ||
        TransactionIdDidCommit(xid) ||
        TransactionIdDidAbort(xid))
        return;

    elog(trace_recovery(DEBUG4),
         "adding recovery lock: db %u rel %u", dbOid, relOid);

    /* dbOid is InvalidOid when we are locking a shared relation. */
    Assert(OidIsValid(relOid));

    newlock = palloc(sizeof(xl_standby_lock));
    newlock->xid = xid;
    newlock->dbOid = dbOid;
    newlock->relOid = relOid;
    RecoveryLockList = lappend(RecoveryLockList, newlock);

    /*
     * Attempt to acquire the lock as requested, if not resolve conflict
     */
    SET_LOCKTAG_RELATION(locktag, newlock->dbOid, newlock->relOid);

    if (LockAcquireExtended(&locktag, AccessExclusiveLock, true, true, false)
        == LOCKACQUIRE_NOT_AVAIL)
        ResolveRecoveryConflictWithLock(newlock->dbOid, newlock->relOid);
}

static void
StandbyReleaseLocks(TransactionId xid)
{
    ListCell   *cell,
               *prev,
               *next;

    /*
     * Release all matching locks and remove them from list
     */
    prev = NULL;
    for (cell = list_head(RecoveryLockList); cell; cell = next)
    {
        xl_standby_lock *lock = (xl_standby_lock *) lfirst(cell);

        next = lnext(cell);

        if (!TransactionIdIsValid(xid) || lock->xid == xid)
        {
            LOCKTAG     locktag;

            elog(trace_recovery(DEBUG4),
                 "releasing recovery lock: xid %u db %u rel %u",
                 lock->xid, lock->dbOid, lock->relOid);
            SET_LOCKTAG_RELATION(locktag, lock->dbOid, lock->relOid);
            if (!LockRelease(&locktag, AccessExclusiveLock, true))
                elog(LOG,
                     "RecoveryLockList contains entry for lock no longer recorded by lock manager: xid %u database %u relation %u",
                     lock->xid, lock->dbOid, lock->relOid);

            RecoveryLockList = list_delete_cell(RecoveryLockList, cell, prev);
            pfree(lock);
        }
        else
            prev = cell;
    }
}

/*
 * Release locks for a transaction tree, starting at xid down, from
 * RecoveryLockList.
 *
 * Called during WAL replay of COMMIT/ROLLBACK when in hot standby mode,
 * to remove any AccessExclusiveLocks requested by a transaction.
 */
void
StandbyReleaseLockTree(TransactionId xid, int nsubxids, TransactionId *subxids)
{
    int         i;

    StandbyReleaseLocks(xid);

    for (i = 0; i < nsubxids; i++)
        StandbyReleaseLocks(subxids[i]);
}

/*
 * Called at end of recovery and when we see a shutdown checkpoint.
 */
void
StandbyReleaseAllLocks(void)
{
    ListCell   *cell,
               *prev,
               *next;
    LOCKTAG     locktag;

    elog(trace_recovery(DEBUG2), "release all standby locks");

    prev = NULL;
    for (cell = list_head(RecoveryLockList); cell; cell = next)
    {
        xl_standby_lock *lock = (xl_standby_lock *) lfirst(cell);

        next = lnext(cell);

        elog(trace_recovery(DEBUG4),
             "releasing recovery lock: xid %u db %u rel %u",
             lock->xid, lock->dbOid, lock->relOid);
        SET_LOCKTAG_RELATION(locktag, lock->dbOid, lock->relOid);
        if (!LockRelease(&locktag, AccessExclusiveLock, true))
            elog(LOG,
                 "RecoveryLockList contains entry for lock no longer recorded by lock manager: xid %u database %u relation %u",
                 lock->xid, lock->dbOid, lock->relOid);
        RecoveryLockList = list_delete_cell(RecoveryLockList, cell, prev);
        pfree(lock);
    }
}

/*
 * StandbyReleaseOldLocks
 *      Release standby locks held by top-level XIDs that aren't running,
 *      as long as they're not prepared transactions.
 */
void
StandbyReleaseOldLocks(int nxids, TransactionId *xids)
{
    ListCell   *cell,
               *prev,
               *next;
    LOCKTAG     locktag;

    prev = NULL;
    for (cell = list_head(RecoveryLockList); cell; cell = next)
    {
        xl_standby_lock *lock = (xl_standby_lock *) lfirst(cell);
        bool        remove = false;

        next = lnext(cell);

        Assert(TransactionIdIsValid(lock->xid));

        if (StandbyTransactionIdIsPrepared(lock->xid))
            remove = false;
        else
        {
            int         i;
            bool        found = false;

            for (i = 0; i < nxids; i++)
            {
                if (lock->xid == xids[i])
                {
                    found = true;
                    break;
                }
            }

            /*
             * If its not a running transaction, remove it.
             */
            if (!found)
                remove = true;
        }

        if (remove)
        {
            elog(trace_recovery(DEBUG4),
                 "releasing recovery lock: xid %u db %u rel %u",
                 lock->xid, lock->dbOid, lock->relOid);
            SET_LOCKTAG_RELATION(locktag, lock->dbOid, lock->relOid);
            if (!LockRelease(&locktag, AccessExclusiveLock, true))
                elog(LOG,
                     "RecoveryLockList contains entry for lock no longer recorded by lock manager: xid %u database %u relation %u",
                     lock->xid, lock->dbOid, lock->relOid);
            RecoveryLockList = list_delete_cell(RecoveryLockList, cell, prev);
            pfree(lock);
        }
        else
            prev = cell;
    }
}

/*
 * --------------------------------------------------------------------
 *      Recovery handling for Rmgr RM_STANDBY_ID
 *
 * These record types will only be created if XLogStandbyInfoActive()
 * --------------------------------------------------------------------
 */

void
standby_redo(XLogRecPtr lsn, XLogRecord *record)
{
    uint8       info = record->xl_info & ~XLR_INFO_MASK;

    /* Backup blocks are not used in standby records */
    Assert(!(record->xl_info & XLR_BKP_BLOCK_MASK));

    /* Do nothing if we're not in hot standby mode */
    if (standbyState == STANDBY_DISABLED)
        return;

    if (info == XLOG_STANDBY_LOCK)
    {
        xl_standby_locks *xlrec = (xl_standby_locks *) XLogRecGetData(record);
        int         i;

        for (i = 0; i < xlrec->nlocks; i++)
            StandbyAcquireAccessExclusiveLock(xlrec->locks[i].xid,
                                              xlrec->locks[i].dbOid,
                                              xlrec->locks[i].relOid);
    }
    else if (info == XLOG_RUNNING_XACTS)
    {
        xl_running_xacts *xlrec = (xl_running_xacts *) XLogRecGetData(record);
        RunningTransactionsData running;

        running.xcnt = xlrec->xcnt;
        running.subxcnt = xlrec->subxcnt;
        running.subxid_overflow = xlrec->subxid_overflow;
        running.nextXid = xlrec->nextXid;
        running.latestCompletedXid = xlrec->latestCompletedXid;
        running.oldestRunningXid = xlrec->oldestRunningXid;
        running.xids = xlrec->xids;

        ProcArrayApplyRecoveryInfo(&running);
    }
    else
        elog(PANIC, "standby_redo: unknown op code %u", info);
}

/*
 * Log details of the current snapshot to WAL. This allows the snapshot state
 * to be reconstructed on the standby.
 *
 * We can move directly to STANDBY_SNAPSHOT_READY at startup if we
 * start from a shutdown checkpoint because we know nothing was running
 * at that time and our recovery snapshot is known empty. In the more
 * typical case of an online checkpoint we need to jump through a few
 * hoops to get a correct recovery snapshot and this requires a two or
 * sometimes a three stage process.
 *
 * The initial snapshot must contain all running xids and all current
 * AccessExclusiveLocks at a point in time on the standby. Assembling
 * that information while the server is running requires many and
 * various LWLocks, so we choose to derive that information piece by
 * piece and then re-assemble that info on the standby. When that
 * information is fully assembled we move to STANDBY_SNAPSHOT_READY.
 *
 * Since locking on the primary when we derive the information is not
 * strict, we note that there is a time window between the derivation and
 * writing to WAL of the derived information. That allows race conditions
 * that we must resolve, since xids and locks may enter or leave the
 * snapshot during that window. This creates the issue that an xid or
 * lock may start *after* the snapshot has been derived yet *before* the
 * snapshot is logged in the running xacts WAL record. We resolve this by
 * starting to accumulate changes at a point just prior to when we derive
 * the snapshot on the primary, then ignore duplicates when we later apply
 * the snapshot from the running xacts record. This is implemented during
 * CreateCheckpoint() where we use the logical checkpoint location as
 * our starting point and then write the running xacts record immediately
 * before writing the main checkpoint WAL record. Since we always start
 * up from a checkpoint and are immediately at our starting point, we
 * unconditionally move to STANDBY_INITIALIZED. After this point we
 * must do 4 things:
 *  * move shared nextXid forwards as we see new xids
 *  * extend the clog and subtrans with each new xid
 *  * keep track of uncommitted known assigned xids
 *  * keep track of uncommitted AccessExclusiveLocks
 *
 * When we see a commit/abort we must remove known assigned xids and locks
 * from the completing transaction. Attempted removals that cannot locate
 * an entry are expected and must not cause an error when we are in state
 * STANDBY_INITIALIZED. This is implemented in StandbyReleaseLocks() and
 * KnownAssignedXidsRemove().
 *
 * Later, when we apply the running xact data we must be careful to ignore
 * transactions already committed, since those commits raced ahead when
 * making WAL entries.
 *
 * The loose timing also means that locks may be recorded that have a
 * zero xid, since xids are removed from procs before locks are removed.
 * So we must prune the lock list down to ensure we hold locks only for
 * currently running xids, performed by StandbyReleaseOldLocks().
 * Zero xids should no longer be possible, but we may be replaying WAL
 * from a time when they were possible.
 */
void
LogStandbySnapshot(void)
{
    RunningTransactions running;
    xl_standby_lock *locks;
    int         nlocks;

    Assert(XLogStandbyInfoActive());

    /*
     * Get details of any AccessExclusiveLocks being held at the moment.
     *
     * XXX GetRunningTransactionLocks() currently holds a lock on all
     * partitions though it is possible to further optimise the locking. By
     * reference counting locks and storing the value on the ProcArray entry
     * for each backend we can easily tell if any locks need recording without
     * trying to acquire the partition locks and scanning the lock table.
     */
    locks = GetRunningTransactionLocks(&nlocks);
    if (nlocks > 0)
        LogAccessExclusiveLocks(nlocks, locks);

    /*
     * Log details of all in-progress transactions. This should be the last
     * record we write, because standby will open up when it sees this.
     */
    running = GetRunningTransactionData();
    LogCurrentRunningXacts(running);
    /* GetRunningTransactionData() acquired XidGenLock, we must release it */
    LWLockRelease(XidGenLock);
}

/*
 * Record an enhanced snapshot of running transactions into WAL.
 *
 * The definitions of RunningTransactionsData and xl_xact_running_xacts
 * are similar. We keep them separate because xl_xact_running_xacts
 * is a contiguous chunk of memory and never exists fully until it is
 * assembled in WAL.
 */
static void
LogCurrentRunningXacts(RunningTransactions CurrRunningXacts)
{
    xl_running_xacts xlrec;
    XLogRecData rdata[2];
    int         lastrdata = 0;
    XLogRecPtr  recptr;

    xlrec.xcnt = CurrRunningXacts->xcnt;
    xlrec.subxcnt = CurrRunningXacts->subxcnt;
    xlrec.subxid_overflow = CurrRunningXacts->subxid_overflow;
    xlrec.nextXid = CurrRunningXacts->nextXid;
    xlrec.oldestRunningXid = CurrRunningXacts->oldestRunningXid;
    xlrec.latestCompletedXid = CurrRunningXacts->latestCompletedXid;

    /* Header */
    rdata[0].data = (char *) (&xlrec);
    rdata[0].len = MinSizeOfXactRunningXacts;
    rdata[0].buffer = InvalidBuffer;

    /* array of TransactionIds */
    if (xlrec.xcnt > 0)
    {
        rdata[0].next = &(rdata[1]);
        rdata[1].data = (char *) CurrRunningXacts->xids;
        rdata[1].len = (xlrec.xcnt + xlrec.subxcnt) * sizeof(TransactionId);
        rdata[1].buffer = InvalidBuffer;
        lastrdata = 1;
    }

    rdata[lastrdata].next = NULL;

    recptr = XLogInsert(RM_STANDBY_ID, XLOG_RUNNING_XACTS, rdata);

    if (CurrRunningXacts->subxid_overflow)
        elog(trace_recovery(DEBUG2),
             "snapshot of %u running transactions overflowed (lsn %X/%X oldest xid %u latest complete %u next xid %u)",
             CurrRunningXacts->xcnt,
             (uint32) (recptr >> 32), (uint32) recptr,
             CurrRunningXacts->oldestRunningXid,
             CurrRunningXacts->latestCompletedXid,
             CurrRunningXacts->nextXid);
    else
        elog(trace_recovery(DEBUG2),
             "snapshot of %u+%u running transaction ids (lsn %X/%X oldest xid %u latest complete %u next xid %u)",
             CurrRunningXacts->xcnt, CurrRunningXacts->subxcnt,
             (uint32) (recptr >> 32), (uint32) recptr,
             CurrRunningXacts->oldestRunningXid,
             CurrRunningXacts->latestCompletedXid,
             CurrRunningXacts->nextXid);
}

/*
 * Wholesale logging of AccessExclusiveLocks. Other lock types need not be
 * logged, as described in backend/storage/lmgr/README.
 */
static void
LogAccessExclusiveLocks(int nlocks, xl_standby_lock *locks)
{
    XLogRecData rdata[2];
    xl_standby_locks xlrec;

    xlrec.nlocks = nlocks;

    rdata[0].data = (char *) &xlrec;
    rdata[0].len = offsetof(xl_standby_locks, locks);
    rdata[0].buffer = InvalidBuffer;
    rdata[0].next = &rdata[1];

    rdata[1].data = (char *) locks;
    rdata[1].len = nlocks * sizeof(xl_standby_lock);
    rdata[1].buffer = InvalidBuffer;
    rdata[1].next = NULL;

    (void) XLogInsert(RM_STANDBY_ID, XLOG_STANDBY_LOCK, rdata);
}

/*
 * Individual logging of AccessExclusiveLocks for use during LockAcquire()
 */
void
LogAccessExclusiveLock(Oid dbOid, Oid relOid)
{
    xl_standby_lock xlrec;

    xlrec.xid = GetTopTransactionId();

    /*
     * Decode the locktag back to the original values, to avoid sending lots
     * of empty bytes with every message.  See lock.h to check how a locktag
     * is defined for LOCKTAG_RELATION
     */
    xlrec.dbOid = dbOid;
    xlrec.relOid = relOid;

    LogAccessExclusiveLocks(1, &xlrec);
}

/*
 * Prepare to log an AccessExclusiveLock, for use during LockAcquire()
 */
void
LogAccessExclusiveLockPrepare(void)
{
    /*
     * Ensure that a TransactionId has been assigned to this transaction, for
     * two reasons, both related to lock release on the standby. First, we
     * must assign an xid so that RecordTransactionCommit() and
     * RecordTransactionAbort() do not optimise away the transaction
     * completion record which recovery relies upon to release locks. It's a
     * hack, but for a corner case not worth adding code for into the main
     * commit path. Second, we must assign an xid before the lock is recorded
     * in shared memory, otherwise a concurrently executing
     * GetRunningTransactionLocks() might see a lock associated with an
     * InvalidTransactionId which we later assert cannot happen.
     */
    (void) GetTopTransactionId();
}