predicate.c

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/*-------------------------------------------------------------------------
 *
 * predicate.c
 *    POSTGRES predicate locking
 *    to support full serializable transaction isolation
 *
 *
 * The approach taken is to implement Serializable Snapshot Isolation (SSI)
 * as initially described in this paper:
 *
 *  Michael J. Cahill, Uwe Röhm, and Alan D. Fekete. 2008.
 *  Serializable isolation for snapshot databases.
 *  In SIGMOD '08: Proceedings of the 2008 ACM SIGMOD
 *  international conference on Management of data,
 *  pages 729-738, New York, NY, USA. ACM.
 *  http://doi.acm.org/10.1145/1376616.1376690
 *
 * and further elaborated in Cahill's doctoral thesis:
 *
 *  Michael James Cahill. 2009.
 *  Serializable Isolation for Snapshot Databases.
 *  Sydney Digital Theses.
 *  University of Sydney, School of Information Technologies.
 *  http://hdl.handle.net/2123/5353
 *
 *
 * Predicate locks for Serializable Snapshot Isolation (SSI) are SIREAD
 * locks, which are so different from normal locks that a distinct set of
 * structures is required to handle them.  They are needed to detect
 * rw-conflicts when the read happens before the write.  (When the write
 * occurs first, the reading transaction can check for a conflict by
 * examining the MVCC data.)
 *
 * (1)  Besides tuples actually read, they must cover ranges of tuples
 *      which would have been read based on the predicate.  This will
 *      require modelling the predicates through locks against database
 *      objects such as pages, index ranges, or entire tables.
 *
 * (2)  They must be kept in RAM for quick access.  Because of this, it
 *      isn't possible to always maintain tuple-level granularity -- when
 *      the space allocated to store these approaches exhaustion, a
 *      request for a lock may need to scan for situations where a single
 *      transaction holds many fine-grained locks which can be coalesced
 *      into a single coarser-grained lock.
 *
 * (3)  They never block anything; they are more like flags than locks
 *      in that regard; although they refer to database objects and are
 *      used to identify rw-conflicts with normal write locks.
 *
 * (4)  While they are associated with a transaction, they must survive
 *      a successful COMMIT of that transaction, and remain until all
 *      overlapping transactions complete.  This even means that they
 *      must survive termination of the transaction's process.  If a
 *      top level transaction is rolled back, however, it is immediately
 *      flagged so that it can be ignored, and its SIREAD locks can be
 *      released any time after that.
 *
 * (5)  The only transactions which create SIREAD locks or check for
 *      conflicts with them are serializable transactions.
 *
 * (6)  When a write lock for a top level transaction is found to cover
 *      an existing SIREAD lock for the same transaction, the SIREAD lock
 *      can be deleted.
 *
 * (7)  A write from a serializable transaction must ensure that a xact
 *      record exists for the transaction, with the same lifespan (until
 *      all concurrent transaction complete or the transaction is rolled
 *      back) so that rw-dependencies to that transaction can be
 *      detected.
 *
 * We use an optimization for read-only transactions. Under certain
 * circumstances, a read-only transaction's snapshot can be shown to
 * never have conflicts with other transactions.  This is referred to
 * as a "safe" snapshot (and one known not to be is "unsafe").
 * However, it can't be determined whether a snapshot is safe until
 * all concurrent read/write transactions complete.
 *
 * Once a read-only transaction is known to have a safe snapshot, it
 * can release its predicate locks and exempt itself from further
 * predicate lock tracking. READ ONLY DEFERRABLE transactions run only
 * on safe snapshots, waiting as necessary for one to be available.
 *
 *
 * Lightweight locks to manage access to the predicate locking shared
 * memory objects must be taken in this order, and should be released in
 * reverse order:
 *
 *  SerializableFinishedListLock
 *      - Protects the list of transactions which have completed but which
 *          may yet matter because they overlap still-active transactions.
 *
 *  SerializablePredicateLockListLock
 *      - Protects the linked list of locks held by a transaction.  Note
 *          that the locks themselves are also covered by the partition
 *          locks of their respective lock targets; this lock only affects
 *          the linked list connecting the locks related to a transaction.
 *      - All transactions share this single lock (with no partitioning).
 *      - There is never a need for a process other than the one running
 *          an active transaction to walk the list of locks held by that
 *          transaction.
 *      - It is relatively infrequent that another process needs to
 *          modify the list for a transaction, but it does happen for such
 *          things as index page splits for pages with predicate locks and
 *          freeing of predicate locked pages by a vacuum process.  When
 *          removing a lock in such cases, the lock itself contains the
 *          pointers needed to remove it from the list.  When adding a
 *          lock in such cases, the lock can be added using the anchor in
 *          the transaction structure.  Neither requires walking the list.
 *      - Cleaning up the list for a terminated transaction is sometimes
 *          not done on a retail basis, in which case no lock is required.
 *      - Due to the above, a process accessing its active transaction's
 *          list always uses a shared lock, regardless of whether it is
 *          walking or maintaining the list.  This improves concurrency
 *          for the common access patterns.
 *      - A process which needs to alter the list of a transaction other
 *          than its own active transaction must acquire an exclusive
 *          lock.
 *
 *  FirstPredicateLockMgrLock based partition locks
 *      - The same lock protects a target, all locks on that target, and
 *          the linked list of locks on the target..
 *      - When more than one is needed, acquire in ascending order.
 *
 *  SerializableXactHashLock
 *      - Protects both PredXact and SerializableXidHash.
 *
 *
 * Portions Copyright (c) 1996-2013, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/predicate.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *
 * housekeeping for setting up shared memory predicate lock structures
 *      InitPredicateLocks(void)
 *      PredicateLockShmemSize(void)
 *
 * predicate lock reporting
 *      GetPredicateLockStatusData(void)
 *      PageIsPredicateLocked(Relation relation, BlockNumber blkno)
 *
 * predicate lock maintenance
 *      GetSerializableTransactionSnapshot(Snapshot snapshot)
 *      SetSerializableTransactionSnapshot(Snapshot snapshot,
 *                                         TransactionId sourcexid)
 *      RegisterPredicateLockingXid(void)
 *      PredicateLockRelation(Relation relation, Snapshot snapshot)
 *      PredicateLockPage(Relation relation, BlockNumber blkno,
 *                      Snapshot snapshot)
 *      PredicateLockTuple(Relation relation, HeapTuple tuple,
 *                      Snapshot snapshot)
 *      PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
 *                             BlockNumber newblkno);
 *      PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
 *                               BlockNumber newblkno);
 *      TransferPredicateLocksToHeapRelation(Relation relation)
 *      ReleasePredicateLocks(bool isCommit)
 *
 * conflict detection (may also trigger rollback)
 *      CheckForSerializableConflictOut(bool visible, Relation relation,
 *                                      HeapTupleData *tup, Buffer buffer,
 *                                      Snapshot snapshot)
 *      CheckForSerializableConflictIn(Relation relation, HeapTupleData *tup,
 *                                     Buffer buffer)
 *      CheckTableForSerializableConflictIn(Relation relation)
 *
 * final rollback checking
 *      PreCommit_CheckForSerializationFailure(void)
 *
 * two-phase commit support
 *      AtPrepare_PredicateLocks(void);
 *      PostPrepare_PredicateLocks(TransactionId xid);
 *      PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
 *      predicatelock_twophase_recover(TransactionId xid, uint16 info,
 *                                     void *recdata, uint32 len);
 */

#include "postgres.h"

#include "access/htup_details.h"
#include "access/slru.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "miscadmin.h"
#include "storage/bufmgr.h"
#include "storage/predicate.h"
#include "storage/predicate_internals.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/tqual.h"

/* Uncomment the next line to test the graceful degradation code. */
/* #define TEST_OLDSERXID */

/*
 * Test the most selective fields first, for performance.
 *
 * a is covered by b if all of the following hold:
 *  1) a.database = b.database
 *  2) a.relation = b.relation
 *  3) b.offset is invalid (b is page-granularity or higher)
 *  4) either of the following:
 *      4a) a.offset is valid (a is tuple-granularity) and a.page = b.page
 *   or 4b) a.offset is invalid and b.page is invalid (a is
 *          page-granularity and b is relation-granularity
 */
#define TargetTagIsCoveredBy(covered_target, covering_target)           \
    ((GET_PREDICATELOCKTARGETTAG_RELATION(covered_target) == /* (2) */  \
      GET_PREDICATELOCKTARGETTAG_RELATION(covering_target))             \
     && (GET_PREDICATELOCKTARGETTAG_OFFSET(covering_target) ==          \
         InvalidOffsetNumber)                                /* (3) */  \
     && (((GET_PREDICATELOCKTARGETTAG_OFFSET(covered_target) !=         \
           InvalidOffsetNumber)                              /* (4a) */ \
          && (GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) ==       \
              GET_PREDICATELOCKTARGETTAG_PAGE(covered_target)))         \
         || ((GET_PREDICATELOCKTARGETTAG_PAGE(covering_target) ==       \
              InvalidBlockNumber)                            /* (4b) */ \
             && (GET_PREDICATELOCKTARGETTAG_PAGE(covered_target)        \
                 != InvalidBlockNumber)))                               \
     && (GET_PREDICATELOCKTARGETTAG_DB(covered_target) ==    /* (1) */  \
         GET_PREDICATELOCKTARGETTAG_DB(covering_target)))

/*
 * The predicate locking target and lock shared hash tables are partitioned to
 * reduce contention.  To determine which partition a given target belongs to,
 * compute the tag's hash code with PredicateLockTargetTagHashCode(), then
 * apply one of these macros.
 * NB: NUM_PREDICATELOCK_PARTITIONS must be a power of 2!
 */
#define PredicateLockHashPartition(hashcode) \
    ((hashcode) % NUM_PREDICATELOCK_PARTITIONS)
#define PredicateLockHashPartitionLock(hashcode) \
    ((LWLockId) (FirstPredicateLockMgrLock + PredicateLockHashPartition(hashcode)))

#define NPREDICATELOCKTARGETENTS() \
    mul_size(max_predicate_locks_per_xact, add_size(MaxBackends, max_prepared_xacts))

#define SxactIsOnFinishedList(sxact) (!SHMQueueIsDetached(&((sxact)->finishedLink)))

/*
 * Note that a sxact is marked "prepared" once it has passed
 * PreCommit_CheckForSerializationFailure, even if it isn't using
 * 2PC. This is the point at which it can no longer be aborted.
 *
 * The PREPARED flag remains set after commit, so SxactIsCommitted
 * implies SxactIsPrepared.
 */
#define SxactIsCommitted(sxact) (((sxact)->flags & SXACT_FLAG_COMMITTED) != 0)
#define SxactIsPrepared(sxact) (((sxact)->flags & SXACT_FLAG_PREPARED) != 0)
#define SxactIsRolledBack(sxact) (((sxact)->flags & SXACT_FLAG_ROLLED_BACK) != 0)
#define SxactIsDoomed(sxact) (((sxact)->flags & SXACT_FLAG_DOOMED) != 0)
#define SxactIsReadOnly(sxact) (((sxact)->flags & SXACT_FLAG_READ_ONLY) != 0)
#define SxactHasSummaryConflictIn(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_IN) != 0)
#define SxactHasSummaryConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_SUMMARY_CONFLICT_OUT) != 0)
/*
 * The following macro actually means that the specified transaction has a
 * conflict out *to a transaction which committed ahead of it*.  It's hard
 * to get that into a name of a reasonable length.
 */
#define SxactHasConflictOut(sxact) (((sxact)->flags & SXACT_FLAG_CONFLICT_OUT) != 0)
#define SxactIsDeferrableWaiting(sxact) (((sxact)->flags & SXACT_FLAG_DEFERRABLE_WAITING) != 0)
#define SxactIsROSafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_SAFE) != 0)
#define SxactIsROUnsafe(sxact) (((sxact)->flags & SXACT_FLAG_RO_UNSAFE) != 0)

/*
 * Compute the hash code associated with a PREDICATELOCKTARGETTAG.
 *
 * To avoid unnecessary recomputations of the hash code, we try to do this
 * just once per function, and then pass it around as needed.  Aside from
 * passing the hashcode to hash_search_with_hash_value(), we can extract
 * the lock partition number from the hashcode.
 */
#define PredicateLockTargetTagHashCode(predicatelocktargettag) \
    (tag_hash((predicatelocktargettag), sizeof(PREDICATELOCKTARGETTAG)))

/*
 * Given a predicate lock tag, and the hash for its target,
 * compute the lock hash.
 *
 * To make the hash code also depend on the transaction, we xor the sxid
 * struct's address into the hash code, left-shifted so that the
 * partition-number bits don't change.  Since this is only a hash, we
 * don't care if we lose high-order bits of the address; use an
 * intermediate variable to suppress cast-pointer-to-int warnings.
 */
#define PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash) \
    ((targethash) ^ ((uint32) PointerGetDatum((predicatelocktag)->myXact)) \
     << LOG2_NUM_PREDICATELOCK_PARTITIONS)


/*
 * The SLRU buffer area through which we access the old xids.
 */
static SlruCtlData OldSerXidSlruCtlData;

#define OldSerXidSlruCtl            (&OldSerXidSlruCtlData)

#define OLDSERXID_PAGESIZE          BLCKSZ
#define OLDSERXID_ENTRYSIZE         sizeof(SerCommitSeqNo)
#define OLDSERXID_ENTRIESPERPAGE    (OLDSERXID_PAGESIZE / OLDSERXID_ENTRYSIZE)

/*
 * Set maximum pages based on the lesser of the number needed to track all
 * transactions and the maximum that SLRU supports.
 */
#define OLDSERXID_MAX_PAGE          Min(SLRU_PAGES_PER_SEGMENT * 0x10000 - 1, \
                                        (MaxTransactionId) / OLDSERXID_ENTRIESPERPAGE)

#define OldSerXidNextPage(page) (((page) >= OLDSERXID_MAX_PAGE) ? 0 : (page) + 1)

#define OldSerXidValue(slotno, xid) (*((SerCommitSeqNo *) \
    (OldSerXidSlruCtl->shared->page_buffer[slotno] + \
    ((((uint32) (xid)) % OLDSERXID_ENTRIESPERPAGE) * OLDSERXID_ENTRYSIZE))))

#define OldSerXidPage(xid)  ((((uint32) (xid)) / OLDSERXID_ENTRIESPERPAGE) % (OLDSERXID_MAX_PAGE + 1))
#define OldSerXidSegment(page)  ((page) / SLRU_PAGES_PER_SEGMENT)

typedef struct OldSerXidControlData
{
    int         headPage;       /* newest initialized page */
    TransactionId headXid;      /* newest valid Xid in the SLRU */
    TransactionId tailXid;      /* oldest xmin we might be interested in */
    bool        warningIssued;  /* have we issued SLRU wrap-around warning? */
}   OldSerXidControlData;

typedef struct OldSerXidControlData *OldSerXidControl;

static OldSerXidControl oldSerXidControl;

/*
 * When the oldest committed transaction on the "finished" list is moved to
 * SLRU, its predicate locks will be moved to this "dummy" transaction,
 * collapsing duplicate targets.  When a duplicate is found, the later
 * commitSeqNo is used.
 */
static SERIALIZABLEXACT *OldCommittedSxact;


/* This configuration variable is used to set the predicate lock table size */
int         max_predicate_locks_per_xact;       /* set by guc.c */

/*
 * This provides a list of objects in order to track transactions
 * participating in predicate locking.  Entries in the list are fixed size,
 * and reside in shared memory.  The memory address of an entry must remain
 * fixed during its lifetime.  The list will be protected from concurrent
 * update externally; no provision is made in this code to manage that.  The
 * number of entries in the list, and the size allowed for each entry is
 * fixed upon creation.
 */
static PredXactList PredXact;

/*
 * This provides a pool of RWConflict data elements to use in conflict lists
 * between transactions.
 */
static RWConflictPoolHeader RWConflictPool;

/*
 * The predicate locking hash tables are in shared memory.
 * Each backend keeps pointers to them.
 */
static HTAB *SerializableXidHash;
static HTAB *PredicateLockTargetHash;
static HTAB *PredicateLockHash;
static SHM_QUEUE *FinishedSerializableTransactions;

/*
 * Tag for a dummy entry in PredicateLockTargetHash. By temporarily removing
 * this entry, you can ensure that there's enough scratch space available for
 * inserting one entry in the hash table. This is an otherwise-invalid tag.
 */
static const PREDICATELOCKTARGETTAG ScratchTargetTag = {0, 0, 0, 0, 0};
static uint32 ScratchTargetTagHash;
static int  ScratchPartitionLock;

/*
 * The local hash table used to determine when to combine multiple fine-
 * grained locks into a single courser-grained lock.
 */
static HTAB *LocalPredicateLockHash = NULL;

/*
 * Keep a pointer to the currently-running serializable transaction (if any)
 * for quick reference. Also, remember if we have written anything that could
 * cause a rw-conflict.
 */
static SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;
static bool MyXactDidWrite = false;

/* local functions */

static SERIALIZABLEXACT *CreatePredXact(void);
static void ReleasePredXact(SERIALIZABLEXACT *sxact);
static SERIALIZABLEXACT *FirstPredXact(void);
static SERIALIZABLEXACT *NextPredXact(SERIALIZABLEXACT *sxact);

static bool RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer);
static void SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
static void SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact, SERIALIZABLEXACT *activeXact);
static void ReleaseRWConflict(RWConflict conflict);
static void FlagSxactUnsafe(SERIALIZABLEXACT *sxact);

static bool OldSerXidPagePrecedesLogically(int p, int q);
static void OldSerXidInit(void);
static void OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo);
static SerCommitSeqNo OldSerXidGetMinConflictCommitSeqNo(TransactionId xid);
static void OldSerXidSetActiveSerXmin(TransactionId xid);

static uint32 predicatelock_hash(const void *key, Size keysize);
static void SummarizeOldestCommittedSxact(void);
static Snapshot GetSafeSnapshot(Snapshot snapshot);
static Snapshot GetSerializableTransactionSnapshotInt(Snapshot snapshot,
                                      TransactionId sourcexid);
static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
                          PREDICATELOCKTARGETTAG *parent);
static bool CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag);
static void RemoveScratchTarget(bool lockheld);
static void RestoreScratchTarget(bool lockheld);
static void RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target,
                           uint32 targettaghash);
static void DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag);
static int  PredicateLockPromotionThreshold(const PREDICATELOCKTARGETTAG *tag);
static bool CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag);
static void DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag);
static void CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
                    uint32 targettaghash,
                    SERIALIZABLEXACT *sxact);
static void DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash);
static bool TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
                                  PREDICATELOCKTARGETTAG newtargettag,
                                  bool removeOld);
static void PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag);
static void DropAllPredicateLocksFromTable(Relation relation,
                               bool transfer);
static void SetNewSxactGlobalXmin(void);
static void ClearOldPredicateLocks(void);
static void ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
                           bool summarize);
static bool XidIsConcurrent(TransactionId xid);
static void CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag);
static void FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer);
static void OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
                                        SERIALIZABLEXACT *writer);


/*------------------------------------------------------------------------*/

/*
 * Does this relation participate in predicate locking? Temporary and system
 * relations are exempt, as are materialized views.
 */
static inline bool
PredicateLockingNeededForRelation(Relation relation)
{
    return !(relation->rd_id < FirstBootstrapObjectId ||
             RelationUsesLocalBuffers(relation) ||
             relation->rd_rel->relkind == RELKIND_MATVIEW);
}

/*
 * When a public interface method is called for a read, this is the test to
 * see if we should do a quick return.
 *
 * Note: this function has side-effects! If this transaction has been flagged
 * as RO-safe since the last call, we release all predicate locks and reset
 * MySerializableXact. That makes subsequent calls to return quickly.
 *
 * This is marked as 'inline' to make to eliminate the function call overhead
 * in the common case that serialization is not needed.
 */
static inline bool
SerializationNeededForRead(Relation relation, Snapshot snapshot)
{
    /* Nothing to do if this is not a serializable transaction */
    if (MySerializableXact == InvalidSerializableXact)
        return false;

    /*
     * Don't acquire locks or conflict when scanning with a special snapshot.
     * This excludes things like CLUSTER and REINDEX. They use the wholesale
     * functions TransferPredicateLocksToHeapRelation() and
     * CheckTableForSerializableConflictIn() to participate serialization, but
     * the scans involved don't need serialization.
     */
    if (!IsMVCCSnapshot(snapshot))
        return false;

    /*
     * Check if we have just become "RO-safe". If we have, immediately release
     * all locks as they're not needed anymore. This also resets
     * MySerializableXact, so that subsequent calls to this function can exit
     * quickly.
     *
     * A transaction is flagged as RO_SAFE if all concurrent R/W transactions
     * commit without having conflicts out to an earlier snapshot, thus
     * ensuring that no conflicts are possible for this transaction.
     */
    if (SxactIsROSafe(MySerializableXact))
    {
        ReleasePredicateLocks(false);
        return false;
    }

    /* Check if the relation doesn't participate in predicate locking */
    if (!PredicateLockingNeededForRelation(relation))
        return false;

    return true;                /* no excuse to skip predicate locking */
}

/*
 * Like SerializationNeededForRead(), but called on writes.
 * The logic is the same, but there is no snapshot and we can't be RO-safe.
 */
static inline bool
SerializationNeededForWrite(Relation relation)
{
    /* Nothing to do if this is not a serializable transaction */
    if (MySerializableXact == InvalidSerializableXact)
        return false;

    /* Check if the relation doesn't participate in predicate locking */
    if (!PredicateLockingNeededForRelation(relation))
        return false;

    return true;                /* no excuse to skip predicate locking */
}


/*------------------------------------------------------------------------*/

/*
 * These functions are a simple implementation of a list for this specific
 * type of struct.  If there is ever a generalized shared memory list, we
 * should probably switch to that.
 */
static SERIALIZABLEXACT *
CreatePredXact(void)
{
    PredXactListElement ptle;

    ptle = (PredXactListElement)
        SHMQueueNext(&PredXact->availableList,
                     &PredXact->availableList,
                     offsetof(PredXactListElementData, link));
    if (!ptle)
        return NULL;

    SHMQueueDelete(&ptle->link);
    SHMQueueInsertBefore(&PredXact->activeList, &ptle->link);
    return &ptle->sxact;
}

static void
ReleasePredXact(SERIALIZABLEXACT *sxact)
{
    PredXactListElement ptle;

    Assert(ShmemAddrIsValid(sxact));

    ptle = (PredXactListElement)
        (((char *) sxact)
         - offsetof(PredXactListElementData, sxact)
         + offsetof(PredXactListElementData, link));
    SHMQueueDelete(&ptle->link);
    SHMQueueInsertBefore(&PredXact->availableList, &ptle->link);
}

static SERIALIZABLEXACT *
FirstPredXact(void)
{
    PredXactListElement ptle;

    ptle = (PredXactListElement)
        SHMQueueNext(&PredXact->activeList,
                     &PredXact->activeList,
                     offsetof(PredXactListElementData, link));
    if (!ptle)
        return NULL;

    return &ptle->sxact;
}

static SERIALIZABLEXACT *
NextPredXact(SERIALIZABLEXACT *sxact)
{
    PredXactListElement ptle;

    Assert(ShmemAddrIsValid(sxact));

    ptle = (PredXactListElement)
        (((char *) sxact)
         - offsetof(PredXactListElementData, sxact)
         + offsetof(PredXactListElementData, link));
    ptle = (PredXactListElement)
        SHMQueueNext(&PredXact->activeList,
                     &ptle->link,
                     offsetof(PredXactListElementData, link));
    if (!ptle)
        return NULL;

    return &ptle->sxact;
}

/*------------------------------------------------------------------------*/

/*
 * These functions manage primitive access to the RWConflict pool and lists.
 */
static bool
RWConflictExists(const SERIALIZABLEXACT *reader, const SERIALIZABLEXACT *writer)
{
    RWConflict  conflict;

    Assert(reader != writer);

    /* Check the ends of the purported conflict first. */
    if (SxactIsDoomed(reader)
        || SxactIsDoomed(writer)
        || SHMQueueEmpty(&reader->outConflicts)
        || SHMQueueEmpty(&writer->inConflicts))
        return false;

    /* A conflict is possible; walk the list to find out. */
    conflict = (RWConflict)
        SHMQueueNext(&reader->outConflicts,
                     &reader->outConflicts,
                     offsetof(RWConflictData, outLink));
    while (conflict)
    {
        if (conflict->sxactIn == writer)
            return true;
        conflict = (RWConflict)
            SHMQueueNext(&reader->outConflicts,
                         &conflict->outLink,
                         offsetof(RWConflictData, outLink));
    }

    /* No conflict found. */
    return false;
}

static void
SetRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
{
    RWConflict  conflict;

    Assert(reader != writer);
    Assert(!RWConflictExists(reader, writer));

    conflict = (RWConflict)
        SHMQueueNext(&RWConflictPool->availableList,
                     &RWConflictPool->availableList,
                     offsetof(RWConflictData, outLink));
    if (!conflict)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("not enough elements in RWConflictPool to record a read/write conflict"),
                 errhint("You might need to run fewer transactions at a time or increase max_connections.")));

    SHMQueueDelete(&conflict->outLink);

    conflict->sxactOut = reader;
    conflict->sxactIn = writer;
    SHMQueueInsertBefore(&reader->outConflicts, &conflict->outLink);
    SHMQueueInsertBefore(&writer->inConflicts, &conflict->inLink);
}

static void
SetPossibleUnsafeConflict(SERIALIZABLEXACT *roXact,
                          SERIALIZABLEXACT *activeXact)
{
    RWConflict  conflict;

    Assert(roXact != activeXact);
    Assert(SxactIsReadOnly(roXact));
    Assert(!SxactIsReadOnly(activeXact));

    conflict = (RWConflict)
        SHMQueueNext(&RWConflictPool->availableList,
                     &RWConflictPool->availableList,
                     offsetof(RWConflictData, outLink));
    if (!conflict)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("not enough elements in RWConflictPool to record a potential read/write conflict"),
                 errhint("You might need to run fewer transactions at a time or increase max_connections.")));

    SHMQueueDelete(&conflict->outLink);

    conflict->sxactOut = activeXact;
    conflict->sxactIn = roXact;
    SHMQueueInsertBefore(&activeXact->possibleUnsafeConflicts,
                         &conflict->outLink);
    SHMQueueInsertBefore(&roXact->possibleUnsafeConflicts,
                         &conflict->inLink);
}

static void
ReleaseRWConflict(RWConflict conflict)
{
    SHMQueueDelete(&conflict->inLink);
    SHMQueueDelete(&conflict->outLink);
    SHMQueueInsertBefore(&RWConflictPool->availableList, &conflict->outLink);
}

static void
FlagSxactUnsafe(SERIALIZABLEXACT *sxact)
{
    RWConflict  conflict,
                nextConflict;

    Assert(SxactIsReadOnly(sxact));
    Assert(!SxactIsROSafe(sxact));

    sxact->flags |= SXACT_FLAG_RO_UNSAFE;

    /*
     * We know this isn't a safe snapshot, so we can stop looking for other
     * potential conflicts.
     */
    conflict = (RWConflict)
        SHMQueueNext(&sxact->possibleUnsafeConflicts,
                     &sxact->possibleUnsafeConflicts,
                     offsetof(RWConflictData, inLink));
    while (conflict)
    {
        nextConflict = (RWConflict)
            SHMQueueNext(&sxact->possibleUnsafeConflicts,
                         &conflict->inLink,
                         offsetof(RWConflictData, inLink));

        Assert(!SxactIsReadOnly(conflict->sxactOut));
        Assert(sxact == conflict->sxactIn);

        ReleaseRWConflict(conflict);

        conflict = nextConflict;
    }
}

/*------------------------------------------------------------------------*/

/*
 * We will work on the page range of 0..OLDSERXID_MAX_PAGE.
 * Compares using wraparound logic, as is required by slru.c.
 */
static bool
OldSerXidPagePrecedesLogically(int p, int q)
{
    int         diff;

    /*
     * We have to compare modulo (OLDSERXID_MAX_PAGE+1)/2.  Both inputs should
     * be in the range 0..OLDSERXID_MAX_PAGE.
     */
    Assert(p >= 0 && p <= OLDSERXID_MAX_PAGE);
    Assert(q >= 0 && q <= OLDSERXID_MAX_PAGE);

    diff = p - q;
    if (diff >= ((OLDSERXID_MAX_PAGE + 1) / 2))
        diff -= OLDSERXID_MAX_PAGE + 1;
    else if (diff < -((int) (OLDSERXID_MAX_PAGE + 1) / 2))
        diff += OLDSERXID_MAX_PAGE + 1;
    return diff < 0;
}

/*
 * Initialize for the tracking of old serializable committed xids.
 */
static void
OldSerXidInit(void)
{
    bool        found;

    /*
     * Set up SLRU management of the pg_serial data.
     */
    OldSerXidSlruCtl->PagePrecedes = OldSerXidPagePrecedesLogically;
    SimpleLruInit(OldSerXidSlruCtl, "OldSerXid SLRU Ctl",
                  NUM_OLDSERXID_BUFFERS, 0, OldSerXidLock, "pg_serial");
    /* Override default assumption that writes should be fsync'd */
    OldSerXidSlruCtl->do_fsync = false;

    /*
     * Create or attach to the OldSerXidControl structure.
     */
    oldSerXidControl = (OldSerXidControl)
        ShmemInitStruct("OldSerXidControlData", sizeof(OldSerXidControlData), &found);

    if (!found)
    {
        /*
         * Set control information to reflect empty SLRU.
         */
        oldSerXidControl->headPage = -1;
        oldSerXidControl->headXid = InvalidTransactionId;
        oldSerXidControl->tailXid = InvalidTransactionId;
        oldSerXidControl->warningIssued = false;
    }
}

/*
 * Record a committed read write serializable xid and the minimum
 * commitSeqNo of any transactions to which this xid had a rw-conflict out.
 * An invalid seqNo means that there were no conflicts out from xid.
 */
static void
OldSerXidAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
{
    TransactionId tailXid;
    int         targetPage;
    int         slotno;
    int         firstZeroPage;
    bool        isNewPage;

    Assert(TransactionIdIsValid(xid));

    targetPage = OldSerXidPage(xid);

    LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);

    /*
     * If no serializable transactions are active, there shouldn't be anything
     * to push out to the SLRU.  Hitting this assert would mean there's
     * something wrong with the earlier cleanup logic.
     */
    tailXid = oldSerXidControl->tailXid;
    Assert(TransactionIdIsValid(tailXid));

    /*
     * If the SLRU is currently unused, zero out the whole active region from
     * tailXid to headXid before taking it into use. Otherwise zero out only
     * any new pages that enter the tailXid-headXid range as we advance
     * headXid.
     */
    if (oldSerXidControl->headPage < 0)
    {
        firstZeroPage = OldSerXidPage(tailXid);
        isNewPage = true;
    }
    else
    {
        firstZeroPage = OldSerXidNextPage(oldSerXidControl->headPage);
        isNewPage = OldSerXidPagePrecedesLogically(oldSerXidControl->headPage,
                                                   targetPage);
    }

    if (!TransactionIdIsValid(oldSerXidControl->headXid)
        || TransactionIdFollows(xid, oldSerXidControl->headXid))
        oldSerXidControl->headXid = xid;
    if (isNewPage)
        oldSerXidControl->headPage = targetPage;

    /*
     * Give a warning if we're about to run out of SLRU pages.
     *
     * slru.c has a maximum of 64k segments, with 32 (SLRU_PAGES_PER_SEGMENT)
     * pages each. We need to store a 64-bit integer for each Xid, and with
     * default 8k block size, 65536*32 pages is only enough to cover 2^30
     * XIDs. If we're about to hit that limit and wrap around, warn the user.
     *
     * To avoid spamming the user, we only give one warning when we've used 1
     * billion XIDs, and stay silent until the situation is fixed and the
     * number of XIDs used falls below 800 million again.
     *
     * XXX: We have no safeguard to actually *prevent* the wrap-around,
     * though. All you get is a warning.
     */
    if (oldSerXidControl->warningIssued)
    {
        TransactionId lowWatermark;

        lowWatermark = tailXid + 800000000;
        if (lowWatermark < FirstNormalTransactionId)
            lowWatermark = FirstNormalTransactionId;
        if (TransactionIdPrecedes(xid, lowWatermark))
            oldSerXidControl->warningIssued = false;
    }
    else
    {
        TransactionId highWatermark;

        highWatermark = tailXid + 1000000000;
        if (highWatermark < FirstNormalTransactionId)
            highWatermark = FirstNormalTransactionId;
        if (TransactionIdFollows(xid, highWatermark))
        {
            oldSerXidControl->warningIssued = true;
            ereport(WARNING,
                    (errmsg("memory for serializable conflict tracking is nearly exhausted"),
                     errhint("There might be an idle transaction or a forgotten prepared transaction causing this.")));
        }
    }

    if (isNewPage)
    {
        /* Initialize intervening pages. */
        while (firstZeroPage != targetPage)
        {
            (void) SimpleLruZeroPage(OldSerXidSlruCtl, firstZeroPage);
            firstZeroPage = OldSerXidNextPage(firstZeroPage);
        }
        slotno = SimpleLruZeroPage(OldSerXidSlruCtl, targetPage);
    }
    else
        slotno = SimpleLruReadPage(OldSerXidSlruCtl, targetPage, true, xid);

    OldSerXidValue(slotno, xid) = minConflictCommitSeqNo;
    OldSerXidSlruCtl->shared->page_dirty[slotno] = true;

    LWLockRelease(OldSerXidLock);
}

/*
 * Get the minimum commitSeqNo for any conflict out for the given xid.  For
 * a transaction which exists but has no conflict out, InvalidSerCommitSeqNo
 * will be returned.
 */
static SerCommitSeqNo
OldSerXidGetMinConflictCommitSeqNo(TransactionId xid)
{
    TransactionId headXid;
    TransactionId tailXid;
    SerCommitSeqNo val;
    int         slotno;

    Assert(TransactionIdIsValid(xid));

    LWLockAcquire(OldSerXidLock, LW_SHARED);
    headXid = oldSerXidControl->headXid;
    tailXid = oldSerXidControl->tailXid;
    LWLockRelease(OldSerXidLock);

    if (!TransactionIdIsValid(headXid))
        return 0;

    Assert(TransactionIdIsValid(tailXid));

    if (TransactionIdPrecedes(xid, tailXid)
        || TransactionIdFollows(xid, headXid))
        return 0;

    /*
     * The following function must be called without holding OldSerXidLock,
     * but will return with that lock held, which must then be released.
     */
    slotno = SimpleLruReadPage_ReadOnly(OldSerXidSlruCtl,
                                        OldSerXidPage(xid), xid);
    val = OldSerXidValue(slotno, xid);
    LWLockRelease(OldSerXidLock);
    return val;
}

/*
 * Call this whenever there is a new xmin for active serializable
 * transactions.  We don't need to keep information on transactions which
 * precede that.  InvalidTransactionId means none active, so everything in
 * the SLRU can be discarded.
 */
static void
OldSerXidSetActiveSerXmin(TransactionId xid)
{
    LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);

    /*
     * When no sxacts are active, nothing overlaps, set the xid values to
     * invalid to show that there are no valid entries.  Don't clear headPage,
     * though.  A new xmin might still land on that page, and we don't want to
     * repeatedly zero out the same page.
     */
    if (!TransactionIdIsValid(xid))
    {
        oldSerXidControl->tailXid = InvalidTransactionId;
        oldSerXidControl->headXid = InvalidTransactionId;
        LWLockRelease(OldSerXidLock);
        return;
    }

    /*
     * When we're recovering prepared transactions, the global xmin might move
     * backwards depending on the order they're recovered. Normally that's not
     * OK, but during recovery no serializable transactions will commit, so
     * the SLRU is empty and we can get away with it.
     */
    if (RecoveryInProgress())
    {
        Assert(oldSerXidControl->headPage < 0);
        if (!TransactionIdIsValid(oldSerXidControl->tailXid)
            || TransactionIdPrecedes(xid, oldSerXidControl->tailXid))
        {
            oldSerXidControl->tailXid = xid;
        }
        LWLockRelease(OldSerXidLock);
        return;
    }

    Assert(!TransactionIdIsValid(oldSerXidControl->tailXid)
           || TransactionIdFollows(xid, oldSerXidControl->tailXid));

    oldSerXidControl->tailXid = xid;

    LWLockRelease(OldSerXidLock);
}

/*
 * Perform a checkpoint --- either during shutdown, or on-the-fly
 *
 * We don't have any data that needs to survive a restart, but this is a
 * convenient place to truncate the SLRU.
 */
void
CheckPointPredicate(void)
{
    int         tailPage;

    LWLockAcquire(OldSerXidLock, LW_EXCLUSIVE);

    /* Exit quickly if the SLRU is currently not in use. */
    if (oldSerXidControl->headPage < 0)
    {
        LWLockRelease(OldSerXidLock);
        return;
    }

    if (TransactionIdIsValid(oldSerXidControl->tailXid))
    {
        /* We can truncate the SLRU up to the page containing tailXid */
        tailPage = OldSerXidPage(oldSerXidControl->tailXid);
    }
    else
    {
        /*
         * The SLRU is no longer needed. Truncate to head before we set head
         * invalid.
         *
         * XXX: It's possible that the SLRU is not needed again until XID
         * wrap-around has happened, so that the segment containing headPage
         * that we leave behind will appear to be new again. In that case it
         * won't be removed until XID horizon advances enough to make it
         * current again.
         */
        tailPage = oldSerXidControl->headPage;
        oldSerXidControl->headPage = -1;
    }

    LWLockRelease(OldSerXidLock);

    /* Truncate away pages that are no longer required */
    SimpleLruTruncate(OldSerXidSlruCtl, tailPage);

    /*
     * Flush dirty SLRU pages to disk
     *
     * This is not actually necessary from a correctness point of view. We do
     * it merely as a debugging aid.
     *
     * We're doing this after the truncation to avoid writing pages right
     * before deleting the file in which they sit, which would be completely
     * pointless.
     */
    SimpleLruFlush(OldSerXidSlruCtl, true);
}

/*------------------------------------------------------------------------*/

/*
 * InitPredicateLocks -- Initialize the predicate locking data structures.
 *
 * This is called from CreateSharedMemoryAndSemaphores(), which see for
 * more comments.  In the normal postmaster case, the shared hash tables
 * are created here.  Backends inherit the pointers
 * to the shared tables via fork().  In the EXEC_BACKEND case, each
 * backend re-executes this code to obtain pointers to the already existing
 * shared hash tables.
 */
void
InitPredicateLocks(void)
{
    HASHCTL     info;
    int         hash_flags;
    long        max_table_size;
    Size        requestSize;
    bool        found;

    /*
     * Compute size of predicate lock target hashtable. Note these
     * calculations must agree with PredicateLockShmemSize!
     */
    max_table_size = NPREDICATELOCKTARGETENTS();

    /*
     * Allocate hash table for PREDICATELOCKTARGET structs.  This stores
     * per-predicate-lock-target information.
     */
    MemSet(&info, 0, sizeof(info));
    info.keysize = sizeof(PREDICATELOCKTARGETTAG);
    info.entrysize = sizeof(PREDICATELOCKTARGET);
    info.hash = tag_hash;
    info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
    hash_flags = (HASH_ELEM | HASH_FUNCTION | HASH_PARTITION | HASH_FIXED_SIZE);

    PredicateLockTargetHash = ShmemInitHash("PREDICATELOCKTARGET hash",
                                            max_table_size,
                                            max_table_size,
                                            &info,
                                            hash_flags);

    /* Assume an average of 2 xacts per target */
    max_table_size *= 2;

    /*
     * Reserve a dummy entry in the hash table; we use it to make sure there's
     * always one entry available when we need to split or combine a page,
     * because running out of space there could mean aborting a
     * non-serializable transaction.
     */
    hash_search(PredicateLockTargetHash, &ScratchTargetTag, HASH_ENTER, NULL);

    /*
     * Allocate hash table for PREDICATELOCK structs.  This stores per
     * xact-lock-of-a-target information.
     */
    MemSet(&info, 0, sizeof(info));
    info.keysize = sizeof(PREDICATELOCKTAG);
    info.entrysize = sizeof(PREDICATELOCK);
    info.hash = predicatelock_hash;
    info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;
    hash_flags = (HASH_ELEM | HASH_FUNCTION | HASH_PARTITION | HASH_FIXED_SIZE);

    PredicateLockHash = ShmemInitHash("PREDICATELOCK hash",
                                      max_table_size,
                                      max_table_size,
                                      &info,
                                      hash_flags);

    /*
     * Compute size for serializable transaction hashtable. Note these
     * calculations must agree with PredicateLockShmemSize!
     */
    max_table_size = (MaxBackends + max_prepared_xacts);

    /*
     * Allocate a list to hold information on transactions participating in
     * predicate locking.
     *
     * Assume an average of 10 predicate locking transactions per backend.
     * This allows aggressive cleanup while detail is present before data must
     * be summarized for storage in SLRU and the "dummy" transaction.
     */
    max_table_size *= 10;

    PredXact = ShmemInitStruct("PredXactList",
                               PredXactListDataSize,
                               &found);
    if (!found)
    {
        int         i;

        SHMQueueInit(&PredXact->availableList);
        SHMQueueInit(&PredXact->activeList);
        PredXact->SxactGlobalXmin = InvalidTransactionId;
        PredXact->SxactGlobalXminCount = 0;
        PredXact->WritableSxactCount = 0;
        PredXact->LastSxactCommitSeqNo = FirstNormalSerCommitSeqNo - 1;
        PredXact->CanPartialClearThrough = 0;
        PredXact->HavePartialClearedThrough = 0;
        requestSize = mul_size((Size) max_table_size,
                               PredXactListElementDataSize);
        PredXact->element = ShmemAlloc(requestSize);
        if (PredXact->element == NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
             errmsg("not enough shared memory for elements of data structure"
                    " \"%s\" (%lu bytes requested)",
                    "PredXactList", (unsigned long) requestSize)));
        /* Add all elements to available list, clean. */
        memset(PredXact->element, 0, requestSize);
        for (i = 0; i < max_table_size; i++)
        {
            SHMQueueInsertBefore(&(PredXact->availableList),
                                 &(PredXact->element[i].link));
        }
        PredXact->OldCommittedSxact = CreatePredXact();
        SetInvalidVirtualTransactionId(PredXact->OldCommittedSxact->vxid);
        PredXact->OldCommittedSxact->prepareSeqNo = 0;
        PredXact->OldCommittedSxact->commitSeqNo = 0;
        PredXact->OldCommittedSxact->SeqNo.lastCommitBeforeSnapshot = 0;
        SHMQueueInit(&PredXact->OldCommittedSxact->outConflicts);
        SHMQueueInit(&PredXact->OldCommittedSxact->inConflicts);
        SHMQueueInit(&PredXact->OldCommittedSxact->predicateLocks);
        SHMQueueInit(&PredXact->OldCommittedSxact->finishedLink);
        SHMQueueInit(&PredXact->OldCommittedSxact->possibleUnsafeConflicts);
        PredXact->OldCommittedSxact->topXid = InvalidTransactionId;
        PredXact->OldCommittedSxact->finishedBefore = InvalidTransactionId;
        PredXact->OldCommittedSxact->xmin = InvalidTransactionId;
        PredXact->OldCommittedSxact->flags = SXACT_FLAG_COMMITTED;
        PredXact->OldCommittedSxact->pid = 0;
    }
    /* This never changes, so let's keep a local copy. */
    OldCommittedSxact = PredXact->OldCommittedSxact;

    /*
     * Allocate hash table for SERIALIZABLEXID structs.  This stores per-xid
     * information for serializable transactions which have accessed data.
     */
    MemSet(&info, 0, sizeof(info));
    info.keysize = sizeof(SERIALIZABLEXIDTAG);
    info.entrysize = sizeof(SERIALIZABLEXID);
    info.hash = tag_hash;
    hash_flags = (HASH_ELEM | HASH_FUNCTION | HASH_FIXED_SIZE);

    SerializableXidHash = ShmemInitHash("SERIALIZABLEXID hash",
                                        max_table_size,
                                        max_table_size,
                                        &info,
                                        hash_flags);

    /*
     * Allocate space for tracking rw-conflicts in lists attached to the
     * transactions.
     *
     * Assume an average of 5 conflicts per transaction.  Calculations suggest
     * that this will prevent resource exhaustion in even the most pessimal
     * loads up to max_connections = 200 with all 200 connections pounding the
     * database with serializable transactions.  Beyond that, there may be
     * occasional transactions canceled when trying to flag conflicts. That's
     * probably OK.
     */
    max_table_size *= 5;

    RWConflictPool = ShmemInitStruct("RWConflictPool",
                                     RWConflictPoolHeaderDataSize,
                                     &found);
    if (!found)
    {
        int         i;

        SHMQueueInit(&RWConflictPool->availableList);
        requestSize = mul_size((Size) max_table_size,
                               RWConflictDataSize);
        RWConflictPool->element = ShmemAlloc(requestSize);
        if (RWConflictPool->element == NULL)
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
             errmsg("not enough shared memory for elements of data structure"
                    " \"%s\" (%lu bytes requested)",
                    "RWConflictPool", (unsigned long) requestSize)));
        /* Add all elements to available list, clean. */
        memset(RWConflictPool->element, 0, requestSize);
        for (i = 0; i < max_table_size; i++)
        {
            SHMQueueInsertBefore(&(RWConflictPool->availableList),
                                 &(RWConflictPool->element[i].outLink));
        }
    }

    /*
     * Create or attach to the header for the list of finished serializable
     * transactions.
     */
    FinishedSerializableTransactions = (SHM_QUEUE *)
        ShmemInitStruct("FinishedSerializableTransactions",
                        sizeof(SHM_QUEUE),
                        &found);
    if (!found)
        SHMQueueInit(FinishedSerializableTransactions);

    /*
     * Initialize the SLRU storage for old committed serializable
     * transactions.
     */
    OldSerXidInit();

    /* Pre-calculate the hash and partition lock of the scratch entry */
    ScratchTargetTagHash = PredicateLockTargetTagHashCode(&ScratchTargetTag);
    ScratchPartitionLock = PredicateLockHashPartitionLock(ScratchTargetTagHash);
}

/*
 * Estimate shared-memory space used for predicate lock table
 */
Size
PredicateLockShmemSize(void)
{
    Size        size = 0;
    long        max_table_size;

    /* predicate lock target hash table */
    max_table_size = NPREDICATELOCKTARGETENTS();
    size = add_size(size, hash_estimate_size(max_table_size,
                                             sizeof(PREDICATELOCKTARGET)));

    /* predicate lock hash table */
    max_table_size *= 2;
    size = add_size(size, hash_estimate_size(max_table_size,
                                             sizeof(PREDICATELOCK)));

    /*
     * Since NPREDICATELOCKTARGETENTS is only an estimate, add 10% safety
     * margin.
     */
    size = add_size(size, size / 10);

    /* transaction list */
    max_table_size = MaxBackends + max_prepared_xacts;
    max_table_size *= 10;
    size = add_size(size, PredXactListDataSize);
    size = add_size(size, mul_size((Size) max_table_size,
                                   PredXactListElementDataSize));

    /* transaction xid table */
    size = add_size(size, hash_estimate_size(max_table_size,
                                             sizeof(SERIALIZABLEXID)));

    /* rw-conflict pool */
    max_table_size *= 5;
    size = add_size(size, RWConflictPoolHeaderDataSize);
    size = add_size(size, mul_size((Size) max_table_size,
                                   RWConflictDataSize));

    /* Head for list of finished serializable transactions. */
    size = add_size(size, sizeof(SHM_QUEUE));

    /* Shared memory structures for SLRU tracking of old committed xids. */
    size = add_size(size, sizeof(OldSerXidControlData));
    size = add_size(size, SimpleLruShmemSize(NUM_OLDSERXID_BUFFERS, 0));

    return size;
}


/*
 * Compute the hash code associated with a PREDICATELOCKTAG.
 *
 * Because we want to use just one set of partition locks for both the
 * PREDICATELOCKTARGET and PREDICATELOCK hash tables, we have to make sure
 * that PREDICATELOCKs fall into the same partition number as their
 * associated PREDICATELOCKTARGETs.  dynahash.c expects the partition number
 * to be the low-order bits of the hash code, and therefore a
 * PREDICATELOCKTAG's hash code must have the same low-order bits as the
 * associated PREDICATELOCKTARGETTAG's hash code.  We achieve this with this
 * specialized hash function.
 */
static uint32
predicatelock_hash(const void *key, Size keysize)
{
    const PREDICATELOCKTAG *predicatelocktag = (const PREDICATELOCKTAG *) key;
    uint32      targethash;

    Assert(keysize == sizeof(PREDICATELOCKTAG));

    /* Look into the associated target object, and compute its hash code */
    targethash = PredicateLockTargetTagHashCode(&predicatelocktag->myTarget->tag);

    return PredicateLockHashCodeFromTargetHashCode(predicatelocktag, targethash);
}


/*
 * GetPredicateLockStatusData
 *      Return a table containing the internal state of the predicate
 *      lock manager for use in pg_lock_status.
 *
 * Like GetLockStatusData, this function tries to hold the partition LWLocks
 * for as short a time as possible by returning two arrays that simply
 * contain the PREDICATELOCKTARGETTAG and SERIALIZABLEXACT for each lock
 * table entry. Multiple copies of the same PREDICATELOCKTARGETTAG and
 * SERIALIZABLEXACT will likely appear.
 */
PredicateLockData *
GetPredicateLockStatusData(void)
{
    PredicateLockData *data;
    int         i;
    int         els,
                el;
    HASH_SEQ_STATUS seqstat;
    PREDICATELOCK *predlock;

    data = (PredicateLockData *) palloc(sizeof(PredicateLockData));

    /*
     * To ensure consistency, take simultaneous locks on all partition locks
     * in ascending order, then SerializableXactHashLock.
     */
    for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
        LWLockAcquire(FirstPredicateLockMgrLock + i, LW_SHARED);
    LWLockAcquire(SerializableXactHashLock, LW_SHARED);

    /* Get number of locks and allocate appropriately-sized arrays. */
    els = hash_get_num_entries(PredicateLockHash);
    data->nelements = els;
    data->locktags = (PREDICATELOCKTARGETTAG *)
        palloc(sizeof(PREDICATELOCKTARGETTAG) * els);
    data->xacts = (SERIALIZABLEXACT *)
        palloc(sizeof(SERIALIZABLEXACT) * els);


    /* Scan through PredicateLockHash and copy contents */
    hash_seq_init(&seqstat, PredicateLockHash);

    el = 0;

    while ((predlock = (PREDICATELOCK *) hash_seq_search(&seqstat)))
    {
        data->locktags[el] = predlock->tag.myTarget->tag;
        data->xacts[el] = *predlock->tag.myXact;
        el++;
    }

    Assert(el == els);

    /* Release locks in reverse order */
    LWLockRelease(SerializableXactHashLock);
    for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
        LWLockRelease(FirstPredicateLockMgrLock + i);

    return data;
}

/*
 * Free up shared memory structures by pushing the oldest sxact (the one at
 * the front of the SummarizeOldestCommittedSxact queue) into summary form.
 * Each call will free exactly one SERIALIZABLEXACT structure and may also
 * free one or more of these structures: SERIALIZABLEXID, PREDICATELOCK,
 * PREDICATELOCKTARGET, RWConflictData.
 */
static void
SummarizeOldestCommittedSxact(void)
{
    SERIALIZABLEXACT *sxact;

    LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);

    /*
     * This function is only called if there are no sxact slots available.
     * Some of them must belong to old, already-finished transactions, so
     * there should be something in FinishedSerializableTransactions list that
     * we can summarize. However, there's a race condition: while we were not
     * holding any locks, a transaction might have ended and cleaned up all
     * the finished sxact entries already, freeing up their sxact slots. In
     * that case, we have nothing to do here. The caller will find one of the
     * slots released by the other backend when it retries.
     */
    if (SHMQueueEmpty(FinishedSerializableTransactions))
    {
        LWLockRelease(SerializableFinishedListLock);
        return;
    }

    /*
     * Grab the first sxact off the finished list -- this will be the earliest
     * commit.  Remove it from the list.
     */
    sxact = (SERIALIZABLEXACT *)
        SHMQueueNext(FinishedSerializableTransactions,
                     FinishedSerializableTransactions,
                     offsetof(SERIALIZABLEXACT, finishedLink));
    SHMQueueDelete(&(sxact->finishedLink));

    /* Add to SLRU summary information. */
    if (TransactionIdIsValid(sxact->topXid) && !SxactIsReadOnly(sxact))
        OldSerXidAdd(sxact->topXid, SxactHasConflictOut(sxact)
           ? sxact->SeqNo.earliestOutConflictCommit : InvalidSerCommitSeqNo);

    /* Summarize and release the detail. */
    ReleaseOneSerializableXact(sxact, false, true);

    LWLockRelease(SerializableFinishedListLock);
}

/*
 * GetSafeSnapshot
 *      Obtain and register a snapshot for a READ ONLY DEFERRABLE
 *      transaction. Ensures that the snapshot is "safe", i.e. a
 *      read-only transaction running on it can execute serializably
 *      without further checks. This requires waiting for concurrent
 *      transactions to complete, and retrying with a new snapshot if
 *      one of them could possibly create a conflict.
 *
 *      As with GetSerializableTransactionSnapshot (which this is a subroutine
 *      for), the passed-in Snapshot pointer should reference a static data
 *      area that can safely be passed to GetSnapshotData.
 */
static Snapshot
GetSafeSnapshot(Snapshot origSnapshot)
{
    Snapshot    snapshot;

    Assert(XactReadOnly && XactDeferrable);

    while (true)
    {
        /*
         * GetSerializableTransactionSnapshotInt is going to call
         * GetSnapshotData, so we need to provide it the static snapshot area
         * our caller passed to us.  The pointer returned is actually the same
         * one passed to it, but we avoid assuming that here.
         */
        snapshot = GetSerializableTransactionSnapshotInt(origSnapshot,
                                                       InvalidTransactionId);

        if (MySerializableXact == InvalidSerializableXact)
            return snapshot;    /* no concurrent r/w xacts; it's safe */

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

        /*
         * Wait for concurrent transactions to finish. Stop early if one of
         * them marked us as conflicted.
         */
        MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;
        while (!(SHMQueueEmpty(&MySerializableXact->possibleUnsafeConflicts) ||
                 SxactIsROUnsafe(MySerializableXact)))
        {
            LWLockRelease(SerializableXactHashLock);
            ProcWaitForSignal();
            LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
        }
        MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;

        if (!SxactIsROUnsafe(MySerializableXact))
        {
            LWLockRelease(SerializableXactHashLock);
            break;              /* success */
        }

        LWLockRelease(SerializableXactHashLock);

        /* else, need to retry... */
        ereport(DEBUG2,
                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                 errmsg("deferrable snapshot was unsafe; trying a new one")));
        ReleasePredicateLocks(false);
    }

    /*
     * Now we have a safe snapshot, so we don't need to do any further checks.
     */
    Assert(SxactIsROSafe(MySerializableXact));
    ReleasePredicateLocks(false);

    return snapshot;
}

/*
 * Acquire a snapshot that can be used for the current transaction.
 *
 * Make sure we have a SERIALIZABLEXACT reference in MySerializableXact.
 * It should be current for this process and be contained in PredXact.
 *
 * The passed-in Snapshot pointer should reference a static data area that
 * can safely be passed to GetSnapshotData.  The return value is actually
 * always this same pointer; no new snapshot data structure is allocated
 * within this function.
 */
Snapshot
GetSerializableTransactionSnapshot(Snapshot snapshot)
{
    Assert(IsolationIsSerializable());

    /*
     * Can't use serializable mode while recovery is still active, as it is,
     * for example, on a hot standby.  We could get here despite the check
     * in check_XactIsoLevel() if default_transaction_isolation is set to
     * serializable, so phrase the hint accordingly.
     */
    if (RecoveryInProgress())
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot use serializable mode in a hot standby"),
                 errdetail("\"default_transaction_isolation\" is set to \"serializable\"."),
                 errhint("You can use \"SET default_transaction_isolation = 'repeatable read'\" to change the default.")));

    /*
     * A special optimization is available for SERIALIZABLE READ ONLY
     * DEFERRABLE transactions -- we can wait for a suitable snapshot and
     * thereby avoid all SSI overhead once it's running.
     */
    if (XactReadOnly && XactDeferrable)
        return GetSafeSnapshot(snapshot);

    return GetSerializableTransactionSnapshotInt(snapshot,
                                                 InvalidTransactionId);
}

/*
 * Import a snapshot to be used for the current transaction.
 *
 * This is nearly the same as GetSerializableTransactionSnapshot, except that
 * we don't take a new snapshot, but rather use the data we're handed.
 *
 * The caller must have verified that the snapshot came from a serializable
 * transaction; and if we're read-write, the source transaction must not be
 * read-only.
 */
void
SetSerializableTransactionSnapshot(Snapshot snapshot,
                                   TransactionId sourcexid)
{
    Assert(IsolationIsSerializable());

    /*
     * We do not allow SERIALIZABLE READ ONLY DEFERRABLE transactions to
     * import snapshots, since there's no way to wait for a safe snapshot when
     * we're using the snap we're told to.  (XXX instead of throwing an error,
     * we could just ignore the XactDeferrable flag?)
     */
    if (XactReadOnly && XactDeferrable)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("a snapshot-importing transaction must not be READ ONLY DEFERRABLE")));

    (void) GetSerializableTransactionSnapshotInt(snapshot, sourcexid);
}

/*
 * Guts of GetSerializableTransactionSnapshot
 *
 * If sourcexid is valid, this is actually an import operation and we should
 * skip calling GetSnapshotData, because the snapshot contents are already
 * loaded up.  HOWEVER: to avoid race conditions, we must check that the
 * source xact is still running after we acquire SerializableXactHashLock.
 * We do that by calling ProcArrayInstallImportedXmin.
 */
static Snapshot
GetSerializableTransactionSnapshotInt(Snapshot snapshot,
                                      TransactionId sourcexid)
{
    PGPROC     *proc;
    VirtualTransactionId vxid;
    SERIALIZABLEXACT *sxact,
               *othersxact;
    HASHCTL     hash_ctl;

    /* We only do this for serializable transactions.  Once. */
    Assert(MySerializableXact == InvalidSerializableXact);

    Assert(!RecoveryInProgress());

    proc = MyProc;
    Assert(proc != NULL);
    GET_VXID_FROM_PGPROC(vxid, *proc);

    /*
     * First we get the sxact structure, which may involve looping and access
     * to the "finished" list to free a structure for use.
     *
     * We must hold SerializableXactHashLock when taking/checking the snapshot
     * to avoid race conditions, for much the same reasons that
     * GetSnapshotData takes the ProcArrayLock.  Since we might have to
     * release SerializableXactHashLock to call SummarizeOldestCommittedSxact,
     * this means we have to create the sxact first, which is a bit annoying
     * (in particular, an elog(ERROR) in procarray.c would cause us to leak
     * the sxact).  Consider refactoring to avoid this.
     */
#ifdef TEST_OLDSERXID
    SummarizeOldestCommittedSxact();
#endif
    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
    do
    {
        sxact = CreatePredXact();
        /* If null, push out committed sxact to SLRU summary & retry. */
        if (!sxact)
        {
            LWLockRelease(SerializableXactHashLock);
            SummarizeOldestCommittedSxact();
            LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
        }
    } while (!sxact);

    /* Get the snapshot, or check that it's safe to use */
    if (!TransactionIdIsValid(sourcexid))
        snapshot = GetSnapshotData(snapshot);
    else if (!ProcArrayInstallImportedXmin(snapshot->xmin, sourcexid))
    {
        ReleasePredXact(sxact);
        LWLockRelease(SerializableXactHashLock);
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not import the requested snapshot"),
               errdetail("The source transaction %u is not running anymore.",
                         sourcexid)));
    }

    /*
     * If there are no serializable transactions which are not read-only, we
     * can "opt out" of predicate locking and conflict checking for a
     * read-only transaction.
     *
     * The reason this is safe is that a read-only transaction can only become
     * part of a dangerous structure if it overlaps a writable transaction
     * which in turn overlaps a writable transaction which committed before
     * the read-only transaction started.  A new writable transaction can
     * overlap this one, but it can't meet the other condition of overlapping
     * a transaction which committed before this one started.
     */
    if (XactReadOnly && PredXact->WritableSxactCount == 0)
    {
        ReleasePredXact(sxact);
        LWLockRelease(SerializableXactHashLock);
        return snapshot;
    }

    /* Maintain serializable global xmin info. */
    if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
    {
        Assert(PredXact->SxactGlobalXminCount == 0);
        PredXact->SxactGlobalXmin = snapshot->xmin;
        PredXact->SxactGlobalXminCount = 1;
        OldSerXidSetActiveSerXmin(snapshot->xmin);
    }
    else if (TransactionIdEquals(snapshot->xmin, PredXact->SxactGlobalXmin))
    {
        Assert(PredXact->SxactGlobalXminCount > 0);
        PredXact->SxactGlobalXminCount++;
    }
    else
    {
        Assert(TransactionIdFollows(snapshot->xmin, PredXact->SxactGlobalXmin));
    }

    /* Initialize the structure. */
    sxact->vxid = vxid;
    sxact->SeqNo.lastCommitBeforeSnapshot = PredXact->LastSxactCommitSeqNo;
    sxact->prepareSeqNo = InvalidSerCommitSeqNo;
    sxact->commitSeqNo = InvalidSerCommitSeqNo;
    SHMQueueInit(&(sxact->outConflicts));
    SHMQueueInit(&(sxact->inConflicts));
    SHMQueueInit(&(sxact->possibleUnsafeConflicts));
    sxact->topXid = GetTopTransactionIdIfAny();
    sxact->finishedBefore = InvalidTransactionId;
    sxact->xmin = snapshot->xmin;
    sxact->pid = MyProcPid;
    SHMQueueInit(&(sxact->predicateLocks));
    SHMQueueElemInit(&(sxact->finishedLink));
    sxact->flags = 0;
    if (XactReadOnly)
    {
        sxact->flags |= SXACT_FLAG_READ_ONLY;

        /*
         * Register all concurrent r/w transactions as possible conflicts; if
         * all of them commit without any outgoing conflicts to earlier
         * transactions then this snapshot can be deemed safe (and we can run
         * without tracking predicate locks).
         */
        for (othersxact = FirstPredXact();
             othersxact != NULL;
             othersxact = NextPredXact(othersxact))
        {
            if (!SxactIsCommitted(othersxact)
                && !SxactIsDoomed(othersxact)
                && !SxactIsReadOnly(othersxact))
            {
                SetPossibleUnsafeConflict(sxact, othersxact);
            }
        }
    }
    else
    {
        ++(PredXact->WritableSxactCount);
        Assert(PredXact->WritableSxactCount <=
               (MaxBackends + max_prepared_xacts));
    }

    MySerializableXact = sxact;
    MyXactDidWrite = false;     /* haven't written anything yet */

    LWLockRelease(SerializableXactHashLock);

    /* Initialize the backend-local hash table of parent locks */
    Assert(LocalPredicateLockHash == NULL);
    MemSet(&hash_ctl, 0, sizeof(hash_ctl));
    hash_ctl.keysize = sizeof(PREDICATELOCKTARGETTAG);
    hash_ctl.entrysize = sizeof(LOCALPREDICATELOCK);
    hash_ctl.hash = tag_hash;
    LocalPredicateLockHash = hash_create("Local predicate lock",
                                         max_predicate_locks_per_xact,
                                         &hash_ctl,
                                         HASH_ELEM | HASH_FUNCTION);

    return snapshot;
}

/*
 * Register the top level XID in SerializableXidHash.
 * Also store it for easy reference in MySerializableXact.
 */
void
RegisterPredicateLockingXid(TransactionId xid)
{
    SERIALIZABLEXIDTAG sxidtag;
    SERIALIZABLEXID *sxid;
    bool        found;

    /*
     * If we're not tracking predicate lock data for this transaction, we
     * should ignore the request and return quickly.
     */
    if (MySerializableXact == InvalidSerializableXact)
        return;

    /* We should have a valid XID and be at the top level. */
    Assert(TransactionIdIsValid(xid));

    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

    /* This should only be done once per transaction. */
    Assert(MySerializableXact->topXid == InvalidTransactionId);

    MySerializableXact->topXid = xid;

    sxidtag.xid = xid;
    sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
                                           &sxidtag,
                                           HASH_ENTER, &found);
    Assert(!found);

    /* Initialize the structure. */
    sxid->myXact = MySerializableXact;
    LWLockRelease(SerializableXactHashLock);
}


/*
 * Check whether there are any predicate locks held by any transaction
 * for the page at the given block number.
 *
 * Note that the transaction may be completed but not yet subject to
 * cleanup due to overlapping serializable transactions.  This must
 * return valid information regardless of transaction isolation level.
 *
 * Also note that this doesn't check for a conflicting relation lock,
 * just a lock specifically on the given page.
 *
 * One use is to support proper behavior during GiST index vacuum.
 */
bool
PageIsPredicateLocked(Relation relation, BlockNumber blkno)
{
    PREDICATELOCKTARGETTAG targettag;
    uint32      targettaghash;
    LWLockId    partitionLock;
    PREDICATELOCKTARGET *target;

    SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
                                    relation->rd_node.dbNode,
                                    relation->rd_id,
                                    blkno);

    targettaghash = PredicateLockTargetTagHashCode(&targettag);
    partitionLock = PredicateLockHashPartitionLock(targettaghash);
    LWLockAcquire(partitionLock, LW_SHARED);
    target = (PREDICATELOCKTARGET *)
        hash_search_with_hash_value(PredicateLockTargetHash,
                                    &targettag, targettaghash,
                                    HASH_FIND, NULL);
    LWLockRelease(partitionLock);

    return (target != NULL);
}


/*
 * Check whether a particular lock is held by this transaction.
 *
 * Important note: this function may return false even if the lock is
 * being held, because it uses the local lock table which is not
 * updated if another transaction modifies our lock list (e.g. to
 * split an index page). It can also return true when a coarser
 * granularity lock that covers this target is being held. Be careful
 * to only use this function in circumstances where such errors are
 * acceptable!
 */
static bool
PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag)
{
    LOCALPREDICATELOCK *lock;

    /* check local hash table */
    lock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
                                              targettag,
                                              HASH_FIND, NULL);

    if (!lock)
        return false;

    /*
     * Found entry in the table, but still need to check whether it's actually
     * held -- it could just be a parent of some held lock.
     */
    return lock->held;
}

/*
 * Return the parent lock tag in the lock hierarchy: the next coarser
 * lock that covers the provided tag.
 *
 * Returns true and sets *parent to the parent tag if one exists,
 * returns false if none exists.
 */
static bool
GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
                          PREDICATELOCKTARGETTAG *parent)
{
    switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
    {
        case PREDLOCKTAG_RELATION:
            /* relation locks have no parent lock */
            return false;

        case PREDLOCKTAG_PAGE:
            /* parent lock is relation lock */
            SET_PREDICATELOCKTARGETTAG_RELATION(*parent,
                                         GET_PREDICATELOCKTARGETTAG_DB(*tag),
                                  GET_PREDICATELOCKTARGETTAG_RELATION(*tag));

            return true;

        case PREDLOCKTAG_TUPLE:
            /* parent lock is page lock */
            SET_PREDICATELOCKTARGETTAG_PAGE(*parent,
                                         GET_PREDICATELOCKTARGETTAG_DB(*tag),
                                   GET_PREDICATELOCKTARGETTAG_RELATION(*tag),
                                      GET_PREDICATELOCKTARGETTAG_PAGE(*tag));
            return true;
    }

    /* not reachable */
    Assert(false);
    return false;
}

/*
 * Check whether the lock we are considering is already covered by a
 * coarser lock for our transaction.
 *
 * Like PredicateLockExists, this function might return a false
 * negative, but it will never return a false positive.
 */
static bool
CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag)
{
    PREDICATELOCKTARGETTAG targettag,
                parenttag;

    targettag = *newtargettag;

    /* check parents iteratively until no more */
    while (GetParentPredicateLockTag(&targettag, &parenttag))
    {
        targettag = parenttag;
        if (PredicateLockExists(&targettag))
            return true;
    }

    /* no more parents to check; lock is not covered */
    return false;
}

/*
 * Remove the dummy entry from the predicate lock target hash, to free up some
 * scratch space. The caller must be holding SerializablePredicateLockListLock,
 * and must restore the entry with RestoreScratchTarget() before releasing the
 * lock.
 *
 * If lockheld is true, the caller is already holding the partition lock
 * of the partition containing the scratch entry.
 */
static void
RemoveScratchTarget(bool lockheld)
{
    bool        found;

    Assert(LWLockHeldByMe(SerializablePredicateLockListLock));

    if (!lockheld)
        LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
    hash_search_with_hash_value(PredicateLockTargetHash,
                                &ScratchTargetTag,
                                ScratchTargetTagHash,
                                HASH_REMOVE, &found);
    Assert(found);
    if (!lockheld)
        LWLockRelease(ScratchPartitionLock);
}

/*
 * Re-insert the dummy entry in predicate lock target hash.
 */
static void
RestoreScratchTarget(bool lockheld)
{
    bool        found;

    Assert(LWLockHeldByMe(SerializablePredicateLockListLock));

    if (!lockheld)
        LWLockAcquire(ScratchPartitionLock, LW_EXCLUSIVE);
    hash_search_with_hash_value(PredicateLockTargetHash,
                                &ScratchTargetTag,
                                ScratchTargetTagHash,
                                HASH_ENTER, &found);
    Assert(!found);
    if (!lockheld)
        LWLockRelease(ScratchPartitionLock);
}

/*
 * Check whether the list of related predicate locks is empty for a
 * predicate lock target, and remove the target if it is.
 */
static void
RemoveTargetIfNoLongerUsed(PREDICATELOCKTARGET *target, uint32 targettaghash)
{
    PREDICATELOCKTARGET *rmtarget PG_USED_FOR_ASSERTS_ONLY;

    Assert(LWLockHeldByMe(SerializablePredicateLockListLock));

    /* Can't remove it until no locks at this target. */
    if (!SHMQueueEmpty(&target->predicateLocks))
        return;

    /* Actually remove the target. */
    rmtarget = hash_search_with_hash_value(PredicateLockTargetHash,
                                           &target->tag,
                                           targettaghash,
                                           HASH_REMOVE, NULL);
    Assert(rmtarget == target);
}

/*
 * Delete child target locks owned by this process.
 * This implementation is assuming that the usage of each target tag field
 * is uniform.  No need to make this hard if we don't have to.
 *
 * We aren't acquiring lightweight locks for the predicate lock or lock
 * target structures associated with this transaction unless we're going
 * to modify them, because no other process is permitted to modify our
 * locks.
 */
static void
DeleteChildTargetLocks(const PREDICATELOCKTARGETTAG *newtargettag)
{
    SERIALIZABLEXACT *sxact;
    PREDICATELOCK *predlock;

    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
    sxact = MySerializableXact;
    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(sxact->predicateLocks),
                     &(sxact->predicateLocks),
                     offsetof(PREDICATELOCK, xactLink));
    while (predlock)
    {
        SHM_QUEUE  *predlocksxactlink;
        PREDICATELOCK *nextpredlock;
        PREDICATELOCKTAG oldlocktag;
        PREDICATELOCKTARGET *oldtarget;
        PREDICATELOCKTARGETTAG oldtargettag;

        predlocksxactlink = &(predlock->xactLink);
        nextpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(sxact->predicateLocks),
                         predlocksxactlink,
                         offsetof(PREDICATELOCK, xactLink));

        oldlocktag = predlock->tag;
        Assert(oldlocktag.myXact == sxact);
        oldtarget = oldlocktag.myTarget;
        oldtargettag = oldtarget->tag;

        if (TargetTagIsCoveredBy(oldtargettag, *newtargettag))
        {
            uint32      oldtargettaghash;
            LWLockId    partitionLock;
            PREDICATELOCK *rmpredlock PG_USED_FOR_ASSERTS_ONLY;

            oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
            partitionLock = PredicateLockHashPartitionLock(oldtargettaghash);

            LWLockAcquire(partitionLock, LW_EXCLUSIVE);

            SHMQueueDelete(predlocksxactlink);
            SHMQueueDelete(&(predlock->targetLink));
            rmpredlock = hash_search_with_hash_value
                (PredicateLockHash,
                 &oldlocktag,
                 PredicateLockHashCodeFromTargetHashCode(&oldlocktag,
                                                         oldtargettaghash),
                 HASH_REMOVE, NULL);
            Assert(rmpredlock == predlock);

            RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);

            LWLockRelease(partitionLock);

            DecrementParentLocks(&oldtargettag);
        }

        predlock = nextpredlock;
    }
    LWLockRelease(SerializablePredicateLockListLock);
}

/*
 * Returns the promotion threshold for a given predicate lock
 * target. This is the number of descendant locks required to promote
 * to the specified tag. Note that the threshold includes non-direct
 * descendants, e.g. both tuples and pages for a relation lock.
 *
 * TODO SSI: We should do something more intelligent about what the
 * thresholds are, either making it proportional to the number of
 * tuples in a page & pages in a relation, or at least making it a
 * GUC. Currently the threshold is 3 for a page lock, and
 * max_pred_locks_per_transaction/2 for a relation lock, chosen
 * entirely arbitrarily (and without benchmarking).
 */
static int
PredicateLockPromotionThreshold(const PREDICATELOCKTARGETTAG *tag)
{
    switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
    {
        case PREDLOCKTAG_RELATION:
            return max_predicate_locks_per_xact / 2;

        case PREDLOCKTAG_PAGE:
            return 3;

        case PREDLOCKTAG_TUPLE:

            /*
             * not reachable: nothing is finer-granularity than a tuple, so we
             * should never try to promote to it.
             */
            Assert(false);
            return 0;
    }

    /* not reachable */
    Assert(false);
    return 0;
}

/*
 * For all ancestors of a newly-acquired predicate lock, increment
 * their child count in the parent hash table. If any of them have
 * more descendants than their promotion threshold, acquire the
 * coarsest such lock.
 *
 * Returns true if a parent lock was acquired and false otherwise.
 */
static bool
CheckAndPromotePredicateLockRequest(const PREDICATELOCKTARGETTAG *reqtag)
{
    PREDICATELOCKTARGETTAG targettag,
                nexttag,
                promotiontag;
    LOCALPREDICATELOCK *parentlock;
    bool        found,
                promote;

    promote = false;

    targettag = *reqtag;

    /* check parents iteratively */
    while (GetParentPredicateLockTag(&targettag, &nexttag))
    {
        targettag = nexttag;
        parentlock = (LOCALPREDICATELOCK *) hash_search(LocalPredicateLockHash,
                                                        &targettag,
                                                        HASH_ENTER,
                                                        &found);
        if (!found)
        {
            parentlock->held = false;
            parentlock->childLocks = 1;
        }
        else
            parentlock->childLocks++;

        if (parentlock->childLocks >=
            PredicateLockPromotionThreshold(&targettag))
        {
            /*
             * We should promote to this parent lock. Continue to check its
             * ancestors, however, both to get their child counts right and to
             * check whether we should just go ahead and promote to one of
             * them.
             */
            promotiontag = targettag;
            promote = true;
        }
    }

    if (promote)
    {
        /* acquire coarsest ancestor eligible for promotion */
        PredicateLockAcquire(&promotiontag);
        return true;
    }
    else
        return false;
}

/*
 * When releasing a lock, decrement the child count on all ancestor
 * locks.
 *
 * This is called only when releasing a lock via
 * DeleteChildTargetLocks (i.e. when a lock becomes redundant because
 * we've acquired its parent, possibly due to promotion) or when a new
 * MVCC write lock makes the predicate lock unnecessary. There's no
 * point in calling it when locks are released at transaction end, as
 * this information is no longer needed.
 */
static void
DecrementParentLocks(const PREDICATELOCKTARGETTAG *targettag)
{
    PREDICATELOCKTARGETTAG parenttag,
                nexttag;

    parenttag = *targettag;

    while (GetParentPredicateLockTag(&parenttag, &nexttag))
    {
        uint32      targettaghash;
        LOCALPREDICATELOCK *parentlock,
                   *rmlock PG_USED_FOR_ASSERTS_ONLY;

        parenttag = nexttag;
        targettaghash = PredicateLockTargetTagHashCode(&parenttag);
        parentlock = (LOCALPREDICATELOCK *)
            hash_search_with_hash_value(LocalPredicateLockHash,
                                        &parenttag, targettaghash,
                                        HASH_FIND, NULL);

        /*
         * There's a small chance the parent lock doesn't exist in the lock
         * table. This can happen if we prematurely removed it because an
         * index split caused the child refcount to be off.
         */
        if (parentlock == NULL)
            continue;

        parentlock->childLocks--;

        /*
         * Under similar circumstances the parent lock's refcount might be
         * zero. This only happens if we're holding that lock (otherwise we
         * would have removed the entry).
         */
        if (parentlock->childLocks < 0)
        {
            Assert(parentlock->held);
            parentlock->childLocks = 0;
        }

        if ((parentlock->childLocks == 0) && (!parentlock->held))
        {
            rmlock = (LOCALPREDICATELOCK *)
                hash_search_with_hash_value(LocalPredicateLockHash,
                                            &parenttag, targettaghash,
                                            HASH_REMOVE, NULL);
            Assert(rmlock == parentlock);
        }
    }
}

/*
 * Indicate that a predicate lock on the given target is held by the
 * specified transaction. Has no effect if the lock is already held.
 *
 * This updates the lock table and the sxact's lock list, and creates
 * the lock target if necessary, but does *not* do anything related to
 * granularity promotion or the local lock table. See
 * PredicateLockAcquire for that.
 */
static void
CreatePredicateLock(const PREDICATELOCKTARGETTAG *targettag,
                    uint32 targettaghash,
                    SERIALIZABLEXACT *sxact)
{
    PREDICATELOCKTARGET *target;
    PREDICATELOCKTAG locktag;
    PREDICATELOCK *lock;
    LWLockId    partitionLock;
    bool        found;

    partitionLock = PredicateLockHashPartitionLock(targettaghash);

    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
    LWLockAcquire(partitionLock, LW_EXCLUSIVE);

    /* Make sure that the target is represented. */
    target = (PREDICATELOCKTARGET *)
        hash_search_with_hash_value(PredicateLockTargetHash,
                                    targettag, targettaghash,
                                    HASH_ENTER_NULL, &found);
    if (!target)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory"),
                 errhint("You might need to increase max_pred_locks_per_transaction.")));
    if (!found)
        SHMQueueInit(&(target->predicateLocks));

    /* We've got the sxact and target, make sure they're joined. */
    locktag.myTarget = target;
    locktag.myXact = sxact;
    lock = (PREDICATELOCK *)
        hash_search_with_hash_value(PredicateLockHash, &locktag,
            PredicateLockHashCodeFromTargetHashCode(&locktag, targettaghash),
                                    HASH_ENTER_NULL, &found);
    if (!lock)
        ereport(ERROR,
                (errcode(ERRCODE_OUT_OF_MEMORY),
                 errmsg("out of shared memory"),
                 errhint("You might need to increase max_pred_locks_per_transaction.")));

    if (!found)
    {
        SHMQueueInsertBefore(&(target->predicateLocks), &(lock->targetLink));
        SHMQueueInsertBefore(&(sxact->predicateLocks),
                             &(lock->xactLink));
        lock->commitSeqNo = InvalidSerCommitSeqNo;
    }

    LWLockRelease(partitionLock);
    LWLockRelease(SerializablePredicateLockListLock);
}

/*
 * Acquire a predicate lock on the specified target for the current
 * connection if not already held. This updates the local lock table
 * and uses it to implement granularity promotion. It will consolidate
 * multiple locks into a coarser lock if warranted, and will release
 * any finer-grained locks covered by the new one.
 */
static void
PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag)
{
    uint32      targettaghash;
    bool        found;
    LOCALPREDICATELOCK *locallock;

    /* Do we have the lock already, or a covering lock? */
    if (PredicateLockExists(targettag))
        return;

    if (CoarserLockCovers(targettag))
        return;

    /* the same hash and LW lock apply to the lock target and the local lock. */
    targettaghash = PredicateLockTargetTagHashCode(targettag);

    /* Acquire lock in local table */
    locallock = (LOCALPREDICATELOCK *)
        hash_search_with_hash_value(LocalPredicateLockHash,
                                    targettag, targettaghash,
                                    HASH_ENTER, &found);
    locallock->held = true;
    if (!found)
        locallock->childLocks = 0;

    /* Actually create the lock */
    CreatePredicateLock(targettag, targettaghash, MySerializableXact);

    /*
     * Lock has been acquired. Check whether it should be promoted to a
     * coarser granularity, or whether there are finer-granularity locks to
     * clean up.
     */
    if (CheckAndPromotePredicateLockRequest(targettag))
    {
        /*
         * Lock request was promoted to a coarser-granularity lock, and that
         * lock was acquired. It will delete this lock and any of its
         * children, so we're done.
         */
    }
    else
    {
        /* Clean up any finer-granularity locks */
        if (GET_PREDICATELOCKTARGETTAG_TYPE(*targettag) != PREDLOCKTAG_TUPLE)
            DeleteChildTargetLocks(targettag);
    }
}


/*
 *      PredicateLockRelation
 *
 * Gets a predicate lock at the relation level.
 * Skip if not in full serializable transaction isolation level.
 * Skip if this is a temporary table.
 * Clear any finer-grained predicate locks this session has on the relation.
 */
void
PredicateLockRelation(Relation relation, Snapshot snapshot)
{
    PREDICATELOCKTARGETTAG tag;

    if (!SerializationNeededForRead(relation, snapshot))
        return;

    SET_PREDICATELOCKTARGETTAG_RELATION(tag,
                                        relation->rd_node.dbNode,
                                        relation->rd_id);
    PredicateLockAcquire(&tag);
}

/*
 *      PredicateLockPage
 *
 * Gets a predicate lock at the page level.
 * Skip if not in full serializable transaction isolation level.
 * Skip if this is a temporary table.
 * Skip if a coarser predicate lock already covers this page.
 * Clear any finer-grained predicate locks this session has on the relation.
 */
void
PredicateLockPage(Relation relation, BlockNumber blkno, Snapshot snapshot)
{
    PREDICATELOCKTARGETTAG tag;

    if (!SerializationNeededForRead(relation, snapshot))
        return;

    SET_PREDICATELOCKTARGETTAG_PAGE(tag,
                                    relation->rd_node.dbNode,
                                    relation->rd_id,
                                    blkno);
    PredicateLockAcquire(&tag);
}

/*
 *      PredicateLockTuple
 *
 * Gets a predicate lock at the tuple level.
 * Skip if not in full serializable transaction isolation level.
 * Skip if this is a temporary table.
 */
void
PredicateLockTuple(Relation relation, HeapTuple tuple, Snapshot snapshot)
{
    PREDICATELOCKTARGETTAG tag;
    ItemPointer tid;
    TransactionId targetxmin;

    if (!SerializationNeededForRead(relation, snapshot))
        return;

    /*
     * If it's a heap tuple, return if this xact wrote it.
     */
    if (relation->rd_index == NULL)
    {
        TransactionId myxid;

        targetxmin = HeapTupleHeaderGetXmin(tuple->t_data);

        myxid = GetTopTransactionIdIfAny();
        if (TransactionIdIsValid(myxid))
        {
            if (TransactionIdFollowsOrEquals(targetxmin, TransactionXmin))
            {
                TransactionId xid = SubTransGetTopmostTransaction(targetxmin);

                if (TransactionIdEquals(xid, myxid))
                {
                    /* We wrote it; we already have a write lock. */
                    return;
                }
            }
        }
    }
    else
        targetxmin = InvalidTransactionId;

    /*
     * Do quick-but-not-definitive test for a relation lock first.  This will
     * never cause a return when the relation is *not* locked, but will
     * occasionally let the check continue when there really *is* a relation
     * level lock.
     */
    SET_PREDICATELOCKTARGETTAG_RELATION(tag,
                                        relation->rd_node.dbNode,
                                        relation->rd_id);
    if (PredicateLockExists(&tag))
        return;

    tid = &(tuple->t_self);
    SET_PREDICATELOCKTARGETTAG_TUPLE(tag,
                                     relation->rd_node.dbNode,
                                     relation->rd_id,
                                     ItemPointerGetBlockNumber(tid),
                                     ItemPointerGetOffsetNumber(tid),
                                     targetxmin);
    PredicateLockAcquire(&tag);
}


/*
 *      DeleteLockTarget
 *
 * Remove a predicate lock target along with any locks held for it.
 *
 * Caller must hold SerializablePredicateLockListLock and the
 * appropriate hash partition lock for the target.
 */
static void
DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash)
{
    PREDICATELOCK *predlock;
    SHM_QUEUE  *predlocktargetlink;
    PREDICATELOCK *nextpredlock;
    bool        found;

    Assert(LWLockHeldByMe(SerializablePredicateLockListLock));
    Assert(LWLockHeldByMe(PredicateLockHashPartitionLock(targettaghash)));

    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(target->predicateLocks),
                     &(target->predicateLocks),
                     offsetof(PREDICATELOCK, targetLink));
    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
    while (predlock)
    {
        predlocktargetlink = &(predlock->targetLink);
        nextpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(target->predicateLocks),
                         predlocktargetlink,
                         offsetof(PREDICATELOCK, targetLink));

        SHMQueueDelete(&(predlock->xactLink));
        SHMQueueDelete(&(predlock->targetLink));

        hash_search_with_hash_value
            (PredicateLockHash,
             &predlock->tag,
             PredicateLockHashCodeFromTargetHashCode(&predlock->tag,
                                                     targettaghash),
             HASH_REMOVE, &found);
        Assert(found);

        predlock = nextpredlock;
    }
    LWLockRelease(SerializableXactHashLock);

    /* Remove the target itself, if possible. */
    RemoveTargetIfNoLongerUsed(target, targettaghash);
}


/*
 *      TransferPredicateLocksToNewTarget
 *
 * Move or copy all the predicate locks for a lock target, for use by
 * index page splits/combines and other things that create or replace
 * lock targets. If 'removeOld' is true, the old locks and the target
 * will be removed.
 *
 * Returns true on success, or false if we ran out of shared memory to
 * allocate the new target or locks. Guaranteed to always succeed if
 * removeOld is set (by using the scratch entry in PredicateLockTargetHash
 * for scratch space).
 *
 * Warning: the "removeOld" option should be used only with care,
 * because this function does not (indeed, can not) update other
 * backends' LocalPredicateLockHash. If we are only adding new
 * entries, this is not a problem: the local lock table is used only
 * as a hint, so missing entries for locks that are held are
 * OK. Having entries for locks that are no longer held, as can happen
 * when using "removeOld", is not in general OK. We can only use it
 * safely when replacing a lock with a coarser-granularity lock that
 * covers it, or if we are absolutely certain that no one will need to
 * refer to that lock in the future.
 *
 * Caller must hold SerializablePredicateLockListLock.
 */
static bool
TransferPredicateLocksToNewTarget(PREDICATELOCKTARGETTAG oldtargettag,
                                  PREDICATELOCKTARGETTAG newtargettag,
                                  bool removeOld)
{
    uint32      oldtargettaghash;
    LWLockId    oldpartitionLock;
    PREDICATELOCKTARGET *oldtarget;
    uint32      newtargettaghash;
    LWLockId    newpartitionLock;
    bool        found;
    bool        outOfShmem = false;

    Assert(LWLockHeldByMe(SerializablePredicateLockListLock));

    oldtargettaghash = PredicateLockTargetTagHashCode(&oldtargettag);
    newtargettaghash = PredicateLockTargetTagHashCode(&newtargettag);
    oldpartitionLock = PredicateLockHashPartitionLock(oldtargettaghash);
    newpartitionLock = PredicateLockHashPartitionLock(newtargettaghash);

    if (removeOld)
    {
        /*
         * Remove the dummy entry to give us scratch space, so we know we'll
         * be able to create the new lock target.
         */
        RemoveScratchTarget(false);
    }

    /*
     * We must get the partition locks in ascending sequence to avoid
     * deadlocks. If old and new partitions are the same, we must request the
     * lock only once.
     */
    if (oldpartitionLock < newpartitionLock)
    {
        LWLockAcquire(oldpartitionLock,
                      (removeOld ? LW_EXCLUSIVE : LW_SHARED));
        LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
    }
    else if (oldpartitionLock > newpartitionLock)
    {
        LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);
        LWLockAcquire(oldpartitionLock,
                      (removeOld ? LW_EXCLUSIVE : LW_SHARED));
    }
    else
        LWLockAcquire(newpartitionLock, LW_EXCLUSIVE);

    /*
     * Look for the old target.  If not found, that's OK; no predicate locks
     * are affected, so we can just clean up and return. If it does exist,
     * walk its list of predicate locks and move or copy them to the new
     * target.
     */
    oldtarget = hash_search_with_hash_value(PredicateLockTargetHash,
                                            &oldtargettag,
                                            oldtargettaghash,
                                            HASH_FIND, NULL);

    if (oldtarget)
    {
        PREDICATELOCKTARGET *newtarget;
        PREDICATELOCK *oldpredlock;
        PREDICATELOCKTAG newpredlocktag;

        newtarget = hash_search_with_hash_value(PredicateLockTargetHash,
                                                &newtargettag,
                                                newtargettaghash,
                                                HASH_ENTER_NULL, &found);

        if (!newtarget)
        {
            /* Failed to allocate due to insufficient shmem */
            outOfShmem = true;
            goto exit;
        }

        /* If we created a new entry, initialize it */
        if (!found)
            SHMQueueInit(&(newtarget->predicateLocks));

        newpredlocktag.myTarget = newtarget;

        /*
         * Loop through all the locks on the old target, replacing them with
         * locks on the new target.
         */
        oldpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(oldtarget->predicateLocks),
                         &(oldtarget->predicateLocks),
                         offsetof(PREDICATELOCK, targetLink));
        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
        while (oldpredlock)
        {
            SHM_QUEUE  *predlocktargetlink;
            PREDICATELOCK *nextpredlock;
            PREDICATELOCK *newpredlock;
            SerCommitSeqNo oldCommitSeqNo = oldpredlock->commitSeqNo;

            predlocktargetlink = &(oldpredlock->targetLink);
            nextpredlock = (PREDICATELOCK *)
                SHMQueueNext(&(oldtarget->predicateLocks),
                             predlocktargetlink,
                             offsetof(PREDICATELOCK, targetLink));
            newpredlocktag.myXact = oldpredlock->tag.myXact;

            if (removeOld)
            {
                SHMQueueDelete(&(oldpredlock->xactLink));
                SHMQueueDelete(&(oldpredlock->targetLink));

                hash_search_with_hash_value
                    (PredicateLockHash,
                     &oldpredlock->tag,
                   PredicateLockHashCodeFromTargetHashCode(&oldpredlock->tag,
                                                           oldtargettaghash),
                     HASH_REMOVE, &found);
                Assert(found);
            }

            newpredlock = (PREDICATELOCK *)
                hash_search_with_hash_value(PredicateLockHash,
                                            &newpredlocktag,
                     PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
                                                           newtargettaghash),
                                            HASH_ENTER_NULL,
                                            &found);
            if (!newpredlock)
            {
                /* Out of shared memory. Undo what we've done so far. */
                LWLockRelease(SerializableXactHashLock);
                DeleteLockTarget(newtarget, newtargettaghash);
                outOfShmem = true;
                goto exit;
            }
            if (!found)
            {
                SHMQueueInsertBefore(&(newtarget->predicateLocks),
                                     &(newpredlock->targetLink));
                SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
                                     &(newpredlock->xactLink));
                newpredlock->commitSeqNo = oldCommitSeqNo;
            }
            else
            {
                if (newpredlock->commitSeqNo < oldCommitSeqNo)
                    newpredlock->commitSeqNo = oldCommitSeqNo;
            }

            Assert(newpredlock->commitSeqNo != 0);
            Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
                   || (newpredlock->tag.myXact == OldCommittedSxact));

            oldpredlock = nextpredlock;
        }
        LWLockRelease(SerializableXactHashLock);

        if (removeOld)
        {
            Assert(SHMQueueEmpty(&oldtarget->predicateLocks));
            RemoveTargetIfNoLongerUsed(oldtarget, oldtargettaghash);
        }
    }


exit:
    /* Release partition locks in reverse order of acquisition. */
    if (oldpartitionLock < newpartitionLock)
    {
        LWLockRelease(newpartitionLock);
        LWLockRelease(oldpartitionLock);
    }
    else if (oldpartitionLock > newpartitionLock)
    {
        LWLockRelease(oldpartitionLock);
        LWLockRelease(newpartitionLock);
    }
    else
        LWLockRelease(newpartitionLock);

    if (removeOld)
    {
        /* We shouldn't run out of memory if we're moving locks */
        Assert(!outOfShmem);

        /* Put the scrach entry back */
        RestoreScratchTarget(false);
    }

    return !outOfShmem;
}

/*
 * Drop all predicate locks of any granularity from the specified relation,
 * which can be a heap relation or an index relation.  If 'transfer' is true,
 * acquire a relation lock on the heap for any transactions with any lock(s)
 * on the specified relation.
 *
 * This requires grabbing a lot of LW locks and scanning the entire lock
 * target table for matches.  That makes this more expensive than most
 * predicate lock management functions, but it will only be called for DDL
 * type commands that are expensive anyway, and there are fast returns when
 * no serializable transactions are active or the relation is temporary.
 *
 * We don't use the TransferPredicateLocksToNewTarget function because it
 * acquires its own locks on the partitions of the two targets involved,
 * and we'll already be holding all partition locks.
 *
 * We can't throw an error from here, because the call could be from a
 * transaction which is not serializable.
 *
 * NOTE: This is currently only called with transfer set to true, but that may
 * change.  If we decide to clean up the locks from a table on commit of a
 * transaction which executed DROP TABLE, the false condition will be useful.
 */
static void
DropAllPredicateLocksFromTable(Relation relation, bool transfer)
{
    HASH_SEQ_STATUS seqstat;
    PREDICATELOCKTARGET *oldtarget;
    PREDICATELOCKTARGET *heaptarget;
    Oid         dbId;
    Oid         relId;
    Oid         heapId;
    int         i;
    bool        isIndex;
    bool        found;
    uint32      heaptargettaghash;

    /*
     * Bail out quickly if there are no serializable transactions running.
     * It's safe to check this without taking locks because the caller is
     * holding an ACCESS EXCLUSIVE lock on the relation.  No new locks which
     * would matter here can be acquired while that is held.
     */
    if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
        return;

    if (!PredicateLockingNeededForRelation(relation))
        return;

    dbId = relation->rd_node.dbNode;
    relId = relation->rd_id;
    if (relation->rd_index == NULL)
    {
        isIndex = false;
        heapId = relId;
    }
    else
    {
        isIndex = true;
        heapId = relation->rd_index->indrelid;
    }
    Assert(heapId != InvalidOid);
    Assert(transfer || !isIndex);       /* index OID only makes sense with
                                         * transfer */

    /* Retrieve first time needed, then keep. */
    heaptargettaghash = 0;
    heaptarget = NULL;

    /* Acquire locks on all lock partitions */
    LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);
    for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
        LWLockAcquire(FirstPredicateLockMgrLock + i, LW_EXCLUSIVE);
    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

    /*
     * Remove the dummy entry to give us scratch space, so we know we'll be
     * able to create the new lock target.
     */
    if (transfer)
        RemoveScratchTarget(true);

    /* Scan through target map */
    hash_seq_init(&seqstat, PredicateLockTargetHash);

    while ((oldtarget = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
    {
        PREDICATELOCK *oldpredlock;

        /*
         * Check whether this is a target which needs attention.
         */
        if (GET_PREDICATELOCKTARGETTAG_RELATION(oldtarget->tag) != relId)
            continue;           /* wrong relation id */
        if (GET_PREDICATELOCKTARGETTAG_DB(oldtarget->tag) != dbId)
            continue;           /* wrong database id */
        if (transfer && !isIndex
            && GET_PREDICATELOCKTARGETTAG_TYPE(oldtarget->tag) == PREDLOCKTAG_RELATION)
            continue;           /* already the right lock */

        /*
         * If we made it here, we have work to do.  We make sure the heap
         * relation lock exists, then we walk the list of predicate locks for
         * the old target we found, moving all locks to the heap relation lock
         * -- unless they already hold that.
         */

        /*
         * First make sure we have the heap relation target.  We only need to
         * do this once.
         */
        if (transfer && heaptarget == NULL)
        {
            PREDICATELOCKTARGETTAG heaptargettag;

            SET_PREDICATELOCKTARGETTAG_RELATION(heaptargettag, dbId, heapId);
            heaptargettaghash = PredicateLockTargetTagHashCode(&heaptargettag);
            heaptarget = hash_search_with_hash_value(PredicateLockTargetHash,
                                                     &heaptargettag,
                                                     heaptargettaghash,
                                                     HASH_ENTER, &found);
            if (!found)
                SHMQueueInit(&heaptarget->predicateLocks);
        }

        /*
         * Loop through all the locks on the old target, replacing them with
         * locks on the new target.
         */
        oldpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(oldtarget->predicateLocks),
                         &(oldtarget->predicateLocks),
                         offsetof(PREDICATELOCK, targetLink));
        while (oldpredlock)
        {
            PREDICATELOCK *nextpredlock;
            PREDICATELOCK *newpredlock;
            SerCommitSeqNo oldCommitSeqNo;
            SERIALIZABLEXACT *oldXact;

            nextpredlock = (PREDICATELOCK *)
                SHMQueueNext(&(oldtarget->predicateLocks),
                             &(oldpredlock->targetLink),
                             offsetof(PREDICATELOCK, targetLink));

            /*
             * Remove the old lock first. This avoids the chance of running
             * out of lock structure entries for the hash table.
             */
            oldCommitSeqNo = oldpredlock->commitSeqNo;
            oldXact = oldpredlock->tag.myXact;

            SHMQueueDelete(&(oldpredlock->xactLink));

            /*
             * No need for retail delete from oldtarget list, we're removing
             * the whole target anyway.
             */
            hash_search(PredicateLockHash,
                        &oldpredlock->tag,
                        HASH_REMOVE, &found);
            Assert(found);

            if (transfer)
            {
                PREDICATELOCKTAG newpredlocktag;

                newpredlocktag.myTarget = heaptarget;
                newpredlocktag.myXact = oldXact;
                newpredlock = (PREDICATELOCK *)
                    hash_search_with_hash_value(PredicateLockHash,
                                                &newpredlocktag,
                     PredicateLockHashCodeFromTargetHashCode(&newpredlocktag,
                                                          heaptargettaghash),
                                                HASH_ENTER,
                                                &found);
                if (!found)
                {
                    SHMQueueInsertBefore(&(heaptarget->predicateLocks),
                                         &(newpredlock->targetLink));
                    SHMQueueInsertBefore(&(newpredlocktag.myXact->predicateLocks),
                                         &(newpredlock->xactLink));
                    newpredlock->commitSeqNo = oldCommitSeqNo;
                }
                else
                {
                    if (newpredlock->commitSeqNo < oldCommitSeqNo)
                        newpredlock->commitSeqNo = oldCommitSeqNo;
                }

                Assert(newpredlock->commitSeqNo != 0);
                Assert((newpredlock->commitSeqNo == InvalidSerCommitSeqNo)
                       || (newpredlock->tag.myXact == OldCommittedSxact));
            }

            oldpredlock = nextpredlock;
        }

        hash_search(PredicateLockTargetHash, &oldtarget->tag, HASH_REMOVE,
                    &found);
        Assert(found);
    }

    /* Put the scratch entry back */
    if (transfer)
        RestoreScratchTarget(true);

    /* Release locks in reverse order */
    LWLockRelease(SerializableXactHashLock);
    for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
        LWLockRelease(FirstPredicateLockMgrLock + i);
    LWLockRelease(SerializablePredicateLockListLock);
}

/*
 * TransferPredicateLocksToHeapRelation
 *      For all transactions, transfer all predicate locks for the given
 *      relation to a single relation lock on the heap.
 */
void
TransferPredicateLocksToHeapRelation(Relation relation)
{
    DropAllPredicateLocksFromTable(relation, true);
}


/*
 *      PredicateLockPageSplit
 *
 * Copies any predicate locks for the old page to the new page.
 * Skip if this is a temporary table or toast table.
 *
 * NOTE: A page split (or overflow) affects all serializable transactions,
 * even if it occurs in the context of another transaction isolation level.
 *
 * NOTE: This currently leaves the local copy of the locks without
 * information on the new lock which is in shared memory.  This could cause
 * problems if enough page splits occur on locked pages without the processes
 * which hold the locks getting in and noticing.
 */
void
PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
                       BlockNumber newblkno)
{
    PREDICATELOCKTARGETTAG oldtargettag;
    PREDICATELOCKTARGETTAG newtargettag;
    bool        success;

    /*
     * Bail out quickly if there are no serializable transactions running.
     *
     * It's safe to do this check without taking any additional locks. Even if
     * a serializable transaction starts concurrently, we know it can't take
     * any SIREAD locks on the page being split because the caller is holding
     * the associated buffer page lock. Memory reordering isn't an issue; the
     * memory barrier in the LWLock acquisition guarantees that this read
     * occurs while the buffer page lock is held.
     */
    if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
        return;

    if (!PredicateLockingNeededForRelation(relation))
        return;

    Assert(oldblkno != newblkno);
    Assert(BlockNumberIsValid(oldblkno));
    Assert(BlockNumberIsValid(newblkno));

    SET_PREDICATELOCKTARGETTAG_PAGE(oldtargettag,
                                    relation->rd_node.dbNode,
                                    relation->rd_id,
                                    oldblkno);
    SET_PREDICATELOCKTARGETTAG_PAGE(newtargettag,
                                    relation->rd_node.dbNode,
                                    relation->rd_id,
                                    newblkno);

    LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);

    /*
     * Try copying the locks over to the new page's tag, creating it if
     * necessary.
     */
    success = TransferPredicateLocksToNewTarget(oldtargettag,
                                                newtargettag,
                                                false);

    if (!success)
    {
        /*
         * No more predicate lock entries are available. Failure isn't an
         * option here, so promote the page lock to a relation lock.
         */

        /* Get the parent relation lock's lock tag */
        success = GetParentPredicateLockTag(&oldtargettag,
                                            &newtargettag);
        Assert(success);

        /*
         * Move the locks to the parent. This shouldn't fail.
         *
         * Note that here we are removing locks held by other backends,
         * leading to a possible inconsistency in their local lock hash table.
         * This is OK because we're replacing it with a lock that covers the
         * old one.
         */
        success = TransferPredicateLocksToNewTarget(oldtargettag,
                                                    newtargettag,
                                                    true);
        Assert(success);
    }

    LWLockRelease(SerializablePredicateLockListLock);
}

/*
 *      PredicateLockPageCombine
 *
 * Combines predicate locks for two existing pages.
 * Skip if this is a temporary table or toast table.
 *
 * NOTE: A page combine affects all serializable transactions, even if it
 * occurs in the context of another transaction isolation level.
 */
void
PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
                         BlockNumber newblkno)
{
    /*
     * Page combines differ from page splits in that we ought to be able to
     * remove the locks on the old page after transferring them to the new
     * page, instead of duplicating them. However, because we can't edit other
     * backends' local lock tables, removing the old lock would leave them
     * with an entry in their LocalPredicateLockHash for a lock they're not
     * holding, which isn't acceptable. So we wind up having to do the same
     * work as a page split, acquiring a lock on the new page and keeping the
     * old page locked too. That can lead to some false positives, but should
     * be rare in practice.
     */
    PredicateLockPageSplit(relation, oldblkno, newblkno);
}

/*
 * Walk the list of in-progress serializable transactions and find the new
 * xmin.
 */
static void
SetNewSxactGlobalXmin(void)
{
    SERIALIZABLEXACT *sxact;

    Assert(LWLockHeldByMe(SerializableXactHashLock));

    PredXact->SxactGlobalXmin = InvalidTransactionId;
    PredXact->SxactGlobalXminCount = 0;

    for (sxact = FirstPredXact(); sxact != NULL; sxact = NextPredXact(sxact))
    {
        if (!SxactIsRolledBack(sxact)
            && !SxactIsCommitted(sxact)
            && sxact != OldCommittedSxact)
        {
            Assert(sxact->xmin != InvalidTransactionId);
            if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
                || TransactionIdPrecedes(sxact->xmin,
                                         PredXact->SxactGlobalXmin))
            {
                PredXact->SxactGlobalXmin = sxact->xmin;
                PredXact->SxactGlobalXminCount = 1;
            }
            else if (TransactionIdEquals(sxact->xmin,
                                         PredXact->SxactGlobalXmin))
                PredXact->SxactGlobalXminCount++;
        }
    }

    OldSerXidSetActiveSerXmin(PredXact->SxactGlobalXmin);
}

/*
 *      ReleasePredicateLocks
 *
 * Releases predicate locks based on completion of the current transaction,
 * whether committed or rolled back.  It can also be called for a read only
 * transaction when it becomes impossible for the transaction to become
 * part of a dangerous structure.
 *
 * We do nothing unless this is a serializable transaction.
 *
 * This method must ensure that shared memory hash tables are cleaned
 * up in some relatively timely fashion.
 *
 * If this transaction is committing and is holding any predicate locks,
 * it must be added to a list of completed serializable transactions still
 * holding locks.
 */
void
ReleasePredicateLocks(bool isCommit)
{
    bool        needToClear;
    RWConflict  conflict,
                nextConflict,
                possibleUnsafeConflict;
    SERIALIZABLEXACT *roXact;

    /*
     * We can't trust XactReadOnly here, because a transaction which started
     * as READ WRITE can show as READ ONLY later, e.g., within
     * substransactions.  We want to flag a transaction as READ ONLY if it
     * commits without writing so that de facto READ ONLY transactions get the
     * benefit of some RO optimizations, so we will use this local variable to
     * get some cleanup logic right which is based on whether the transaction
     * was declared READ ONLY at the top level.
     */
    bool        topLevelIsDeclaredReadOnly;

    if (MySerializableXact == InvalidSerializableXact)
    {
        Assert(LocalPredicateLockHash == NULL);
        return;
    }

    Assert(!isCommit || SxactIsPrepared(MySerializableXact));
    Assert(!isCommit || !SxactIsDoomed(MySerializableXact));
    Assert(!SxactIsCommitted(MySerializableXact));
    Assert(!SxactIsRolledBack(MySerializableXact));

    /* may not be serializable during COMMIT/ROLLBACK PREPARED */
    if (MySerializableXact->pid != 0)
        Assert(IsolationIsSerializable());

    /* We'd better not already be on the cleanup list. */
    Assert(!SxactIsOnFinishedList(MySerializableXact));

    topLevelIsDeclaredReadOnly = SxactIsReadOnly(MySerializableXact);

    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

    /*
     * We don't hold XidGenLock lock here, assuming that TransactionId is
     * atomic!
     *
     * If this value is changing, we don't care that much whether we get the
     * old or new value -- it is just used to determine how far
     * GlobalSerizableXmin must advance before this transaction can be fully
     * cleaned up.  The worst that could happen is we wait for one more
     * transaction to complete before freeing some RAM; correctness of visible
     * behavior is not affected.
     */
    MySerializableXact->finishedBefore = ShmemVariableCache->nextXid;

    /*
     * If it's not a commit it's a rollback, and we can clear our locks
     * immediately.
     */
    if (isCommit)
    {
        MySerializableXact->flags |= SXACT_FLAG_COMMITTED;
        MySerializableXact->commitSeqNo = ++(PredXact->LastSxactCommitSeqNo);
        /* Recognize implicit read-only transaction (commit without write). */
        if (!MyXactDidWrite)
            MySerializableXact->flags |= SXACT_FLAG_READ_ONLY;
    }
    else
    {
        /*
         * The DOOMED flag indicates that we intend to roll back this
         * transaction and so it should not cause serialization failures for
         * other transactions that conflict with it. Note that this flag might
         * already be set, if another backend marked this transaction for
         * abort.
         *
         * The ROLLED_BACK flag further indicates that ReleasePredicateLocks
         * has been called, and so the SerializableXact is eligible for
         * cleanup. This means it should not be considered when calculating
         * SxactGlobalXmin.
         */
        MySerializableXact->flags |= SXACT_FLAG_DOOMED;
        MySerializableXact->flags |= SXACT_FLAG_ROLLED_BACK;

        /*
         * If the transaction was previously prepared, but is now failing due
         * to a ROLLBACK PREPARED or (hopefully very rare) error after the
         * prepare, clear the prepared flag.  This simplifies conflict
         * checking.
         */
        MySerializableXact->flags &= ~SXACT_FLAG_PREPARED;
    }

    if (!topLevelIsDeclaredReadOnly)
    {
        Assert(PredXact->WritableSxactCount > 0);
        if (--(PredXact->WritableSxactCount) == 0)
        {
            /*
             * Release predicate locks and rw-conflicts in for all committed
             * transactions.  There are no longer any transactions which might
             * conflict with the locks and no chance for new transactions to
             * overlap.  Similarly, existing conflicts in can't cause pivots,
             * and any conflicts in which could have completed a dangerous
             * structure would already have caused a rollback, so any
             * remaining ones must be benign.
             */
            PredXact->CanPartialClearThrough = PredXact->LastSxactCommitSeqNo;
        }
    }
    else
    {
        /*
         * Read-only transactions: clear the list of transactions that might
         * make us unsafe. Note that we use 'inLink' for the iteration as
         * opposed to 'outLink' for the r/w xacts.
         */
        possibleUnsafeConflict = (RWConflict)
            SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
                         &MySerializableXact->possibleUnsafeConflicts,
                         offsetof(RWConflictData, inLink));
        while (possibleUnsafeConflict)
        {
            nextConflict = (RWConflict)
                SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
                             &possibleUnsafeConflict->inLink,
                             offsetof(RWConflictData, inLink));

            Assert(!SxactIsReadOnly(possibleUnsafeConflict->sxactOut));
            Assert(MySerializableXact == possibleUnsafeConflict->sxactIn);

            ReleaseRWConflict(possibleUnsafeConflict);

            possibleUnsafeConflict = nextConflict;
        }
    }

    /* Check for conflict out to old committed transactions. */
    if (isCommit
        && !SxactIsReadOnly(MySerializableXact)
        && SxactHasSummaryConflictOut(MySerializableXact))
    {
        /*
         * we don't know which old committed transaction we conflicted with,
         * so be conservative and use FirstNormalSerCommitSeqNo here
         */
        MySerializableXact->SeqNo.earliestOutConflictCommit =
            FirstNormalSerCommitSeqNo;
        MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
    }

    /*
     * Release all outConflicts to committed transactions.  If we're rolling
     * back clear them all.  Set SXACT_FLAG_CONFLICT_OUT if any point to
     * previously committed transactions.
     */
    conflict = (RWConflict)
        SHMQueueNext(&MySerializableXact->outConflicts,
                     &MySerializableXact->outConflicts,
                     offsetof(RWConflictData, outLink));
    while (conflict)
    {
        nextConflict = (RWConflict)
            SHMQueueNext(&MySerializableXact->outConflicts,
                         &conflict->outLink,
                         offsetof(RWConflictData, outLink));

        if (isCommit
            && !SxactIsReadOnly(MySerializableXact)
            && SxactIsCommitted(conflict->sxactIn))
        {
            if ((MySerializableXact->flags & SXACT_FLAG_CONFLICT_OUT) == 0
                || conflict->sxactIn->prepareSeqNo < MySerializableXact->SeqNo.earliestOutConflictCommit)
                MySerializableXact->SeqNo.earliestOutConflictCommit = conflict->sxactIn->prepareSeqNo;
            MySerializableXact->flags |= SXACT_FLAG_CONFLICT_OUT;
        }

        if (!isCommit
            || SxactIsCommitted(conflict->sxactIn)
            || (conflict->sxactIn->SeqNo.lastCommitBeforeSnapshot >= PredXact->LastSxactCommitSeqNo))
            ReleaseRWConflict(conflict);

        conflict = nextConflict;
    }

    /*
     * Release all inConflicts from committed and read-only transactions. If
     * we're rolling back, clear them all.
     */
    conflict = (RWConflict)
        SHMQueueNext(&MySerializableXact->inConflicts,
                     &MySerializableXact->inConflicts,
                     offsetof(RWConflictData, inLink));
    while (conflict)
    {
        nextConflict = (RWConflict)
            SHMQueueNext(&MySerializableXact->inConflicts,
                         &conflict->inLink,
                         offsetof(RWConflictData, inLink));

        if (!isCommit
            || SxactIsCommitted(conflict->sxactOut)
            || SxactIsReadOnly(conflict->sxactOut))
            ReleaseRWConflict(conflict);

        conflict = nextConflict;
    }

    if (!topLevelIsDeclaredReadOnly)
    {
        /*
         * Remove ourselves from the list of possible conflicts for concurrent
         * READ ONLY transactions, flagging them as unsafe if we have a
         * conflict out. If any are waiting DEFERRABLE transactions, wake them
         * up if they are known safe or known unsafe.
         */
        possibleUnsafeConflict = (RWConflict)
            SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
                         &MySerializableXact->possibleUnsafeConflicts,
                         offsetof(RWConflictData, outLink));
        while (possibleUnsafeConflict)
        {
            nextConflict = (RWConflict)
                SHMQueueNext(&MySerializableXact->possibleUnsafeConflicts,
                             &possibleUnsafeConflict->outLink,
                             offsetof(RWConflictData, outLink));

            roXact = possibleUnsafeConflict->sxactIn;
            Assert(MySerializableXact == possibleUnsafeConflict->sxactOut);
            Assert(SxactIsReadOnly(roXact));

            /* Mark conflicted if necessary. */
            if (isCommit
                && MyXactDidWrite
                && SxactHasConflictOut(MySerializableXact)
                && (MySerializableXact->SeqNo.earliestOutConflictCommit
                    <= roXact->SeqNo.lastCommitBeforeSnapshot))
            {
                /*
                 * This releases possibleUnsafeConflict (as well as all other
                 * possible conflicts for roXact)
                 */
                FlagSxactUnsafe(roXact);
            }
            else
            {
                ReleaseRWConflict(possibleUnsafeConflict);

                /*
                 * If we were the last possible conflict, flag it safe. The
                 * transaction can now safely release its predicate locks (but
                 * that transaction's backend has to do that itself).
                 */
                if (SHMQueueEmpty(&roXact->possibleUnsafeConflicts))
                    roXact->flags |= SXACT_FLAG_RO_SAFE;
            }

            /*
             * Wake up the process for a waiting DEFERRABLE transaction if we
             * now know it's either safe or conflicted.
             */
            if (SxactIsDeferrableWaiting(roXact) &&
                (SxactIsROUnsafe(roXact) || SxactIsROSafe(roXact)))
                ProcSendSignal(roXact->pid);

            possibleUnsafeConflict = nextConflict;
        }
    }

    /*
     * Check whether it's time to clean up old transactions. This can only be
     * done when the last serializable transaction with the oldest xmin among
     * serializable transactions completes.  We then find the "new oldest"
     * xmin and purge any transactions which finished before this transaction
     * was launched.
     */
    needToClear = false;
    if (TransactionIdEquals(MySerializableXact->xmin, PredXact->SxactGlobalXmin))
    {
        Assert(PredXact->SxactGlobalXminCount > 0);
        if (--(PredXact->SxactGlobalXminCount) == 0)
        {
            SetNewSxactGlobalXmin();
            needToClear = true;
        }
    }

    LWLockRelease(SerializableXactHashLock);

    LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);

    /* Add this to the list of transactions to check for later cleanup. */
    if (isCommit)
        SHMQueueInsertBefore(FinishedSerializableTransactions,
                             &MySerializableXact->finishedLink);

    if (!isCommit)
        ReleaseOneSerializableXact(MySerializableXact, false, false);

    LWLockRelease(SerializableFinishedListLock);

    if (needToClear)
        ClearOldPredicateLocks();

    MySerializableXact = InvalidSerializableXact;
    MyXactDidWrite = false;

    /* Delete per-transaction lock table */
    if (LocalPredicateLockHash != NULL)
    {
        hash_destroy(LocalPredicateLockHash);
        LocalPredicateLockHash = NULL;
    }
}

/*
 * Clear old predicate locks, belonging to committed transactions that are no
 * longer interesting to any in-progress transaction.
 */
static void
ClearOldPredicateLocks(void)
{
    SERIALIZABLEXACT *finishedSxact;
    PREDICATELOCK *predlock;

    /*
     * Loop through finished transactions. They are in commit order, so we can
     * stop as soon as we find one that's still interesting.
     */
    LWLockAcquire(SerializableFinishedListLock, LW_EXCLUSIVE);
    finishedSxact = (SERIALIZABLEXACT *)
        SHMQueueNext(FinishedSerializableTransactions,
                     FinishedSerializableTransactions,
                     offsetof(SERIALIZABLEXACT, finishedLink));
    LWLockAcquire(SerializableXactHashLock, LW_SHARED);
    while (finishedSxact)
    {
        SERIALIZABLEXACT *nextSxact;

        nextSxact = (SERIALIZABLEXACT *)
            SHMQueueNext(FinishedSerializableTransactions,
                         &(finishedSxact->finishedLink),
                         offsetof(SERIALIZABLEXACT, finishedLink));
        if (!TransactionIdIsValid(PredXact->SxactGlobalXmin)
            || TransactionIdPrecedesOrEquals(finishedSxact->finishedBefore,
                                             PredXact->SxactGlobalXmin))
        {
            /*
             * This transaction committed before any in-progress transaction
             * took its snapshot. It's no longer interesting.
             */
            LWLockRelease(SerializableXactHashLock);
            SHMQueueDelete(&(finishedSxact->finishedLink));
            ReleaseOneSerializableXact(finishedSxact, false, false);
            LWLockAcquire(SerializableXactHashLock, LW_SHARED);
        }
        else if (finishedSxact->commitSeqNo > PredXact->HavePartialClearedThrough
           && finishedSxact->commitSeqNo <= PredXact->CanPartialClearThrough)
        {
            /*
             * Any active transactions that took their snapshot before this
             * transaction committed are read-only, so we can clear part of
             * its state.
             */
            LWLockRelease(SerializableXactHashLock);

            if (SxactIsReadOnly(finishedSxact))
            {
                /* A read-only transaction can be removed entirely */
                SHMQueueDelete(&(finishedSxact->finishedLink));
                ReleaseOneSerializableXact(finishedSxact, false, false);
            }
            else
            {
                /*
                 * A read-write transaction can only be partially cleared. We
                 * need to keep the SERIALIZABLEXACT but can release the
                 * SIREAD locks and conflicts in.
                 */
                ReleaseOneSerializableXact(finishedSxact, true, false);
            }

            PredXact->HavePartialClearedThrough = finishedSxact->commitSeqNo;
            LWLockAcquire(SerializableXactHashLock, LW_SHARED);
        }
        else
        {
            /* Still interesting. */
            break;
        }
        finishedSxact = nextSxact;
    }
    LWLockRelease(SerializableXactHashLock);

    /*
     * Loop through predicate locks on dummy transaction for summarized data.
     */
    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
    predlock = (PREDICATELOCK *)
        SHMQueueNext(&OldCommittedSxact->predicateLocks,
                     &OldCommittedSxact->predicateLocks,
                     offsetof(PREDICATELOCK, xactLink));
    while (predlock)
    {
        PREDICATELOCK *nextpredlock;
        bool        canDoPartialCleanup;

        nextpredlock = (PREDICATELOCK *)
            SHMQueueNext(&OldCommittedSxact->predicateLocks,
                         &predlock->xactLink,
                         offsetof(PREDICATELOCK, xactLink));

        LWLockAcquire(SerializableXactHashLock, LW_SHARED);
        Assert(predlock->commitSeqNo != 0);
        Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
        canDoPartialCleanup = (predlock->commitSeqNo <= PredXact->CanPartialClearThrough);
        LWLockRelease(SerializableXactHashLock);

        /*
         * If this lock originally belonged to an old enough transaction, we
         * can release it.
         */
        if (canDoPartialCleanup)
        {
            PREDICATELOCKTAG tag;
            PREDICATELOCKTARGET *target;
            PREDICATELOCKTARGETTAG targettag;
            uint32      targettaghash;
            LWLockId    partitionLock;

            tag = predlock->tag;
            target = tag.myTarget;
            targettag = target->tag;
            targettaghash = PredicateLockTargetTagHashCode(&targettag);
            partitionLock = PredicateLockHashPartitionLock(targettaghash);

            LWLockAcquire(partitionLock, LW_EXCLUSIVE);

            SHMQueueDelete(&(predlock->targetLink));
            SHMQueueDelete(&(predlock->xactLink));

            hash_search_with_hash_value(PredicateLockHash, &tag,
                                PredicateLockHashCodeFromTargetHashCode(&tag,
                                                              targettaghash),
                                        HASH_REMOVE, NULL);
            RemoveTargetIfNoLongerUsed(target, targettaghash);

            LWLockRelease(partitionLock);
        }

        predlock = nextpredlock;
    }

    LWLockRelease(SerializablePredicateLockListLock);
    LWLockRelease(SerializableFinishedListLock);
}

/*
 * This is the normal way to delete anything from any of the predicate
 * locking hash tables.  Given a transaction which we know can be deleted:
 * delete all predicate locks held by that transaction and any predicate
 * lock targets which are now unreferenced by a lock; delete all conflicts
 * for the transaction; delete all xid values for the transaction; then
 * delete the transaction.
 *
 * When the partial flag is set, we can release all predicate locks and
 * in-conflict information -- we've established that there are no longer
 * any overlapping read write transactions for which this transaction could
 * matter -- but keep the transaction entry itself and any outConflicts.
 *
 * When the summarize flag is set, we've run short of room for sxact data
 * and must summarize to the SLRU.  Predicate locks are transferred to a
 * dummy "old" transaction, with duplicate locks on a single target
 * collapsing to a single lock with the "latest" commitSeqNo from among
 * the conflicting locks..
 */
static void
ReleaseOneSerializableXact(SERIALIZABLEXACT *sxact, bool partial,
                           bool summarize)
{
    PREDICATELOCK *predlock;
    SERIALIZABLEXIDTAG sxidtag;
    RWConflict  conflict,
                nextConflict;

    Assert(sxact != NULL);
    Assert(SxactIsRolledBack(sxact) || SxactIsCommitted(sxact));
    Assert(partial || !SxactIsOnFinishedList(sxact));
    Assert(LWLockHeldByMe(SerializableFinishedListLock));

    /*
     * First release all the predicate locks held by this xact (or transfer
     * them to OldCommittedSxact if summarize is true)
     */
    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(sxact->predicateLocks),
                     &(sxact->predicateLocks),
                     offsetof(PREDICATELOCK, xactLink));
    while (predlock)
    {
        PREDICATELOCK *nextpredlock;
        PREDICATELOCKTAG tag;
        SHM_QUEUE  *targetLink;
        PREDICATELOCKTARGET *target;
        PREDICATELOCKTARGETTAG targettag;
        uint32      targettaghash;
        LWLockId    partitionLock;

        nextpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(sxact->predicateLocks),
                         &(predlock->xactLink),
                         offsetof(PREDICATELOCK, xactLink));

        tag = predlock->tag;
        targetLink = &(predlock->targetLink);
        target = tag.myTarget;
        targettag = target->tag;
        targettaghash = PredicateLockTargetTagHashCode(&targettag);
        partitionLock = PredicateLockHashPartitionLock(targettaghash);

        LWLockAcquire(partitionLock, LW_EXCLUSIVE);

        SHMQueueDelete(targetLink);

        hash_search_with_hash_value(PredicateLockHash, &tag,
                                PredicateLockHashCodeFromTargetHashCode(&tag,
                                                              targettaghash),
                                    HASH_REMOVE, NULL);
        if (summarize)
        {
            bool        found;

            /* Fold into dummy transaction list. */
            tag.myXact = OldCommittedSxact;
            predlock = hash_search_with_hash_value(PredicateLockHash, &tag,
                                PredicateLockHashCodeFromTargetHashCode(&tag,
                                                              targettaghash),
                                                   HASH_ENTER_NULL, &found);
            if (!predlock)
                ereport(ERROR,
                        (errcode(ERRCODE_OUT_OF_MEMORY),
                         errmsg("out of shared memory"),
                         errhint("You might need to increase max_pred_locks_per_transaction.")));
            if (found)
            {
                Assert(predlock->commitSeqNo != 0);
                Assert(predlock->commitSeqNo != InvalidSerCommitSeqNo);
                if (predlock->commitSeqNo < sxact->commitSeqNo)
                    predlock->commitSeqNo = sxact->commitSeqNo;
            }
            else
            {
                SHMQueueInsertBefore(&(target->predicateLocks),
                                     &(predlock->targetLink));
                SHMQueueInsertBefore(&(OldCommittedSxact->predicateLocks),
                                     &(predlock->xactLink));
                predlock->commitSeqNo = sxact->commitSeqNo;
            }
        }
        else
            RemoveTargetIfNoLongerUsed(target, targettaghash);

        LWLockRelease(partitionLock);

        predlock = nextpredlock;
    }

    /*
     * Rather than retail removal, just re-init the head after we've run
     * through the list.
     */
    SHMQueueInit(&sxact->predicateLocks);

    LWLockRelease(SerializablePredicateLockListLock);

    sxidtag.xid = sxact->topXid;
    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

    /* Release all outConflicts (unless 'partial' is true) */
    if (!partial)
    {
        conflict = (RWConflict)
            SHMQueueNext(&sxact->outConflicts,
                         &sxact->outConflicts,
                         offsetof(RWConflictData, outLink));
        while (conflict)
        {
            nextConflict = (RWConflict)
                SHMQueueNext(&sxact->outConflicts,
                             &conflict->outLink,
                             offsetof(RWConflictData, outLink));
            if (summarize)
                conflict->sxactIn->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
            ReleaseRWConflict(conflict);
            conflict = nextConflict;
        }
    }

    /* Release all inConflicts. */
    conflict = (RWConflict)
        SHMQueueNext(&sxact->inConflicts,
                     &sxact->inConflicts,
                     offsetof(RWConflictData, inLink));
    while (conflict)
    {
        nextConflict = (RWConflict)
            SHMQueueNext(&sxact->inConflicts,
                         &conflict->inLink,
                         offsetof(RWConflictData, inLink));
        if (summarize)
            conflict->sxactOut->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
        ReleaseRWConflict(conflict);
        conflict = nextConflict;
    }

    /* Finally, get rid of the xid and the record of the transaction itself. */
    if (!partial)
    {
        if (sxidtag.xid != InvalidTransactionId)
            hash_search(SerializableXidHash, &sxidtag, HASH_REMOVE, NULL);
        ReleasePredXact(sxact);
    }

    LWLockRelease(SerializableXactHashLock);
}

/*
 * Tests whether the given top level transaction is concurrent with
 * (overlaps) our current transaction.
 *
 * We need to identify the top level transaction for SSI, anyway, so pass
 * that to this function to save the overhead of checking the snapshot's
 * subxip array.
 */
static bool
XidIsConcurrent(TransactionId xid)
{
    Snapshot    snap;
    uint32      i;

    Assert(TransactionIdIsValid(xid));
    Assert(!TransactionIdEquals(xid, GetTopTransactionIdIfAny()));

    snap = GetTransactionSnapshot();

    if (TransactionIdPrecedes(xid, snap->xmin))
        return false;

    if (TransactionIdFollowsOrEquals(xid, snap->xmax))
        return true;

    for (i = 0; i < snap->xcnt; i++)
    {
        if (xid == snap->xip[i])
            return true;
    }

    return false;
}

/*
 * CheckForSerializableConflictOut
 *      We are reading a tuple which has been modified.  If it is visible to
 *      us but has been deleted, that indicates a rw-conflict out.  If it's
 *      not visible and was created by a concurrent (overlapping)
 *      serializable transaction, that is also a rw-conflict out,
 *
 * We will determine the top level xid of the writing transaction with which
 * we may be in conflict, and check for overlap with our own transaction.
 * If the transactions overlap (i.e., they cannot see each other's writes),
 * then we have a conflict out.
 *
 * This function should be called just about anywhere in heapam.c where a
 * tuple has been read. The caller must hold at least a shared lock on the
 * buffer, because this function might set hint bits on the tuple. There is
 * currently no known reason to call this function from an index AM.
 */
void
CheckForSerializableConflictOut(bool visible, Relation relation,
                                HeapTuple tuple, Buffer buffer,
                                Snapshot snapshot)
{
    TransactionId xid;
    SERIALIZABLEXIDTAG sxidtag;
    SERIALIZABLEXID *sxid;
    SERIALIZABLEXACT *sxact;
    HTSV_Result htsvResult;

    if (!SerializationNeededForRead(relation, snapshot))
        return;

    /* Check if someone else has already decided that we need to die */
    if (SxactIsDoomed(MySerializableXact))
    {
        ereport(ERROR,
                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict out checking."),
                 errhint("The transaction might succeed if retried.")));
    }

    /*
     * Check to see whether the tuple has been written to by a concurrent
     * transaction, either to create it not visible to us, or to delete it
     * while it is visible to us.  The "visible" bool indicates whether the
     * tuple is visible to us, while HeapTupleSatisfiesVacuum checks what else
     * is going on with it.
     */
    htsvResult = HeapTupleSatisfiesVacuum(tuple->t_data, TransactionXmin, buffer);
    switch (htsvResult)
    {
        case HEAPTUPLE_LIVE:
            if (visible)
                return;
            xid = HeapTupleHeaderGetXmin(tuple->t_data);
            break;
        case HEAPTUPLE_RECENTLY_DEAD:
            if (!visible)
                return;
            xid = HeapTupleHeaderGetUpdateXid(tuple->t_data);
            break;
        case HEAPTUPLE_DELETE_IN_PROGRESS:
            xid = HeapTupleHeaderGetUpdateXid(tuple->t_data);
            break;
        case HEAPTUPLE_INSERT_IN_PROGRESS:
            xid = HeapTupleHeaderGetXmin(tuple->t_data);
            break;
        case HEAPTUPLE_DEAD:
            return;
        default:

            /*
             * The only way to get to this default clause is if a new value is
             * added to the enum type without adding it to this switch
             * statement.  That's a bug, so elog.
             */
            elog(ERROR, "unrecognized return value from HeapTupleSatisfiesVacuum: %u", htsvResult);

            /*
             * In spite of having all enum values covered and calling elog on
             * this default, some compilers think this is a code path which
             * allows xid to be used below without initialization. Silence
             * that warning.
             */
            xid = InvalidTransactionId;
    }
    Assert(TransactionIdIsValid(xid));
    Assert(TransactionIdFollowsOrEquals(xid, TransactionXmin));

    /*
     * Find top level xid.  Bail out if xid is too early to be a conflict, or
     * if it's our own xid.
     */
    if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
        return;
    xid = SubTransGetTopmostTransaction(xid);
    if (TransactionIdPrecedes(xid, TransactionXmin))
        return;
    if (TransactionIdEquals(xid, GetTopTransactionIdIfAny()))
        return;

    /*
     * Find sxact or summarized info for the top level xid.
     */
    sxidtag.xid = xid;
    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
    sxid = (SERIALIZABLEXID *)
        hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
    if (!sxid)
    {
        /*
         * Transaction not found in "normal" SSI structures.  Check whether it
         * got pushed out to SLRU storage for "old committed" transactions.
         */
        SerCommitSeqNo conflictCommitSeqNo;

        conflictCommitSeqNo = OldSerXidGetMinConflictCommitSeqNo(xid);
        if (conflictCommitSeqNo != 0)
        {
            if (conflictCommitSeqNo != InvalidSerCommitSeqNo
                && (!SxactIsReadOnly(MySerializableXact)
                    || conflictCommitSeqNo
                    <= MySerializableXact->SeqNo.lastCommitBeforeSnapshot))
                ereport(ERROR,
                        (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                         errmsg("could not serialize access due to read/write dependencies among transactions"),
                         errdetail_internal("Reason code: Canceled on conflict out to old pivot %u.", xid),
                      errhint("The transaction might succeed if retried.")));

            if (SxactHasSummaryConflictIn(MySerializableXact)
                || !SHMQueueEmpty(&MySerializableXact->inConflicts))
                ereport(ERROR,
                        (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                         errmsg("could not serialize access due to read/write dependencies among transactions"),
                         errdetail_internal("Reason code: Canceled on identification as a pivot, with conflict out to old committed transaction %u.", xid),
                      errhint("The transaction might succeed if retried.")));

            MySerializableXact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
        }

        /* It's not serializable or otherwise not important. */
        LWLockRelease(SerializableXactHashLock);
        return;
    }
    sxact = sxid->myXact;
    Assert(TransactionIdEquals(sxact->topXid, xid));
    if (sxact == MySerializableXact || SxactIsDoomed(sxact))
    {
        /* Can't conflict with ourself or a transaction that will roll back. */
        LWLockRelease(SerializableXactHashLock);
        return;
    }

    /*
     * We have a conflict out to a transaction which has a conflict out to a
     * summarized transaction.  That summarized transaction must have
     * committed first, and we can't tell when it committed in relation to our
     * snapshot acquisition, so something needs to be canceled.
     */
    if (SxactHasSummaryConflictOut(sxact))
    {
        if (!SxactIsPrepared(sxact))
        {
            sxact->flags |= SXACT_FLAG_DOOMED;
            LWLockRelease(SerializableXactHashLock);
            return;
        }
        else
        {
            LWLockRelease(SerializableXactHashLock);
            ereport(ERROR,
                    (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                     errmsg("could not serialize access due to read/write dependencies among transactions"),
                     errdetail_internal("Reason code: Canceled on conflict out to old pivot."),
                     errhint("The transaction might succeed if retried.")));
        }
    }

    /*
     * If this is a read-only transaction and the writing transaction has
     * committed, and it doesn't have a rw-conflict to a transaction which
     * committed before it, no conflict.
     */
    if (SxactIsReadOnly(MySerializableXact)
        && SxactIsCommitted(sxact)
        && !SxactHasSummaryConflictOut(sxact)
        && (!SxactHasConflictOut(sxact)
            || MySerializableXact->SeqNo.lastCommitBeforeSnapshot < sxact->SeqNo.earliestOutConflictCommit))
    {
        /* Read-only transaction will appear to run first.  No conflict. */
        LWLockRelease(SerializableXactHashLock);
        return;
    }

    if (!XidIsConcurrent(xid))
    {
        /* This write was already in our snapshot; no conflict. */
        LWLockRelease(SerializableXactHashLock);
        return;
    }

    if (RWConflictExists(MySerializableXact, sxact))
    {
        /* We don't want duplicate conflict records in the list. */
        LWLockRelease(SerializableXactHashLock);
        return;
    }

    /*
     * Flag the conflict.  But first, if this conflict creates a dangerous
     * structure, ereport an error.
     */
    FlagRWConflict(MySerializableXact, sxact);
    LWLockRelease(SerializableXactHashLock);
}

/*
 * Check a particular target for rw-dependency conflict in. A subroutine of
 * CheckForSerializableConflictIn().
 */
static void
CheckTargetForConflictsIn(PREDICATELOCKTARGETTAG *targettag)
{
    uint32      targettaghash;
    LWLockId    partitionLock;
    PREDICATELOCKTARGET *target;
    PREDICATELOCK *predlock;
    PREDICATELOCK *mypredlock = NULL;
    PREDICATELOCKTAG mypredlocktag;

    Assert(MySerializableXact != InvalidSerializableXact);

    /*
     * The same hash and LW lock apply to the lock target and the lock itself.
     */
    targettaghash = PredicateLockTargetTagHashCode(targettag);
    partitionLock = PredicateLockHashPartitionLock(targettaghash);
    LWLockAcquire(partitionLock, LW_SHARED);
    target = (PREDICATELOCKTARGET *)
        hash_search_with_hash_value(PredicateLockTargetHash,
                                    targettag, targettaghash,
                                    HASH_FIND, NULL);
    if (!target)
    {
        /* Nothing has this target locked; we're done here. */
        LWLockRelease(partitionLock);
        return;
    }

    /*
     * Each lock for an overlapping transaction represents a conflict: a
     * rw-dependency in to this transaction.
     */
    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(target->predicateLocks),
                     &(target->predicateLocks),
                     offsetof(PREDICATELOCK, targetLink));
    LWLockAcquire(SerializableXactHashLock, LW_SHARED);
    while (predlock)
    {
        SHM_QUEUE  *predlocktargetlink;
        PREDICATELOCK *nextpredlock;
        SERIALIZABLEXACT *sxact;

        predlocktargetlink = &(predlock->targetLink);
        nextpredlock = (PREDICATELOCK *)
            SHMQueueNext(&(target->predicateLocks),
                         predlocktargetlink,
                         offsetof(PREDICATELOCK, targetLink));

        sxact = predlock->tag.myXact;
        if (sxact == MySerializableXact)
        {
            /*
             * If we're getting a write lock on a tuple, we don't need a
             * predicate (SIREAD) lock on the same tuple. We can safely remove
             * our SIREAD lock, but we'll defer doing so until after the loop
             * because that requires upgrading to an exclusive partition lock.
             *
             * We can't use this optimization within a subtransaction because
             * the subtransaction could roll back, and we would be left
             * without any lock at the top level.
             */
            if (!IsSubTransaction()
                && GET_PREDICATELOCKTARGETTAG_OFFSET(*targettag))
            {
                mypredlock = predlock;
                mypredlocktag = predlock->tag;
            }
        }
        else if (!SxactIsDoomed(sxact)
                 && (!SxactIsCommitted(sxact)
                     || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                              sxact->finishedBefore))
                 && !RWConflictExists(sxact, MySerializableXact))
        {
            LWLockRelease(SerializableXactHashLock);
            LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

            /*
             * Re-check after getting exclusive lock because the other
             * transaction may have flagged a conflict.
             */
            if (!SxactIsDoomed(sxact)
                && (!SxactIsCommitted(sxact)
                    || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                             sxact->finishedBefore))
                && !RWConflictExists(sxact, MySerializableXact))
            {
                FlagRWConflict(sxact, MySerializableXact);
            }

            LWLockRelease(SerializableXactHashLock);
            LWLockAcquire(SerializableXactHashLock, LW_SHARED);
        }

        predlock = nextpredlock;
    }
    LWLockRelease(SerializableXactHashLock);
    LWLockRelease(partitionLock);

    /*
     * If we found one of our own SIREAD locks to remove, remove it now.
     *
     * At this point our transaction already has an ExclusiveRowLock on the
     * relation, so we are OK to drop the predicate lock on the tuple, if
     * found, without fearing that another write against the tuple will occur
     * before the MVCC information makes it to the buffer.
     */
    if (mypredlock != NULL)
    {
        uint32      predlockhashcode;
        PREDICATELOCK *rmpredlock;

        LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);
        LWLockAcquire(partitionLock, LW_EXCLUSIVE);
        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

        /*
         * Remove the predicate lock from shared memory, if it wasn't removed
         * while the locks were released.  One way that could happen is from
         * autovacuum cleaning up an index.
         */
        predlockhashcode = PredicateLockHashCodeFromTargetHashCode
            (&mypredlocktag, targettaghash);
        rmpredlock = (PREDICATELOCK *)
            hash_search_with_hash_value(PredicateLockHash,
                                        &mypredlocktag,
                                        predlockhashcode,
                                        HASH_FIND, NULL);
        if (rmpredlock != NULL)
        {
            Assert(rmpredlock == mypredlock);

            SHMQueueDelete(&(mypredlock->targetLink));
            SHMQueueDelete(&(mypredlock->xactLink));

            rmpredlock = (PREDICATELOCK *)
                hash_search_with_hash_value(PredicateLockHash,
                                            &mypredlocktag,
                                            predlockhashcode,
                                            HASH_REMOVE, NULL);
            Assert(rmpredlock == mypredlock);

            RemoveTargetIfNoLongerUsed(target, targettaghash);
        }

        LWLockRelease(SerializableXactHashLock);
        LWLockRelease(partitionLock);
        LWLockRelease(SerializablePredicateLockListLock);

        if (rmpredlock != NULL)
        {
            /*
             * Remove entry in local lock table if it exists. It's OK if it
             * doesn't exist; that means the lock was transferred to a new
             * target by a different backend.
             */
            hash_search_with_hash_value(LocalPredicateLockHash,
                                        targettag, targettaghash,
                                        HASH_REMOVE, NULL);

            DecrementParentLocks(targettag);
        }
    }
}

/*
 * CheckForSerializableConflictIn
 *      We are writing the given tuple.  If that indicates a rw-conflict
 *      in from another serializable transaction, take appropriate action.
 *
 * Skip checking for any granularity for which a parameter is missing.
 *
 * A tuple update or delete is in conflict if we have a predicate lock
 * against the relation or page in which the tuple exists, or against the
 * tuple itself.
 */
void
CheckForSerializableConflictIn(Relation relation, HeapTuple tuple,
                               Buffer buffer)
{
    PREDICATELOCKTARGETTAG targettag;

    if (!SerializationNeededForWrite(relation))
        return;

    /* Check if someone else has already decided that we need to die */
    if (SxactIsDoomed(MySerializableXact))
        ereport(ERROR,
                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                 errdetail_internal("Reason code: Canceled on identification as a pivot, during conflict in checking."),
                 errhint("The transaction might succeed if retried.")));

    /*
     * We're doing a write which might cause rw-conflicts now or later.
     * Memorize that fact.
     */
    MyXactDidWrite = true;

    /*
     * It is important that we check for locks from the finest granularity to
     * the coarsest granularity, so that granularity promotion doesn't cause
     * us to miss a lock.  The new (coarser) lock will be acquired before the
     * old (finer) locks are released.
     *
     * It is not possible to take and hold a lock across the checks for all
     * granularities because each target could be in a separate partition.
     */
    if (tuple != NULL)
    {
        SET_PREDICATELOCKTARGETTAG_TUPLE(targettag,
                                         relation->rd_node.dbNode,
                                         relation->rd_id,
                         ItemPointerGetBlockNumber(&(tuple->t_data->t_ctid)),
                        ItemPointerGetOffsetNumber(&(tuple->t_data->t_ctid)),
                                      HeapTupleHeaderGetXmin(tuple->t_data));
        CheckTargetForConflictsIn(&targettag);
    }

    if (BufferIsValid(buffer))
    {
        SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
                                        relation->rd_node.dbNode,
                                        relation->rd_id,
                                        BufferGetBlockNumber(buffer));
        CheckTargetForConflictsIn(&targettag);
    }

    SET_PREDICATELOCKTARGETTAG_RELATION(targettag,
                                        relation->rd_node.dbNode,
                                        relation->rd_id);
    CheckTargetForConflictsIn(&targettag);
}

/*
 * CheckTableForSerializableConflictIn
 *      The entire table is going through a DDL-style logical mass delete
 *      like TRUNCATE or DROP TABLE.  If that causes a rw-conflict in from
 *      another serializable transaction, take appropriate action.
 *
 * While these operations do not operate entirely within the bounds of
 * snapshot isolation, they can occur inside a serializable transaction, and
 * will logically occur after any reads which saw rows which were destroyed
 * by these operations, so we do what we can to serialize properly under
 * SSI.
 *
 * The relation passed in must be a heap relation. Any predicate lock of any
 * granularity on the heap will cause a rw-conflict in to this transaction.
 * Predicate locks on indexes do not matter because they only exist to guard
 * against conflicting inserts into the index, and this is a mass *delete*.
 * When a table is truncated or dropped, the index will also be truncated
 * or dropped, and we'll deal with locks on the index when that happens.
 *
 * Dropping or truncating a table also needs to drop any existing predicate
 * locks on heap tuples or pages, because they're about to go away. This
 * should be done before altering the predicate locks because the transaction
 * could be rolled back because of a conflict, in which case the lock changes
 * are not needed. (At the moment, we don't actually bother to drop the
 * existing locks on a dropped or truncated table at the moment. That might
 * lead to some false positives, but it doesn't seem worth the trouble.)
 */
void
CheckTableForSerializableConflictIn(Relation relation)
{
    HASH_SEQ_STATUS seqstat;
    PREDICATELOCKTARGET *target;
    Oid         dbId;
    Oid         heapId;
    int         i;

    /*
     * Bail out quickly if there are no serializable transactions running.
     * It's safe to check this without taking locks because the caller is
     * holding an ACCESS EXCLUSIVE lock on the relation.  No new locks which
     * would matter here can be acquired while that is held.
     */
    if (!TransactionIdIsValid(PredXact->SxactGlobalXmin))
        return;

    if (!SerializationNeededForWrite(relation))
        return;

    /*
     * We're doing a write which might cause rw-conflicts now or later.
     * Memorize that fact.
     */
    MyXactDidWrite = true;

    Assert(relation->rd_index == NULL); /* not an index relation */

    dbId = relation->rd_node.dbNode;
    heapId = relation->rd_id;

    LWLockAcquire(SerializablePredicateLockListLock, LW_EXCLUSIVE);
    for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
        LWLockAcquire(FirstPredicateLockMgrLock + i, LW_SHARED);
    LWLockAcquire(SerializableXactHashLock, LW_SHARED);

    /* Scan through target list */
    hash_seq_init(&seqstat, PredicateLockTargetHash);

    while ((target = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
    {
        PREDICATELOCK *predlock;

        /*
         * Check whether this is a target which needs attention.
         */
        if (GET_PREDICATELOCKTARGETTAG_RELATION(target->tag) != heapId)
            continue;           /* wrong relation id */
        if (GET_PREDICATELOCKTARGETTAG_DB(target->tag) != dbId)
            continue;           /* wrong database id */

        /*
         * Loop through locks for this target and flag conflicts.
         */
        predlock = (PREDICATELOCK *)
            SHMQueueNext(&(target->predicateLocks),
                         &(target->predicateLocks),
                         offsetof(PREDICATELOCK, targetLink));
        while (predlock)
        {
            PREDICATELOCK *nextpredlock;

            nextpredlock = (PREDICATELOCK *)
                SHMQueueNext(&(target->predicateLocks),
                             &(predlock->targetLink),
                             offsetof(PREDICATELOCK, targetLink));

            if (predlock->tag.myXact != MySerializableXact
              && !RWConflictExists(predlock->tag.myXact, MySerializableXact))
            {
                FlagRWConflict(predlock->tag.myXact, MySerializableXact);
            }

            predlock = nextpredlock;
        }
    }

    /* Release locks in reverse order */
    LWLockRelease(SerializableXactHashLock);
    for (i = NUM_PREDICATELOCK_PARTITIONS - 1; i >= 0; i--)
        LWLockRelease(FirstPredicateLockMgrLock + i);
    LWLockRelease(SerializablePredicateLockListLock);
}


/*
 * Flag a rw-dependency between two serializable transactions.
 *
 * The caller is responsible for ensuring that we have a LW lock on
 * the transaction hash table.
 */
static void
FlagRWConflict(SERIALIZABLEXACT *reader, SERIALIZABLEXACT *writer)
{
    Assert(reader != writer);

    /* First, see if this conflict causes failure. */
    OnConflict_CheckForSerializationFailure(reader, writer);

    /* Actually do the conflict flagging. */
    if (reader == OldCommittedSxact)
        writer->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
    else if (writer == OldCommittedSxact)
        reader->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
    else
        SetRWConflict(reader, writer);
}

/*----------------------------------------------------------------------------
 * We are about to add a RW-edge to the dependency graph - check that we don't
 * introduce a dangerous structure by doing so, and abort one of the
 * transactions if so.
 *
 * A serialization failure can only occur if there is a dangerous structure
 * in the dependency graph:
 *
 *      Tin ------> Tpivot ------> Tout
 *            rw             rw
 *
 * Furthermore, Tout must commit first.
 *
 * One more optimization is that if Tin is declared READ ONLY (or commits
 * without writing), we can only have a problem if Tout committed before Tin
 * acquired its snapshot.
 *----------------------------------------------------------------------------
 */
static void
OnConflict_CheckForSerializationFailure(const SERIALIZABLEXACT *reader,
                                        SERIALIZABLEXACT *writer)
{
    bool        failure;
    RWConflict  conflict;

    Assert(LWLockHeldByMe(SerializableXactHashLock));

    failure = false;

    /*------------------------------------------------------------------------
     * Check for already-committed writer with rw-conflict out flagged
     * (conflict-flag on W means that T2 committed before W):
     *
     *      R ------> W ------> T2
     *          rw        rw
     *
     * That is a dangerous structure, so we must abort. (Since the writer
     * has already committed, we must be the reader)
     *------------------------------------------------------------------------
     */
    if (SxactIsCommitted(writer)
      && (SxactHasConflictOut(writer) || SxactHasSummaryConflictOut(writer)))
        failure = true;

    /*------------------------------------------------------------------------
     * Check whether the writer has become a pivot with an out-conflict
     * committed transaction (T2), and T2 committed first:
     *
     *      R ------> W ------> T2
     *          rw        rw
     *
     * Because T2 must've committed first, there is no anomaly if:
     * - the reader committed before T2
     * - the writer committed before T2
     * - the reader is a READ ONLY transaction and the reader was concurrent
     *   with T2 (= reader acquired its snapshot before T2 committed)
     *
     * We also handle the case that T2 is prepared but not yet committed
     * here. In that case T2 has already checked for conflicts, so if it
     * commits first, making the above conflict real, it's too late for it
     * to abort.
     *------------------------------------------------------------------------
     */
    if (!failure)
    {
        if (SxactHasSummaryConflictOut(writer))
        {
            failure = true;
            conflict = NULL;
        }
        else
            conflict = (RWConflict)
                SHMQueueNext(&writer->outConflicts,
                             &writer->outConflicts,
                             offsetof(RWConflictData, outLink));
        while (conflict)
        {
            SERIALIZABLEXACT *t2 = conflict->sxactIn;

            if (SxactIsPrepared(t2)
                && (!SxactIsCommitted(reader)
                    || t2->prepareSeqNo <= reader->commitSeqNo)
                && (!SxactIsCommitted(writer)
                    || t2->prepareSeqNo <= writer->commitSeqNo)
                && (!SxactIsReadOnly(reader)
              || t2->prepareSeqNo <= reader->SeqNo.lastCommitBeforeSnapshot))
            {
                failure = true;
                break;
            }
            conflict = (RWConflict)
                SHMQueueNext(&writer->outConflicts,
                             &conflict->outLink,
                             offsetof(RWConflictData, outLink));
        }
    }

    /*------------------------------------------------------------------------
     * Check whether the reader has become a pivot with a writer
     * that's committed (or prepared):
     *
     *      T0 ------> R ------> W
     *           rw        rw
     *
     * Because W must've committed first for an anomaly to occur, there is no
     * anomaly if:
     * - T0 committed before the writer
     * - T0 is READ ONLY, and overlaps the writer
     *------------------------------------------------------------------------
     */
    if (!failure && SxactIsPrepared(writer) && !SxactIsReadOnly(reader))
    {
        if (SxactHasSummaryConflictIn(reader))
        {
            failure = true;
            conflict = NULL;
        }
        else
            conflict = (RWConflict)
                SHMQueueNext(&reader->inConflicts,
                             &reader->inConflicts,
                             offsetof(RWConflictData, inLink));
        while (conflict)
        {
            SERIALIZABLEXACT *t0 = conflict->sxactOut;

            if (!SxactIsDoomed(t0)
                && (!SxactIsCommitted(t0)
                    || t0->commitSeqNo >= writer->prepareSeqNo)
                && (!SxactIsReadOnly(t0)
              || t0->SeqNo.lastCommitBeforeSnapshot >= writer->prepareSeqNo))
            {
                failure = true;
                break;
            }
            conflict = (RWConflict)
                SHMQueueNext(&reader->inConflicts,
                             &conflict->inLink,
                             offsetof(RWConflictData, inLink));
        }
    }

    if (failure)
    {
        /*
         * We have to kill a transaction to avoid a possible anomaly from
         * occurring. If the writer is us, we can just ereport() to cause a
         * transaction abort. Otherwise we flag the writer for termination,
         * causing it to abort when it tries to commit. However, if the writer
         * is a prepared transaction, already prepared, we can't abort it
         * anymore, so we have to kill the reader instead.
         */
        if (MySerializableXact == writer)
        {
            LWLockRelease(SerializableXactHashLock);
            ereport(ERROR,
                    (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                     errmsg("could not serialize access due to read/write dependencies among transactions"),
                     errdetail_internal("Reason code: Canceled on identification as a pivot, during write."),
                     errhint("The transaction might succeed if retried.")));
        }
        else if (SxactIsPrepared(writer))
        {
            LWLockRelease(SerializableXactHashLock);

            /* if we're not the writer, we have to be the reader */
            Assert(MySerializableXact == reader);
            ereport(ERROR,
                    (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                     errmsg("could not serialize access due to read/write dependencies among transactions"),
                     errdetail_internal("Reason code: Canceled on conflict out to pivot %u, during read.", writer->topXid),
                     errhint("The transaction might succeed if retried.")));
        }
        writer->flags |= SXACT_FLAG_DOOMED;
    }
}

/*
 * PreCommit_CheckForSerializableConflicts
 *      Check for dangerous structures in a serializable transaction
 *      at commit.
 *
 * We're checking for a dangerous structure as each conflict is recorded.
 * The only way we could have a problem at commit is if this is the "out"
 * side of a pivot, and neither the "in" side nor the pivot has yet
 * committed.
 *
 * If a dangerous structure is found, the pivot (the near conflict) is
 * marked for death, because rolling back another transaction might mean
 * that we flail without ever making progress.  This transaction is
 * committing writes, so letting it commit ensures progress.  If we
 * canceled the far conflict, it might immediately fail again on retry.
 */
void
PreCommit_CheckForSerializationFailure(void)
{
    RWConflict  nearConflict;

    if (MySerializableXact == InvalidSerializableXact)
        return;

    Assert(IsolationIsSerializable());

    LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

    /* Check if someone else has already decided that we need to die */
    if (SxactIsDoomed(MySerializableXact))
    {
        LWLockRelease(SerializableXactHashLock);
        ereport(ERROR,
                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                 errdetail_internal("Reason code: Canceled on identification as a pivot, during commit attempt."),
                 errhint("The transaction might succeed if retried.")));
    }

    nearConflict = (RWConflict)
        SHMQueueNext(&MySerializableXact->inConflicts,
                     &MySerializableXact->inConflicts,
                     offsetof(RWConflictData, inLink));
    while (nearConflict)
    {
        if (!SxactIsCommitted(nearConflict->sxactOut)
            && !SxactIsDoomed(nearConflict->sxactOut))
        {
            RWConflict  farConflict;

            farConflict = (RWConflict)
                SHMQueueNext(&nearConflict->sxactOut->inConflicts,
                             &nearConflict->sxactOut->inConflicts,
                             offsetof(RWConflictData, inLink));
            while (farConflict)
            {
                if (farConflict->sxactOut == MySerializableXact
                    || (!SxactIsCommitted(farConflict->sxactOut)
                        && !SxactIsReadOnly(farConflict->sxactOut)
                        && !SxactIsDoomed(farConflict->sxactOut)))
                {
                    /*
                     * Normally, we kill the pivot transaction to make sure we
                     * make progress if the failing transaction is retried.
                     * However, we can't kill it if it's already prepared, so
                     * in that case we commit suicide instead.
                     */
                    if (SxactIsPrepared(nearConflict->sxactOut))
                    {
                        LWLockRelease(SerializableXactHashLock);
                        ereport(ERROR,
                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                                 errdetail_internal("Reason code: Canceled on commit attempt with conflict in from prepared pivot."),
                                 errhint("The transaction might succeed if retried.")));
                    }
                    nearConflict->sxactOut->flags |= SXACT_FLAG_DOOMED;
                    break;
                }
                farConflict = (RWConflict)
                    SHMQueueNext(&nearConflict->sxactOut->inConflicts,
                                 &farConflict->inLink,
                                 offsetof(RWConflictData, inLink));
            }
        }

        nearConflict = (RWConflict)
            SHMQueueNext(&MySerializableXact->inConflicts,
                         &nearConflict->inLink,
                         offsetof(RWConflictData, inLink));
    }

    MySerializableXact->prepareSeqNo = ++(PredXact->LastSxactCommitSeqNo);
    MySerializableXact->flags |= SXACT_FLAG_PREPARED;

    LWLockRelease(SerializableXactHashLock);
}

/*------------------------------------------------------------------------*/

/*
 * Two-phase commit support
 */

/*
 * AtPrepare_Locks
 *      Do the preparatory work for a PREPARE: make 2PC state file
 *      records for all predicate locks currently held.
 */
void
AtPrepare_PredicateLocks(void)
{
    PREDICATELOCK *predlock;
    SERIALIZABLEXACT *sxact;
    TwoPhasePredicateRecord record;
    TwoPhasePredicateXactRecord *xactRecord;
    TwoPhasePredicateLockRecord *lockRecord;

    sxact = MySerializableXact;
    xactRecord = &(record.data.xactRecord);
    lockRecord = &(record.data.lockRecord);

    if (MySerializableXact == InvalidSerializableXact)
        return;

    /* Generate a xact record for our SERIALIZABLEXACT */
    record.type = TWOPHASEPREDICATERECORD_XACT;
    xactRecord->xmin = MySerializableXact->xmin;
    xactRecord->flags = MySerializableXact->flags;

    /*
     * Note that we don't include the list of conflicts in our out in the
     * statefile, because new conflicts can be added even after the
     * transaction prepares. We'll just make a conservative assumption during
     * recovery instead.
     */

    RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
                           &record, sizeof(record));

    /*
     * Generate a lock record for each lock.
     *
     * To do this, we need to walk the predicate lock list in our sxact rather
     * than using the local predicate lock table because the latter is not
     * guaranteed to be accurate.
     */
    LWLockAcquire(SerializablePredicateLockListLock, LW_SHARED);

    predlock = (PREDICATELOCK *)
        SHMQueueNext(&(sxact->predicateLocks),
                     &(sxact->predicateLocks),
                     offsetof(PREDICATELOCK, xactLink));

    while (predlock != NULL)
    {
        record.type = TWOPHASEPREDICATERECORD_LOCK;
        lockRecord->target = predlock->tag.myTarget->tag;

        RegisterTwoPhaseRecord(TWOPHASE_RM_PREDICATELOCK_ID, 0,
                               &record, sizeof(record));

        predlock = (PREDICATELOCK *)
            SHMQueueNext(&(sxact->predicateLocks),
                         &(predlock->xactLink),
                         offsetof(PREDICATELOCK, xactLink));
    }

    LWLockRelease(SerializablePredicateLockListLock);
}

/*
 * PostPrepare_Locks
 *      Clean up after successful PREPARE. Unlike the non-predicate
 *      lock manager, we do not need to transfer locks to a dummy
 *      PGPROC because our SERIALIZABLEXACT will stay around
 *      anyway. We only need to clean up our local state.
 */
void
PostPrepare_PredicateLocks(TransactionId xid)
{
    if (MySerializableXact == InvalidSerializableXact)
        return;

    Assert(SxactIsPrepared(MySerializableXact));

    MySerializableXact->pid = 0;

    hash_destroy(LocalPredicateLockHash);
    LocalPredicateLockHash = NULL;

    MySerializableXact = InvalidSerializableXact;
    MyXactDidWrite = false;
}

/*
 * PredicateLockTwoPhaseFinish
 *      Release a prepared transaction's predicate locks once it
 *      commits or aborts.
 */
void
PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit)
{
    SERIALIZABLEXID *sxid;
    SERIALIZABLEXIDTAG sxidtag;

    sxidtag.xid = xid;

    LWLockAcquire(SerializableXactHashLock, LW_SHARED);
    sxid = (SERIALIZABLEXID *)
        hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
    LWLockRelease(SerializableXactHashLock);

    /* xid will not be found if it wasn't a serializable transaction */
    if (sxid == NULL)
        return;

    /* Release its locks */
    MySerializableXact = sxid->myXact;
    MyXactDidWrite = true;      /* conservatively assume that we wrote
                                 * something */
    ReleasePredicateLocks(isCommit);
}

/*
 * Re-acquire a predicate lock belonging to a transaction that was prepared.
 */
void
predicatelock_twophase_recover(TransactionId xid, uint16 info,
                               void *recdata, uint32 len)
{
    TwoPhasePredicateRecord *record;

    Assert(len == sizeof(TwoPhasePredicateRecord));

    record = (TwoPhasePredicateRecord *) recdata;

    Assert((record->type == TWOPHASEPREDICATERECORD_XACT) ||
           (record->type == TWOPHASEPREDICATERECORD_LOCK));

    if (record->type == TWOPHASEPREDICATERECORD_XACT)
    {
        /* Per-transaction record. Set up a SERIALIZABLEXACT. */
        TwoPhasePredicateXactRecord *xactRecord;
        SERIALIZABLEXACT *sxact;
        SERIALIZABLEXID *sxid;
        SERIALIZABLEXIDTAG sxidtag;
        bool        found;

        xactRecord = (TwoPhasePredicateXactRecord *) &record->data.xactRecord;

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);
        sxact = CreatePredXact();
        if (!sxact)
            ereport(ERROR,
                    (errcode(ERRCODE_OUT_OF_MEMORY),
                     errmsg("out of shared memory")));

        /* vxid for a prepared xact is InvalidBackendId/xid; no pid */
        sxact->vxid.backendId = InvalidBackendId;
        sxact->vxid.localTransactionId = (LocalTransactionId) xid;
        sxact->pid = 0;

        /* a prepared xact hasn't committed yet */
        sxact->prepareSeqNo = RecoverySerCommitSeqNo;
        sxact->commitSeqNo = InvalidSerCommitSeqNo;
        sxact->finishedBefore = InvalidTransactionId;

        sxact->SeqNo.lastCommitBeforeSnapshot = RecoverySerCommitSeqNo;

        /*
         * Don't need to track this; no transactions running at the time the
         * recovered xact started are still active, except possibly other
         * prepared xacts and we don't care whether those are RO_SAFE or not.
         */
        SHMQueueInit(&(sxact->possibleUnsafeConflicts));

        SHMQueueInit(&(sxact->predicateLocks));
        SHMQueueElemInit(&(sxact->finishedLink));

        sxact->topXid = xid;
        sxact->xmin = xactRecord->xmin;
        sxact->flags = xactRecord->flags;
        Assert(SxactIsPrepared(sxact));
        if (!SxactIsReadOnly(sxact))
        {
            ++(PredXact->WritableSxactCount);
            Assert(PredXact->WritableSxactCount <=
                   (MaxBackends + max_prepared_xacts));
        }

        /*
         * We don't know whether the transaction had any conflicts or not, so
         * we'll conservatively assume that it had both a conflict in and a
         * conflict out, and represent that with the summary conflict flags.
         */
        SHMQueueInit(&(sxact->outConflicts));
        SHMQueueInit(&(sxact->inConflicts));
        sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
        sxact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;

        /* Register the transaction's xid */
        sxidtag.xid = xid;
        sxid = (SERIALIZABLEXID *) hash_search(SerializableXidHash,
                                               &sxidtag,
                                               HASH_ENTER, &found);
        Assert(sxid != NULL);
        Assert(!found);
        sxid->myXact = (SERIALIZABLEXACT *) sxact;

        /*
         * Update global xmin. Note that this is a special case compared to
         * registering a normal transaction, because the global xmin might go
         * backwards. That's OK, because until recovery is over we're not
         * going to complete any transactions or create any non-prepared
         * transactions, so there's no danger of throwing away.
         */
        if ((!TransactionIdIsValid(PredXact->SxactGlobalXmin)) ||
            (TransactionIdFollows(PredXact->SxactGlobalXmin, sxact->xmin)))
        {
            PredXact->SxactGlobalXmin = sxact->xmin;
            PredXact->SxactGlobalXminCount = 1;
            OldSerXidSetActiveSerXmin(sxact->xmin);
        }
        else if (TransactionIdEquals(sxact->xmin, PredXact->SxactGlobalXmin))
        {
            Assert(PredXact->SxactGlobalXminCount > 0);
            PredXact->SxactGlobalXminCount++;
        }

        LWLockRelease(SerializableXactHashLock);
    }
    else if (record->type == TWOPHASEPREDICATERECORD_LOCK)
    {
        /* Lock record. Recreate the PREDICATELOCK */
        TwoPhasePredicateLockRecord *lockRecord;
        SERIALIZABLEXID *sxid;
        SERIALIZABLEXACT *sxact;
        SERIALIZABLEXIDTAG sxidtag;
        uint32      targettaghash;

        lockRecord = (TwoPhasePredicateLockRecord *) &record->data.lockRecord;
        targettaghash = PredicateLockTargetTagHashCode(&lockRecord->target);

        LWLockAcquire(SerializableXactHashLock, LW_SHARED);
        sxidtag.xid = xid;
        sxid = (SERIALIZABLEXID *)
            hash_search(SerializableXidHash, &sxidtag, HASH_FIND, NULL);
        LWLockRelease(SerializableXactHashLock);

        Assert(sxid != NULL);
        sxact = sxid->myXact;
        Assert(sxact != InvalidSerializableXact);

        CreatePredicateLock(&lockRecord->target, targettaghash, sxact);
    }
}