Make ascii-art in comments pgindent-safe, and some other formatting changes.

[postgresql] / src / backend / storage / lmgr / predicate.c

/*-------------------------------------------------------------------------
 *
 * 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) 1996-2011, 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
 *              RegisterSerializableTransaction(Snapshot snapshot)
 *              RegisterPredicateLockingXid(void)
 *              PredicateLockRelation(Relation relation)
 *              PredicateLockPage(Relation relation, BlockNumber blkno)
 *              PredicateLockTuple(Relation relation, HeapTuple tuple)
 *              PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
 *                                                         BlockNumber newblkno);
 *              PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
 *                                                               BlockNumber newblkno);
 *              ReleasePredicateLocks(bool isCommit)
 *
 * conflict detection (may also trigger rollback)
 *              CheckForSerializableConflictOut(bool visible, Relation relation,
 *                                                                              HeapTupleData *tup, Buffer buffer)
 *              CheckForSerializableConflictIn(Relation relation, HeapTupleData *tup,
 *                                                                         Buffer buffer)
 *
 * 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/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/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)))

#define SxactIsPrepared(sxact) (((sxact)->flags & SXACT_FLAG_PREPARED) != 0)
#define SxactIsCommitted(sxact) (((sxact)->flags & SXACT_FLAG_COMMITTED) != 0)
#define SxactIsRolledBack(sxact) (((sxact)->flags & SXACT_FLAG_ROLLED_BACK) != 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)
#define SxactIsMarkedForDeath(sxact) (((sxact)->flags & SXACT_FLAG_MARKED_FOR_DEATH) != 0)

/*
 * When a public interface method is called for a split on an index relation,
 * this is the test to see if we should do a quick return.
 */
#define SkipSplitTracking(relation) \
        (((relation)->rd_id < FirstBootstrapObjectId) \
        || RelationUsesLocalBuffers(relation))

/*
 * When a public interface method is called for serializing a relation within
 * the current transaction, this is the test to see if we should do a quick
 * return.
 */
#define SkipSerialization(relation) \
        ((!IsolationIsSerializable()) \
        || ((MySerializableXact == InvalidSerializableXact)) \
        || ReleasePredicateLocksIfROSafe() \
        || SkipSplitTracking(relation))


/*
 * 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)
#define OLDSERXID_MAX_PAGE                      (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)

#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;
}       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 reserved entry in PredicateLockTargetHash; used to ensure
 * there's an element available for scratch space if we need it,
 * e.g. in PredicateLockPageSplit. This is an otherwise-invalid tag.
 */
static const PREDICATELOCKTARGETTAG ReservedTargetTag = {0, 0, 0, 0, 0};

/*
 * 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.
 * TODO SSI: Remove volatile qualifier and the then-unnecessary casts?
 */
static volatile SERIALIZABLEXACT *MySerializableXact = InvalidSerializableXact;

/* 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 RegisterSerializableTransactionInt(Snapshot snapshot);
static bool PredicateLockExists(const PREDICATELOCKTARGETTAG *targettag);
static bool GetParentPredicateLockTag(const PREDICATELOCKTARGETTAG *tag,
                                                  PREDICATELOCKTARGETTAG *parent);
static bool CoarserLockCovers(const PREDICATELOCKTARGETTAG *newtargettag);
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(const PREDICATELOCKTARGETTAG oldtargettag,
                                                                  const PREDICATELOCKTARGETTAG newtargettag,
                                                                  bool removeOld);
static void PredicateLockAcquire(const PREDICATELOCKTARGETTAG *targettag);
static void SetNewSxactGlobalXmin(void);
static bool ReleasePredicateLocksIfROSafe(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);

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

/*
 * 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 (SxactIsRolledBack(reader)
                || SxactIsRolledBack(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 rw-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 rw-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 < -((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.
 * A zero 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;
        int                     xidSpread;
        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;

        xidSpread = (((uint32) xid) - ((uint32) tailXid));
        if (oldSerXidControl->warningIssued)
        {
                if (xidSpread < 800000000)
                        oldSerXidControl->warningIssued = false;
        }
        else if (xidSpread >= 1000000000)
        {
                oldSerXidControl->warningIssued = true;
                ereport(WARNING,
                                (errmsg("memory for serializable conflict tracking is nearly exhausted"),
                                 errhint("There may 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;

        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 everything.  If we try to
                 * leave the head page around to avoid re-zeroing it, we might not use
                 * the SLRU again until we're past the wrap-around point, which makes
                 * SLRU unhappy.
                 *
                 * While the API asks you to specify truncation by page, it silently
                 * ignores the request unless the specified page is in a segment past
                 * some allocated portion of the SLRU.  We don't care which page in a
                 * later segment we hit, so just add the number of pages per segment
                 * to the head page to land us *somewhere* in the next segment.
                 */
                tailPage = oldSerXidControl->headPage + SLRU_PAGES_PER_SEGMENT;
                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 an 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, &ReservedTargetTag,
                                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->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
         * occassional 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();
}

/*
 * 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.
 */
static Snapshot
GetSafeSnapshot(Snapshot origSnapshot)
{
        Snapshot        snapshot;

        Assert(XactReadOnly && XactDeferrable);

        while (true)
        {
                /*
                 * RegisterSerializableTransactionInt is going to call
                 * GetSnapshotData, so we need to provide it the static snapshot our
                 * caller passed to us. It returns a copy of that snapshot and
                 * registers it on TopTransactionResourceOwner.
                 */
                snapshot = RegisterSerializableTransactionInt(origSnapshot);

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

                MySerializableXact->flags |= SXACT_FLAG_DEFERRABLE_WAITING;

                /*
                 * Wait for concurrent transactions to finish. Stop early if one of
                 * them marked us as conflicted.
                 */
                while (!(SHMQueueEmpty((SHM_QUEUE *)
                                                         &MySerializableXact->possibleUnsafeConflicts) ||
                                 SxactIsROUnsafe(MySerializableXact)))
                        ProcWaitForSignal();

                MySerializableXact->flags &= ~SXACT_FLAG_DEFERRABLE_WAITING;
                if (!SxactIsROUnsafe(MySerializableXact))
                        break;                          /* success */

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

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

        return snapshot;
}

/*
 * Acquire and register a snapshot which can be used for this transaction..
 * Make sure we have a SERIALIZABLEXACT reference in MySerializableXact.
 * It should be current for this process and be contained in PredXact.
 */
Snapshot
RegisterSerializableTransaction(Snapshot snapshot)
{
        Assert(IsolationIsSerializable());

        /*
         * 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 RegisterSerializableTransactionInt(snapshot);
}

static Snapshot
RegisterSerializableTransactionInt(Snapshot snapshot)
{
        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.
         */
#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 and register a snapshot */
        snapshot = GetSnapshotData(snapshot);
        snapshot = RegisterSnapshotOnOwner(snapshot, TopTransactionResourceOwner);

        /*
         * 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->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 (!SxactIsOnFinishedList(othersxact) &&
                                !SxactIsReadOnly(othersxact))
                        {
                                SetPossibleUnsafeConflict(sxact, othersxact);
                        }
                }
        }
        else
        {
                ++(PredXact->WritableSxactCount);
                Assert(PredXact->WritableSxactCount <=
                           (MaxBackends + max_prepared_xacts));
        }

        MySerializableXact = sxact;

        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(const 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;

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

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

        MySerializableXact->topXid = xid;

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

        /* Initialize the structure. */
        sxid->myXact = (SERIALIZABLEXACT *) 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(const Relation relation, const 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;
}

/*
 * 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;

        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 = (SERIALIZABLEXACT *) 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;

                        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;

                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,
                                                (SERIALIZABLEXACT *) 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(const Relation relation)
{
        PREDICATELOCKTARGETTAG tag;

        if (SkipSerialization(relation))
                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(const Relation relation, const BlockNumber blkno)
{
        PREDICATELOCKTARGETTAG tag;

        if (SkipSerialization(relation))
                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(const Relation relation, const HeapTuple tuple)
{
        PREDICATELOCKTARGETTAG tag;
        ItemPointer tid;
        TransactionId targetxmin;

        if (SkipSerialization(relation))
                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 reserved 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(const PREDICATELOCKTARGETTAG oldtargettag,
                                                                  const PREDICATELOCKTARGETTAG newtargettag,
                                                                  bool removeOld)
{
        uint32          oldtargettaghash;
        LWLockId        oldpartitionLock;
        PREDICATELOCKTARGET *oldtarget;
        uint32          newtargettaghash;
        LWLockId        newpartitionLock;
        bool            found;
        bool            outOfShmem = false;
        uint32          reservedtargettaghash;
        LWLockId        reservedpartitionLock;


        Assert(LWLockHeldByMe(SerializablePredicateLockListLock));

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

        reservedtargettaghash = 0;      /* Quiet compiler warnings. */
        reservedpartitionLock = 0;      /* Quiet compiler warnings. */

        if (removeOld)
        {
                /*
                 * Remove the reserved entry to give us scratch space, so we know
                 * we'll be able to create the new lock target.
                 */
                reservedtargettaghash = PredicateLockTargetTagHashCode(&ReservedTargetTag);
                reservedpartitionLock = PredicateLockHashPartitionLock(reservedtargettaghash);
                LWLockAcquire(reservedpartitionLock, LW_EXCLUSIVE);
                hash_search_with_hash_value(PredicateLockTargetHash,
                                                                        &ReservedTargetTag,
                                                                        reservedtargettaghash,
                                                                        HASH_REMOVE, &found);
                Assert(found);
                LWLockRelease(reservedpartitionLock);
        }

        /*
         * 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;

                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 reserved entry back */
                LWLockAcquire(reservedpartitionLock, LW_EXCLUSIVE);
                hash_search_with_hash_value(PredicateLockTargetHash,
                                                                        &ReservedTargetTag,
                                                                        reservedtargettaghash,
                                                                        HASH_ENTER, &found);
                Assert(!found);
                LWLockRelease(reservedpartitionLock);
        }

        return !outOfShmem;
}


/*
 *              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(const Relation relation, const BlockNumber oldblkno,
                                           const 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 (SkipSplitTracking(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(const Relation relation, const BlockNumber oldblkno,
                                                 const 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 hash table 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(const 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(!SxactIsRolledBack(MySerializableXact));
        Assert(!SxactIsCommitted(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((SERIALIZABLEXACT *) 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 (!(MySerializableXact->flags & SXACT_FLAG_DID_WRITE))
                        MySerializableXact->flags |= SXACT_FLAG_READ_ONLY;
        }
        else
        {
                MySerializableXact->flags |= SXACT_FLAG_ROLLED_BACK;
        }

        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((SHM_QUEUE *) &MySerializableXact->possibleUnsafeConflicts,
                                  (SHM_QUEUE *) &MySerializableXact->possibleUnsafeConflicts,
                                                 offsetof(RWConflictData, inLink));
                while (possibleUnsafeConflict)
                {
                        nextConflict = (RWConflict)
                                SHMQueueNext((SHM_QUEUE *) &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))
        {
                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((SHM_QUEUE *) &MySerializableXact->outConflicts,
                                         (SHM_QUEUE *) &MySerializableXact->outConflicts,
                                         offsetof(RWConflictData, outLink));
        while (conflict)
        {
                nextConflict = (RWConflict)
                        SHMQueueNext((SHM_QUEUE *) &MySerializableXact->outConflicts,
                                                 &conflict->outLink,
                                                 offsetof(RWConflictData, outLink));

                if (isCommit
                        && !SxactIsReadOnly(MySerializableXact)
                        && SxactIsCommitted(conflict->sxactIn))
                {
                        if ((MySerializableXact->flags & SXACT_FLAG_CONFLICT_OUT) == 0
                                || conflict->sxactIn->commitSeqNo < MySerializableXact->SeqNo.earliestOutConflictCommit)
                                MySerializableXact->SeqNo.earliestOutConflictCommit = conflict->sxactIn->commitSeqNo;
                        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((SHM_QUEUE *) &MySerializableXact->inConflicts,
                                         (SHM_QUEUE *) &MySerializableXact->inConflicts,
                                         offsetof(RWConflictData, inLink));
        while (conflict)
        {
                nextConflict = (RWConflict)
                        SHMQueueNext((SHM_QUEUE *) &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((SHM_QUEUE *) &MySerializableXact->possibleUnsafeConflicts,
                                  (SHM_QUEUE *) &MySerializableXact->possibleUnsafeConflicts,
                                                 offsetof(RWConflictData, outLink));
                while (possibleUnsafeConflict)
                {
                        nextConflict = (RWConflict)
                                SHMQueueNext((SHM_QUEUE *) &MySerializableXact->possibleUnsafeConflicts,
                                                         &possibleUnsafeConflict->outLink,
                                                         offsetof(RWConflictData, outLink));

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

                        /* Mark conflicted if necessary. */
                        if (isCommit
                                && (MySerializableXact->flags & SXACT_FLAG_DID_WRITE)
                                && 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,
                                                  (SHM_QUEUE *) &(MySerializableXact->finishedLink));

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

        LWLockRelease(SerializableFinishedListLock);

        if (needToClear)
                ClearOldPredicateLocks();

        MySerializableXact = InvalidSerializableXact;

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

/*
 * ReleasePredicateLocksIfROSafe
 *              Check if the current transaction is read only and operating on
 *              a safe snapshot. If so, release predicate locks and return
 *              true.
 *
 * 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. Thus, we call this function as part of the
 * SkipSerialization check on all public interface methods.
 */
static bool
ReleasePredicateLocksIfROSafe(void)
{
        if (SxactIsROSafe(MySerializableXact))
        {
                ReleasePredicateLocks(false);
                return true;
        }
        else
                return false;
}

/*
 * Clear old predicate locks.
 */
static void
ClearOldPredicateLocks(void)
{
        SERIALIZABLEXACT *finishedSxact;
        PREDICATELOCK *predlock;

        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))
                {
                        LWLockRelease(SerializableXactHashLock);
                        SHMQueueDelete(&(finishedSxact->finishedLink));
                        ReleaseOneSerializableXact(finishedSxact, false, false);
                        LWLockAcquire(SerializableXactHashLock, LW_SHARED);
                }
                else if (finishedSxact->commitSeqNo > PredXact->HavePartialClearedThrough
                   && finishedSxact->commitSeqNo <= PredXact->CanPartialClearThrough)
                {
                        LWLockRelease(SerializableXactHashLock);
                        ReleaseOneSerializableXact(finishedSxact,
                                                                           !SxactIsReadOnly(finishedSxact),
                                                                           false);
                        PredXact->HavePartialClearedThrough = finishedSxact->commitSeqNo;
                        LWLockAcquire(SerializableXactHashLock, LW_SHARED);
                }
                else
                        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 (canDoPartialCleanup)
                {
                        PREDICATELOCKTAG tag;
                        SHM_QUEUE  *targetLink;
                        PREDICATELOCKTARGET *target;
                        PREDICATELOCKTARGETTAG targettag;
                        uint32          targettaghash;
                        LWLockId        partitionLock;

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

                        LWLockAcquire(partitionLock, LW_EXCLUSIVE);

                        SHMQueueDelete(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
 * out-conflict information -- we've established that there are no longer
 * any overlapping read write transactions for which this transaction could
 * matter.
 *
 * 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(LWLockHeldByMe(SerializableFinishedListLock));

        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);

        if (!partial)
        {
                /* Release all outConflicts. */
                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;
        }

        if (!partial)
        {
                /* Get rid of the xid and the record of the transaction itself. */
                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(const bool visible, const Relation relation,
                                                                const HeapTuple tuple, const Buffer buffer)
{
        TransactionId xid;
        SERIALIZABLEXIDTAG sxidtag;
        SERIALIZABLEXID *sxid;
        SERIALIZABLEXACT *sxact;
        HTSV_Result htsvResult;

        if (SkipSerialization(relation))
                return;

        if (SxactIsMarkedForDeath(MySerializableXact))
        {
                ereport(ERROR,
                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                                 errdetail("Cancelled 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 = HeapTupleHeaderGetXmax(tuple->t_data);
                        break;
                case HEAPTUPLE_DELETE_IN_PROGRESS:
                        xid = HeapTupleHeaderGetXmax(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("Cancelled on conflict out to old pivot %u.", xid),
                                          errhint("The transaction might succeed if retried.")));

                        if (SxactHasSummaryConflictIn(MySerializableXact)
                        || !SHMQueueEmpty((SHM_QUEUE *) &MySerializableXact->inConflicts))
                                ereport(ERROR,
                                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                                                 errdetail("Cancelled 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
                || SxactIsRolledBack(sxact)
                || SxactIsMarkedForDeath(sxact))
        {
                /* We can't conflict with our own transaction or one rolled 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 cancelled.
         */
        if (SxactHasSummaryConflictOut(sxact))
        {
                if (!SxactIsPrepared(sxact))
                {
                        sxact->flags |= SXACT_FLAG_MARKED_FOR_DEATH;
                        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("Cancelled 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((SERIALIZABLEXACT *) 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((SERIALIZABLEXACT *) MySerializableXact, sxact);
        LWLockRelease(SerializableXactHashLock);
}

/*
 * Check a particular target for rw-dependency conflict in.
 */
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 (!SxactIsRolledBack(sxact)
                                 && (!SxactIsCommitted(sxact)
                                         || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                                                                          sxact->finishedBefore))
                && !RWConflictExists(sxact, (SERIALIZABLEXACT *) MySerializableXact))
                {
                        LWLockRelease(SerializableXactHashLock);
                        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

                        /*
                         * Re-check after getting exclusive lock because the other
                         * transaction may have flagged a conflict.
                         */
                        if (!SxactIsRolledBack(sxact)
                                && (!SxactIsCommitted(sxact)
                                        || TransactionIdPrecedes(GetTransactionSnapshot()->xmin,
                                                                                         sxact->finishedBefore))
                                && !RWConflictExists(sxact,
                                                                         (SERIALIZABLEXACT *) MySerializableXact))
                        {
                                FlagRWConflict(sxact, (SERIALIZABLEXACT *) 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(const Relation relation, const HeapTuple tuple,
                                                           const Buffer buffer)
{
        PREDICATELOCKTARGETTAG targettag;

        if (SkipSerialization(relation))
                return;

        if (SxactIsMarkedForDeath(MySerializableXact))
                ereport(ERROR,
                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                                 errdetail("Cancelled on identification as a pivot, during conflict in checking."),
                                 errhint("The transaction might succeed if retried.")));

        MySerializableXact->flags |= SXACT_FLAG_DID_WRITE;

        /*
         * 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);
}

/*
 * 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)
         *------------------------------------------------------------------------
         */
        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 (SxactIsCommitted(t2)
                                && (!SxactIsCommitted(reader)
                                        || t2->commitSeqNo <= reader->commitSeqNo)
                                && (!SxactIsCommitted(writer)
                                        || t2->commitSeqNo <= writer->commitSeqNo)
                                && (!SxactIsReadOnly(reader)
                           || t2->commitSeqNo <= 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 committed writer:
         *
         *              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 && SxactIsCommitted(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 (!SxactIsRolledBack(t0)
                                && (!SxactIsCommitted(t0)
                                        || t0->commitSeqNo >= writer->commitSeqNo)
                                && (!SxactIsReadOnly(t0)
                           || t0->SeqNo.lastCommitBeforeSnapshot >= writer->commitSeqNo))
                        {
                                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("Cancelled 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("Cancelled on conflict out to pivot %u, during read.", writer->topXid),
                                         errhint("The transaction might succeed if retried.")));
                }
                writer->flags |= SXACT_FLAG_MARKED_FOR_DEATH;
        }
}

/*
 * 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
 * cancelled 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);

        if (SxactIsMarkedForDeath(MySerializableXact))
        {
                LWLockRelease(SerializableXactHashLock);
                ereport(ERROR,
                                (errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
                                 errmsg("could not serialize access due to read/write dependencies among transactions"),
                                 errdetail("Cancelled on identification as a pivot, during commit attempt."),
                                 errhint("The transaction might succeed if retried.")));
        }

        nearConflict = (RWConflict)
                SHMQueueNext((SHM_QUEUE *) &MySerializableXact->inConflicts,
                                         (SHM_QUEUE *) &MySerializableXact->inConflicts,
                                         offsetof(RWConflictData, inLink));
        while (nearConflict)
        {
                if (!SxactIsCommitted(nearConflict->sxactOut)
                        && !SxactIsRolledBack(nearConflict->sxactOut)
                        && !SxactIsMarkedForDeath(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)
                                                && !SxactIsRolledBack(farConflict->sxactOut)
                                                && !SxactIsMarkedForDeath(farConflict->sxactOut)))
                                {
                                        nearConflict->sxactOut->flags |= SXACT_FLAG_MARKED_FOR_DEATH;
                                        break;
                                }
                                farConflict = (RWConflict)
                                        SHMQueueNext(&nearConflict->sxactOut->inConflicts,
                                                                 &farConflict->inLink,
                                                                 offsetof(RWConflictData, inLink));
                        }
                }

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

        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 = (SERIALIZABLEXACT *) 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;

        /*
         * Tweak the flags. Since we're not going to output the inConflicts and
         * outConflicts lists, if they're non-empty we'll represent that by
         * setting the appropriate summary conflict flags.
         */
        if (!SHMQueueEmpty((SHM_QUEUE *) &MySerializableXact->inConflicts))
                xactRecord->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
        if (!SHMQueueEmpty((SHM_QUEUE *) &MySerializableXact->outConflicts))
                xactRecord->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;

        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;
}

/*
 * 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;
        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->commitSeqNo = InvalidSerCommitSeqNo;
                sxact->finishedBefore = InvalidTransactionId;

                sxact->SeqNo.lastCommitBeforeSnapshot = RecoverySerCommitSeqNo;


                /*
                 * We don't need the details of a prepared transaction's conflicts,
                 * just whether it had conflicts in or out (which we get from the
                 * flags)
                 */
                SHMQueueInit(&(sxact->outConflicts));
                SHMQueueInit(&(sxact->inConflicts));

                /*
                 * 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));
                }

                /* 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);
        }
}