Rework lwlocknames.txt to become lwlocklist.h

[pgsql.git] / 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 an 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.
 *
 *      SerializablePredicateListLock
 *              - 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, except parallel query workers sharing the leader's
 *                      transaction.  In the parallel case, an extra per-sxact lock is
 *                      taken; see below.
 *              - 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.
 *
 *      SERIALIZABLEXACT's member 'perXactPredicateListLock'
 *              - Protects the linked list of predicate locks held by a transaction.
 *                      Only needed for parallel mode, where multiple backends share the
 *                      same SERIALIZABLEXACT object.  Not needed if
 *                      SerializablePredicateListLock is held exclusively.
 *
 *      PredicateLockHashPartitionLock(hashcode)
 *              - 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 address order.
 *              - When all are needed (rare), acquire in ascending index order with
 *                      PredicateLockHashPartitionLockByIndex(index).
 *
 *      SerializableXactHashLock
 *              - Protects both PredXact and SerializableXidHash.
 *
 *      SerialControlLock
 *              - Protects SerialControlData members
 *
 *      SerialSLRULock
 *              - Protects SerialSlruCtl
 *
 * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        src/backend/storage/lmgr/predicate.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *
 * housekeeping for setting up shared memory predicate lock structures
 *              InitPredicateLocks(void)
 *              PredicateLockShmemSize(void)
 *
 * predicate lock reporting
 *              GetPredicateLockStatusData(void)
 *              PageIsPredicateLocked(Relation relation, BlockNumber blkno)
 *
 * predicate lock maintenance
 *              GetSerializableTransactionSnapshot(Snapshot snapshot)
 *              SetSerializableTransactionSnapshot(Snapshot snapshot,
 *                                                                                 VirtualTransactionId *sourcevxid)
 *              RegisterPredicateLockingXid(void)
 *              PredicateLockRelation(Relation relation, Snapshot snapshot)
 *              PredicateLockPage(Relation relation, BlockNumber blkno,
 *                                              Snapshot snapshot)
 *              PredicateLockTID(Relation relation, ItemPointer tid, Snapshot snapshot,
 *                                               TransactionId tuple_xid)
 *              PredicateLockPageSplit(Relation relation, BlockNumber oldblkno,
 *                                                         BlockNumber newblkno)
 *              PredicateLockPageCombine(Relation relation, BlockNumber oldblkno,
 *                                                               BlockNumber newblkno)
 *              TransferPredicateLocksToHeapRelation(Relation relation)
 *              ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
 *
 * conflict detection (may also trigger rollback)
 *              CheckForSerializableConflictOut(Relation relation, TransactionId xid,
 *                                                                              Snapshot snapshot)
 *              CheckForSerializableConflictIn(Relation relation, ItemPointer tid,
 *                                                                         BlockNumber blkno)
 *              CheckTableForSerializableConflictIn(Relation relation)
 *
 * final rollback checking
 *              PreCommit_CheckForSerializationFailure(void)
 *
 * two-phase commit support
 *              AtPrepare_PredicateLocks(void);
 *              PostPrepare_PredicateLocks(TransactionId xid);
 *              PredicateLockTwoPhaseFinish(TransactionId xid, bool isCommit);
 *              predicatelock_twophase_recover(TransactionId xid, uint16 info,
 *                                                                         void *recdata, uint32 len);
 */

#include "postgres.h"

#include "access/parallel.h"
#include "access/slru.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/twophase_rmgr.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/pg_lfind.h"
#include "storage/predicate.h"
#include "storage/predicate_internals.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "utils/guc_hooks.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"

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

/*
 * 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) \
        (&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + \
                PredicateLockHashPartition(hashcode)].lock)
#define PredicateLockHashPartitionLockByIndex(i) \
        (&MainLWLockArray[PREDICATELOCK_MANAGER_LWLOCK_OFFSET + (i)].lock)

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

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

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

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

#define SerialSlruCtl                   (&SerialSlruCtlData)

#define SERIAL_PAGESIZE                 BLCKSZ
#define SERIAL_ENTRYSIZE                        sizeof(SerCommitSeqNo)
#define SERIAL_ENTRIESPERPAGE   (SERIAL_PAGESIZE / SERIAL_ENTRYSIZE)

/*
 * Set maximum pages based on the number needed to track all transactions.
 */
#define SERIAL_MAX_PAGE                 (MaxTransactionId / SERIAL_ENTRIESPERPAGE)

#define SerialNextPage(page) (((page) >= SERIAL_MAX_PAGE) ? 0 : (page) + 1)

#define SerialValue(slotno, xid) (*((SerCommitSeqNo *) \
        (SerialSlruCtl->shared->page_buffer[slotno] + \
        ((((uint32) (xid)) % SERIAL_ENTRIESPERPAGE) * SERIAL_ENTRYSIZE))))

#define SerialPage(xid) (((uint32) (xid)) / SERIAL_ENTRIESPERPAGE)

typedef struct SerialControlData
{
        int                     headPage;               /* newest initialized page */
        TransactionId headXid;          /* newest valid Xid in the SLRU */
        TransactionId tailXid;          /* oldest xmin we might be interested in */
}                       SerialControlData;

typedef struct SerialControlData *SerialControl;

static SerialControl serialControl;

/*
 * 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;


/*
 * These configuration variables are used to set the predicate lock table size
 * and to control promotion of predicate locks to coarser granularity in an
 * attempt to degrade performance (mostly as false positive serialization
 * failure) gracefully in the face of memory pressure.
 */
int                     max_predicate_locks_per_xact;   /* in guc_tables.c */
int                     max_predicate_locks_per_relation;       /* in guc_tables.c */
int                     max_predicate_locks_per_page;   /* in guc_tables.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 dlist_head *FinishedSerializableTransactions;

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

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

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

/*
 * The SXACT_FLAG_RO_UNSAFE optimization might lead us to release
 * MySerializableXact early.  If that happens in a parallel query, the leader
 * needs to defer the destruction of the SERIALIZABLEXACT until end of
 * transaction, because the workers still have a reference to it.  In that
 * case, the leader stores it here.
 */
static SERIALIZABLEXACT *SavedSerializableXact = InvalidSerializableXact;

/* local functions */

static SERIALIZABLEXACT *CreatePredXact(void);
static void ReleasePredXact(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 SerialPagePrecedesLogically(int64 page1, int64 page2);
static void SerialInit(void);
static void SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo);
static SerCommitSeqNo SerialGetMinConflictCommitSeqNo(TransactionId xid);
static void SerialSetActiveSerXmin(TransactionId xid);

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


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

/*
 * Does this relation participate in predicate locking? Temporary and system
 * relations are exempt.
 */
static inline bool
PredicateLockingNeededForRelation(Relation relation)
{
        return !(relation->rd_id < FirstUnpinnedObjectId ||
                         RelationUsesLocalBuffers(relation));
}

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

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

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

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

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

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

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

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


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

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

        if (dlist_is_empty(&PredXact->availableList))
                return NULL;

        sxact = dlist_container(SERIALIZABLEXACT, xactLink,
                                                        dlist_pop_head_node(&PredXact->availableList));
        dlist_push_tail(&PredXact->activeList, &sxact->xactLink);
        return sxact;
}

static void
ReleasePredXact(SERIALIZABLEXACT *sxact)
{
        Assert(ShmemAddrIsValid(sxact));

        dlist_delete(&sxact->xactLink);
        dlist_push_tail(&PredXact->availableList, &sxact->xactLink);
}

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

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

        Assert(reader != writer);

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

        /*
         * A conflict is possible; walk the list to find out.
         *
         * The unconstify is needed as we have no const version of
         * dlist_foreach().
         */
        dlist_foreach(iter, &unconstify(SERIALIZABLEXACT *, reader)->outConflicts)
        {
                RWConflict      conflict =
                        dlist_container(RWConflictData, outLink, iter.cur);

                if (conflict->sxactIn == writer)
                        return true;
        }

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

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

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

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

        conflict = dlist_head_element(RWConflictData, outLink, &RWConflictPool->availableList);
        dlist_delete(&conflict->outLink);

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

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

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

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

        conflict = dlist_head_element(RWConflictData, outLink, &RWConflictPool->availableList);
        dlist_delete(&conflict->outLink);

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

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

static void
FlagSxactUnsafe(SERIALIZABLEXACT *sxact)
{
        dlist_mutable_iter iter;

        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.
         */
        dlist_foreach_modify(iter, &sxact->possibleUnsafeConflicts)
        {
                RWConflict      conflict =
                        dlist_container(RWConflictData, inLink, iter.cur);

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

                ReleaseRWConflict(conflict);
        }
}

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

/*
 * Decide whether a Serial page number is "older" for truncation purposes.
 * Analogous to CLOGPagePrecedes().
 */
static bool
SerialPagePrecedesLogically(int64 page1, int64 page2)
{
        TransactionId xid1;
        TransactionId xid2;

        xid1 = ((TransactionId) page1) * SERIAL_ENTRIESPERPAGE;
        xid1 += FirstNormalTransactionId + 1;
        xid2 = ((TransactionId) page2) * SERIAL_ENTRIESPERPAGE;
        xid2 += FirstNormalTransactionId + 1;

        return (TransactionIdPrecedes(xid1, xid2) &&
                        TransactionIdPrecedes(xid1, xid2 + SERIAL_ENTRIESPERPAGE - 1));
}

#ifdef USE_ASSERT_CHECKING
static void
SerialPagePrecedesLogicallyUnitTests(void)
{
        int                     per_page = SERIAL_ENTRIESPERPAGE,
                                offset = per_page / 2;
        int64           newestPage,
                                oldestPage,
                                headPage,
                                targetPage;
        TransactionId newestXact,
                                oldestXact;

        /* GetNewTransactionId() has assigned the last XID it can safely use. */
        newestPage = 2 * SLRU_PAGES_PER_SEGMENT - 1;    /* nothing special */
        newestXact = newestPage * per_page + offset;
        Assert(newestXact / per_page == newestPage);
        oldestXact = newestXact + 1;
        oldestXact -= 1U << 31;
        oldestPage = oldestXact / per_page;

        /*
         * In this scenario, the SLRU headPage pertains to the last ~1000 XIDs
         * assigned.  oldestXact finishes, ~2B XIDs having elapsed since it
         * started.  Further transactions cause us to summarize oldestXact to
         * tailPage.  Function must return false so SerialAdd() doesn't zero
         * tailPage (which may contain entries for other old, recently-finished
         * XIDs) and half the SLRU.  Reaching this requires burning ~2B XIDs in
         * single-user mode, a negligible possibility.
         */
        headPage = newestPage;
        targetPage = oldestPage;
        Assert(!SerialPagePrecedesLogically(headPage, targetPage));

        /*
         * In this scenario, the SLRU headPage pertains to oldestXact.  We're
         * summarizing an XID near newestXact.  (Assume few other XIDs used
         * SERIALIZABLE, hence the minimal headPage advancement.  Assume
         * oldestXact was long-running and only recently reached the SLRU.)
         * Function must return true to make SerialAdd() create targetPage.
         *
         * Today's implementation mishandles this case, but it doesn't matter
         * enough to fix.  Verify that the defect affects just one page by
         * asserting correct treatment of its prior page.  Reaching this case
         * requires burning ~2B XIDs in single-user mode, a negligible
         * possibility.  Moreover, if it does happen, the consequence would be
         * mild, namely a new transaction failing in SimpleLruReadPage().
         */
        headPage = oldestPage;
        targetPage = newestPage;
        Assert(SerialPagePrecedesLogically(headPage, targetPage - 1));
#if 0
        Assert(SerialPagePrecedesLogically(headPage, targetPage));
#endif
}
#endif

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

        /*
         * Set up SLRU management of the pg_serial data.
         */
        SerialSlruCtl->PagePrecedes = SerialPagePrecedesLogically;
        SimpleLruInit(SerialSlruCtl, "serializable",
                                  serializable_buffers, 0, "pg_serial",
                                  LWTRANCHE_SERIAL_BUFFER, LWTRANCHE_SERIAL_SLRU,
                                  SYNC_HANDLER_NONE, false);
#ifdef USE_ASSERT_CHECKING
        SerialPagePrecedesLogicallyUnitTests();
#endif
        SlruPagePrecedesUnitTests(SerialSlruCtl, SERIAL_ENTRIESPERPAGE);

        /*
         * Create or attach to the SerialControl structure.
         */
        serialControl = (SerialControl)
                ShmemInitStruct("SerialControlData", sizeof(SerialControlData), &found);

        Assert(found == IsUnderPostmaster);
        if (!found)
        {
                /*
                 * Set control information to reflect empty SLRU.
                 */
                LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
                serialControl->headPage = -1;
                serialControl->headXid = InvalidTransactionId;
                serialControl->tailXid = InvalidTransactionId;
                LWLockRelease(SerialControlLock);
        }
}

/*
 * GUC check_hook for serializable_buffers
 */
bool
check_serial_buffers(int *newval, void **extra, GucSource source)
{
        return check_slru_buffers("serializable_buffers", newval);
}

/*
 * Record a committed read write serializable xid and the minimum
 * commitSeqNo of any transactions to which this xid had a rw-conflict out.
 * An invalid commitSeqNo means that there were no conflicts out from xid.
 */
static void
SerialAdd(TransactionId xid, SerCommitSeqNo minConflictCommitSeqNo)
{
        TransactionId tailXid;
        int64           targetPage;
        int                     slotno;
        int64           firstZeroPage;
        bool            isNewPage;
        LWLock     *lock;

        Assert(TransactionIdIsValid(xid));

        targetPage = SerialPage(xid);
        lock = SimpleLruGetBankLock(SerialSlruCtl, targetPage);

        /*
         * In this routine, we must hold both SerialControlLock and the SLRU bank
         * lock simultaneously while making the SLRU data catch up with the new
         * state that we determine.
         */
        LWLockAcquire(SerialControlLock, 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 = serialControl->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 (serialControl->headPage < 0)
        {
                firstZeroPage = SerialPage(tailXid);
                isNewPage = true;
        }
        else
        {
                firstZeroPage = SerialNextPage(serialControl->headPage);
                isNewPage = SerialPagePrecedesLogically(serialControl->headPage,
                                                                                                targetPage);
        }

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

        LWLockAcquire(lock, LW_EXCLUSIVE);

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

        SerialValue(slotno, xid) = minConflictCommitSeqNo;
        SerialSlruCtl->shared->page_dirty[slotno] = true;

        LWLockRelease(lock);
        LWLockRelease(SerialControlLock);
}

/*
 * 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
SerialGetMinConflictCommitSeqNo(TransactionId xid)
{
        TransactionId headXid;
        TransactionId tailXid;
        SerCommitSeqNo val;
        int                     slotno;

        Assert(TransactionIdIsValid(xid));

        LWLockAcquire(SerialControlLock, LW_SHARED);
        headXid = serialControl->headXid;
        tailXid = serialControl->tailXid;
        LWLockRelease(SerialControlLock);

        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 SLRU bank lock,
         * but will return with that lock held, which must then be released.
         */
        slotno = SimpleLruReadPage_ReadOnly(SerialSlruCtl,
                                                                                SerialPage(xid), xid);
        val = SerialValue(slotno, xid);
        LWLockRelease(SimpleLruGetBankLock(SerialSlruCtl, SerialPage(xid)));
        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
SerialSetActiveSerXmin(TransactionId xid)
{
        LWLockAcquire(SerialControlLock, 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))
        {
                serialControl->tailXid = InvalidTransactionId;
                serialControl->headXid = InvalidTransactionId;
                LWLockRelease(SerialControlLock);
                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(serialControl->headPage < 0);
                if (!TransactionIdIsValid(serialControl->tailXid)
                        || TransactionIdPrecedes(xid, serialControl->tailXid))
                {
                        serialControl->tailXid = xid;
                }
                LWLockRelease(SerialControlLock);
                return;
        }

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

        serialControl->tailXid = xid;

        LWLockRelease(SerialControlLock);
}

/*
 * 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                     truncateCutoffPage;

        LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);

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

        if (TransactionIdIsValid(serialControl->tailXid))
        {
                int                     tailPage;

                tailPage = SerialPage(serialControl->tailXid);

                /*
                 * It is possible for the tailXid to be ahead of the headXid.  This
                 * occurs if we checkpoint while there are in-progress serializable
                 * transaction(s) advancing the tail but we are yet to summarize the
                 * transactions.  In this case, we cutoff up to the headPage and the
                 * next summary will advance the headXid.
                 */
                if (SerialPagePrecedesLogically(tailPage, serialControl->headPage))
                {
                        /* We can truncate the SLRU up to the page containing tailXid */
                        truncateCutoffPage = tailPage;
                }
                else
                        truncateCutoffPage = serialControl->headPage;
        }
        else
        {
                /*----------
                 * The SLRU is no longer needed. Truncate to head before we set head
                 * invalid.
                 *
                 * XXX: It's possible that the SLRU is not needed again until XID
                 * wrap-around has happened, so that the segment containing headPage
                 * that we leave behind will appear to be new again. In that case it
                 * won't be removed until XID horizon advances enough to make it
                 * current again.
                 *
                 * XXX: This should happen in vac_truncate_clog(), not in checkpoints.
                 * Consider this scenario, starting from a system with no in-progress
                 * transactions and VACUUM FREEZE having maximized oldestXact:
                 * - Start a SERIALIZABLE transaction.
                 * - Start, finish, and summarize a SERIALIZABLE transaction, creating
                 *   one SLRU page.
                 * - Consume XIDs to reach xidStopLimit.
                 * - Finish all transactions.  Due to the long-running SERIALIZABLE
                 *   transaction, earlier checkpoints did not touch headPage.  The
                 *   next checkpoint will change it, but that checkpoint happens after
                 *   the end of the scenario.
                 * - VACUUM to advance XID limits.
                 * - Consume ~2M XIDs, crossing the former xidWrapLimit.
                 * - Start, finish, and summarize a SERIALIZABLE transaction.
                 *   SerialAdd() declines to create the targetPage, because headPage
                 *   is not regarded as in the past relative to that targetPage.  The
                 *   transaction instigating the summarize fails in
                 *   SimpleLruReadPage().
                 */
                truncateCutoffPage = serialControl->headPage;
                serialControl->headPage = -1;
        }

        LWLockRelease(SerialControlLock);

        /*
         * Truncate away pages that are no longer required.  Note that no
         * additional locking is required, because this is only called as part of
         * a checkpoint, and the validity limits have already been determined.
         */
        SimpleLruTruncate(SerialSlruCtl, truncateCutoffPage);

        /*
         * Write 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.
         */
        SimpleLruWriteAll(SerialSlruCtl, 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;
        long            max_table_size;
        Size            requestSize;
        bool            found;

#ifndef EXEC_BACKEND
        Assert(!IsUnderPostmaster);
#endif

        /*
         * 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.
         */
        info.keysize = sizeof(PREDICATELOCKTARGETTAG);
        info.entrysize = sizeof(PREDICATELOCKTARGET);
        info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;

        PredicateLockTargetHash = ShmemInitHash("PREDICATELOCKTARGET hash",
                                                                                        max_table_size,
                                                                                        max_table_size,
                                                                                        &info,
                                                                                        HASH_ELEM | HASH_BLOBS |
                                                                                        HASH_PARTITION | HASH_FIXED_SIZE);

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

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

        /*
         * Allocate hash table for PREDICATELOCK structs.  This stores per
         * xact-lock-of-a-target information.
         */
        info.keysize = sizeof(PREDICATELOCKTAG);
        info.entrysize = sizeof(PREDICATELOCK);
        info.hash = predicatelock_hash;
        info.num_partitions = NUM_PREDICATELOCK_PARTITIONS;

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

        PredicateLockHash = ShmemInitHash("PREDICATELOCK hash",
                                                                          max_table_size,
                                                                          max_table_size,
                                                                          &info,
                                                                          HASH_ELEM | HASH_FUNCTION |
                                                                          HASH_PARTITION | HASH_FIXED_SIZE);

        /*
         * 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);
        Assert(found == IsUnderPostmaster);
        if (!found)
        {
                int                     i;

                dlist_init(&PredXact->availableList);
                dlist_init(&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,
                                                           sizeof(SERIALIZABLEXACT));
                PredXact->element = ShmemAlloc(requestSize);
                /* Add all elements to available list, clean. */
                memset(PredXact->element, 0, requestSize);
                for (i = 0; i < max_table_size; i++)
                {
                        LWLockInitialize(&PredXact->element[i].perXactPredicateListLock,
                                                         LWTRANCHE_PER_XACT_PREDICATE_LIST);
                        dlist_push_tail(&PredXact->availableList, &PredXact->element[i].xactLink);
                }
                PredXact->OldCommittedSxact = CreatePredXact();
                SetInvalidVirtualTransactionId(PredXact->OldCommittedSxact->vxid);
                PredXact->OldCommittedSxact->prepareSeqNo = 0;
                PredXact->OldCommittedSxact->commitSeqNo = 0;
                PredXact->OldCommittedSxact->SeqNo.lastCommitBeforeSnapshot = 0;
                dlist_init(&PredXact->OldCommittedSxact->outConflicts);
                dlist_init(&PredXact->OldCommittedSxact->inConflicts);
                dlist_init(&PredXact->OldCommittedSxact->predicateLocks);
                dlist_node_init(&PredXact->OldCommittedSxact->finishedLink);
                dlist_init(&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;
                PredXact->OldCommittedSxact->pgprocno = INVALID_PROC_NUMBER;
        }
        /* 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.
         */
        info.keysize = sizeof(SERIALIZABLEXIDTAG);
        info.entrysize = sizeof(SERIALIZABLEXID);

        SerializableXidHash = ShmemInitHash("SERIALIZABLEXID hash",
                                                                                max_table_size,
                                                                                max_table_size,
                                                                                &info,
                                                                                HASH_ELEM | HASH_BLOBS |
                                                                                HASH_FIXED_SIZE);

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

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

                dlist_init(&RWConflictPool->availableList);
                requestSize = mul_size((Size) max_table_size,
                                                           RWConflictDataSize);
                RWConflictPool->element = ShmemAlloc(requestSize);
                /* Add all elements to available list, clean. */
                memset(RWConflictPool->element, 0, requestSize);
                for (i = 0; i < max_table_size; i++)
                {
                        dlist_push_tail(&RWConflictPool->availableList,
                                                        &RWConflictPool->element[i].outLink);
                }
        }

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

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

/*
 * 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,
                                                                   sizeof(SERIALIZABLEXACT)));

        /* 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(dlist_head));

        /* Shared memory structures for SLRU tracking of old committed xids. */
        size = add_size(size, sizeof(SerialControlData));
        size = add_size(size, SimpleLruShmemSize(serializable_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(PredicateLockHashPartitionLockByIndex(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(PredicateLockHashPartitionLockByIndex(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 (dlist_is_empty(FinishedSerializableTransactions))
        {
                LWLockRelease(SerializableFinishedListLock);
                return;
        }

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

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

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

        LWLockRelease(SerializableFinishedListLock);
}

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

        Assert(XactReadOnly && XactDeferrable);

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

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

                LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

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

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

                LWLockRelease(SerializableXactHashLock);

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

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

        return snapshot;
}

/*
 * GetSafeSnapshotBlockingPids
 *              If the specified process is currently blocked in GetSafeSnapshot,
 *              write the process IDs of all processes that it is blocked by
 *              into the caller-supplied buffer output[].  The list is truncated at
 *              output_size, and the number of PIDs written into the buffer is
 *              returned.  Returns zero if the given PID is not currently blocked
 *              in GetSafeSnapshot.
 */
int
GetSafeSnapshotBlockingPids(int blocked_pid, int *output, int output_size)
{
        int                     num_written = 0;
        dlist_iter      iter;
        SERIALIZABLEXACT *blocking_sxact = NULL;

        LWLockAcquire(SerializableXactHashLock, LW_SHARED);

        /* Find blocked_pid's SERIALIZABLEXACT by linear search. */
        dlist_foreach(iter, &PredXact->activeList)
        {
                SERIALIZABLEXACT *sxact =
                        dlist_container(SERIALIZABLEXACT, xactLink, iter.cur);

                if (sxact->pid == blocked_pid)
                {
                        blocking_sxact = sxact;
                        break;
                }
        }

        /* Did we find it, and is it currently waiting in GetSafeSnapshot? */
        if (blocking_sxact != NULL && SxactIsDeferrableWaiting(blocking_sxact))
        {
                /* Traverse the list of possible unsafe conflicts collecting PIDs. */
                dlist_foreach(iter, &blocking_sxact->possibleUnsafeConflicts)
                {
                        RWConflict      possibleUnsafeConflict =
                                dlist_container(RWConflictData, inLink, iter.cur);

                        output[num_written++] = possibleUnsafeConflict->sxactOut->pid;

                        if (num_written >= output_size)
                                break;
                }
        }

        LWLockRelease(SerializableXactHashLock);

        return num_written;
}

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

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

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

        return GetSerializableTransactionSnapshotInt(snapshot,
                                                                                                 NULL, InvalidPid);
}

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

        /*
         * If this is called by parallel.c in a parallel worker, we don't want to
         * create a SERIALIZABLEXACT just yet because the leader's
         * SERIALIZABLEXACT will be installed with AttachSerializableXact().  We
         * also don't want to reject SERIALIZABLE READ ONLY DEFERRABLE in this
         * case, because the leader has already determined that the snapshot it
         * has passed us is safe.  So there is nothing for us to do.
         */
        if (IsParallelWorker())
                return;

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

        (void) GetSerializableTransactionSnapshotInt(snapshot, sourcevxid,
                                                                                                 sourcepid);
}

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

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

        Assert(!RecoveryInProgress());

        /*
         * Since all parts of a serializable transaction must use the same
         * snapshot, it is too late to establish one after a parallel operation
         * has begun.
         */
        if (IsInParallelMode())
                elog(ERROR, "cannot establish serializable snapshot during a parallel operation");

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

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

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

        /*
         * 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;
        }

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

                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).
                 */
                dlist_foreach(iter, &PredXact->activeList)
                {
                        othersxact = dlist_container(SERIALIZABLEXACT, xactLink, iter.cur);

                        if (!SxactIsCommitted(othersxact)
                                && !SxactIsDoomed(othersxact)
                                && !SxactIsReadOnly(othersxact))
                        {
                                SetPossibleUnsafeConflict(sxact, othersxact);
                        }
                }

                /*
                 * If we didn't find any possibly unsafe conflicts because every
                 * uncommitted writable transaction turned out to be doomed, then we
                 * can "opt out" immediately.  See comments above the earlier check
                 * for PredXact->WritableSxactCount == 0.
                 */
                if (dlist_is_empty(&sxact->possibleUnsafeConflicts))
                {
                        ReleasePredXact(sxact);
                        LWLockRelease(SerializableXactHashLock);
                        return snapshot;
                }
        }
        else
        {
                ++(PredXact->WritableSxactCount);
                Assert(PredXact->WritableSxactCount <=
                           (MaxBackends + max_prepared_xacts));
        }

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

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

        LWLockRelease(SerializableXactHashLock);

        CreateLocalPredicateLockHash();

        return snapshot;
}

static void
CreateLocalPredicateLockHash(void)
{
        HASHCTL         hash_ctl;

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

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

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

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

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

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

        MySerializableXact->topXid = xid;

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

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


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

        SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
                                                                        relation->rd_locator.dbOid,
                                                                        relation->rd_id,
                                                                        blkno);

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

        return (target != NULL);
}


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

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

        if (!lock)
                return false;

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

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

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

                        return true;

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

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

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

        targettag = *newtargettag;

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

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

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

        Assert(LWLockHeldByMe(SerializablePredicateListLock));

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

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

        Assert(LWLockHeldByMe(SerializablePredicateListLock));

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

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

        Assert(LWLockHeldByMe(SerializablePredicateListLock));

        /* Can't remove it until no locks at this target. */
        if (!dlist_is_empty(&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 acquire an LWLock in the case of parallel mode, because worker
 * backends have access to the leader's SERIALIZABLEXACT.  Otherwise,
 * we aren't acquiring LWLocks 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;
        dlist_mutable_iter iter;

        LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
        sxact = MySerializableXact;
        if (IsInParallelMode())
                LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);

        dlist_foreach_modify(iter, &sxact->predicateLocks)
        {
                PREDICATELOCKTAG oldlocktag;
                PREDICATELOCKTARGET *oldtarget;
                PREDICATELOCKTARGETTAG oldtargettag;

                predlock = dlist_container(PREDICATELOCK, xactLink, iter.cur);

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

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

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

                        LWLockAcquire(partitionLock, LW_EXCLUSIVE);

                        dlist_delete(&predlock->xactLink);
                        dlist_delete(&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);
                }
        }
        if (IsInParallelMode())
                LWLockRelease(&sxact->perXactPredicateListLock);
        LWLockRelease(SerializablePredicateListLock);
}

/*
 * Returns the promotion limit for a given predicate lock target.  This is the
 * max number of descendant locks allowed before promoting to the specified
 * tag. Note that the limit includes non-direct descendants (e.g., both tuples
 * and pages for a relation lock).
 *
 * Currently the default limit is 2 for a page lock, and half of the value of
 * max_pred_locks_per_transaction - 1 for a relation lock, to match behavior
 * of earlier releases when upgrading.
 *
 * TODO SSI: We should probably add additional GUCs to allow a maximum ratio
 * of page and tuple locks based on the pages in a relation, and the maximum
 * ratio of tuple locks to tuples in a page.  This would provide more
 * generally "balanced" allocation of locks to where they are most useful,
 * while still allowing the absolute numbers to prevent one relation from
 * tying up all predicate lock resources.
 */
static int
MaxPredicateChildLocks(const PREDICATELOCKTARGETTAG *tag)
{
        switch (GET_PREDICATELOCKTARGETTAG_TYPE(*tag))
        {
                case PREDLOCKTAG_RELATION:
                        return max_predicate_locks_per_relation < 0
                                ? (max_predicate_locks_per_xact
                                   / (-max_predicate_locks_per_relation)) - 1
                                : max_predicate_locks_per_relation;

                case PREDLOCKTAG_PAGE:
                        return max_predicate_locks_per_page;

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

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

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

        parenttag = *targettag;

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

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

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

                parentlock->childLocks--;

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

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

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

        partitionLock = PredicateLockHashPartitionLock(targettaghash);

        LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
        if (IsInParallelMode())
                LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);
        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 %s.", "max_pred_locks_per_transaction")));
        if (!found)
                dlist_init(&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 %s.", "max_pred_locks_per_transaction")));

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

        LWLockRelease(partitionLock);
        if (IsInParallelMode())
                LWLockRelease(&sxact->perXactPredicateListLock);
        LWLockRelease(SerializablePredicateListLock);
}

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

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

        if (CoarserLockCovers(targettag))
                return;

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

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

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

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


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

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

        SET_PREDICATELOCKTARGETTAG_RELATION(tag,
                                                                                relation->rd_locator.dbOid,
                                                                                relation->rd_id);
        PredicateLockAcquire(&tag);
}

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

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

        SET_PREDICATELOCKTARGETTAG_PAGE(tag,
                                                                        relation->rd_locator.dbOid,
                                                                        relation->rd_id,
                                                                        blkno);
        PredicateLockAcquire(&tag);
}

/*
 *              PredicateLockTID
 *
 * 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
PredicateLockTID(Relation relation, ItemPointer tid, Snapshot snapshot,
                                 TransactionId tuple_xid)
{
        PREDICATELOCKTARGETTAG tag;

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

        /*
         * Return if this xact wrote it.
         */
        if (relation->rd_index == NULL)
        {
                /* If we wrote it; we already have a write lock. */
                if (TransactionIdIsCurrentTransactionId(tuple_xid))
                        return;
        }

        /*
         * 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_locator.dbOid,
                                                                                relation->rd_id);
        if (PredicateLockExists(&tag))
                return;

        SET_PREDICATELOCKTARGETTAG_TUPLE(tag,
                                                                         relation->rd_locator.dbOid,
                                                                         relation->rd_id,
                                                                         ItemPointerGetBlockNumber(tid),
                                                                         ItemPointerGetOffsetNumber(tid));
        PredicateLockAcquire(&tag);
}


/*
 *              DeleteLockTarget
 *
 * Remove a predicate lock target along with any locks held for it.
 *
 * Caller must hold SerializablePredicateListLock and the
 * appropriate hash partition lock for the target.
 */
static void
DeleteLockTarget(PREDICATELOCKTARGET *target, uint32 targettaghash)
{
        dlist_mutable_iter iter;

        Assert(LWLockHeldByMeInMode(SerializablePredicateListLock,
                                                                LW_EXCLUSIVE));
        Assert(LWLockHeldByMe(PredicateLockHashPartitionLock(targettaghash)));

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

        dlist_foreach_modify(iter, &target->predicateLocks)
        {
                PREDICATELOCK *predlock =
                        dlist_container(PREDICATELOCK, targetLink, iter.cur);
                bool            found;

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

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

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


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

        Assert(LWLockHeldByMeInMode(SerializablePredicateListLock,
                                                                LW_EXCLUSIVE));

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

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

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

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

        if (oldtarget)
        {
                PREDICATELOCKTARGET *newtarget;
                PREDICATELOCKTAG newpredlocktag;
                dlist_mutable_iter iter;

                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)
                        dlist_init(&newtarget->predicateLocks);

                newpredlocktag.myTarget = newtarget;

                /*
                 * Loop through all the locks on the old target, replacing them with
                 * locks on the new target.
                 */
                LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

                dlist_foreach_modify(iter, &oldtarget->predicateLocks)
                {
                        PREDICATELOCK *oldpredlock =
                                dlist_container(PREDICATELOCK, targetLink, iter.cur);
                        PREDICATELOCK *newpredlock;
                        SerCommitSeqNo oldCommitSeqNo = oldpredlock->commitSeqNo;

                        newpredlocktag.myXact = oldpredlock->tag.myXact;

                        if (removeOld)
                        {
                                dlist_delete(&(oldpredlock->xactLink));
                                dlist_delete(&(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)
                        {
                                dlist_push_tail(&(newtarget->predicateLocks),
                                                                &(newpredlock->targetLink));
                                dlist_push_tail(&(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));
                }
                LWLockRelease(SerializableXactHashLock);

                if (removeOld)
                {
                        Assert(dlist_is_empty(&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 scratch entry back */
                RestoreScratchTarget(false);
        }

        return !outOfShmem;
}

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

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

        if (!PredicateLockingNeededForRelation(relation))
                return;

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

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

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

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

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

        while ((oldtarget = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
        {
                dlist_mutable_iter iter;

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

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

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

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

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

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

                        dlist_delete(&(oldpredlock->xactLink));

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

                        if (transfer)
                        {
                                PREDICATELOCKTAG newpredlocktag;

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

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

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

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

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


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

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

        if (!PredicateLockingNeededForRelation(relation))
                return;

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

        SET_PREDICATELOCKTARGETTAG_PAGE(oldtargettag,
                                                                        relation->rd_locator.dbOid,
                                                                        relation->rd_id,
                                                                        oldblkno);
        SET_PREDICATELOCKTARGETTAG_PAGE(newtargettag,
                                                                        relation->rd_locator.dbOid,
                                                                        relation->rd_id,
                                                                        newblkno);

        LWLockAcquire(SerializablePredicateListLock, 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(SerializablePredicateListLock);
}

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

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

        Assert(LWLockHeldByMe(SerializableXactHashLock));

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

        dlist_foreach(iter, &PredXact->activeList)
        {
                SERIALIZABLEXACT *sxact =
                        dlist_container(SERIALIZABLEXACT, xactLink, iter.cur);

                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++;
                }
        }

        SerialSetActiveSerXmin(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.
 *
 * If isReadOnlySafe is true, then predicate locks are being released before
 * the end of the transaction because MySerializableXact has been determined
 * to be RO_SAFE.  In non-parallel mode we can release it completely, but it
 * in parallel mode we partially release the SERIALIZABLEXACT and keep it
 * around until the end of the transaction, allowing each backend to clear its
 * MySerializableXact variable and benefit from the optimization in its own
 * time.
 */
void
ReleasePredicateLocks(bool isCommit, bool isReadOnlySafe)
{
        bool            partiallyReleasing = false;
        bool            needToClear;
        SERIALIZABLEXACT *roXact;
        dlist_mutable_iter iter;

        /*
         * We can't trust XactReadOnly here, because a transaction which started
         * as READ WRITE can show as READ ONLY later, e.g., within
         * subtransactions.  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;

        /* We can't be both committing and releasing early due to RO_SAFE. */
        Assert(!(isCommit && isReadOnlySafe));

        /* Are we at the end of a transaction, that is, a commit or abort? */
        if (!isReadOnlySafe)
        {
                /*
                 * Parallel workers mustn't release predicate locks at the end of
                 * their transaction.  The leader will do that at the end of its
                 * transaction.
                 */
                if (IsParallelWorker())
                {
                        ReleasePredicateLocksLocal();
                        return;
                }

                /*
                 * By the time the leader in a parallel query reaches end of
                 * transaction, it has waited for all workers to exit.
                 */
                Assert(!ParallelContextActive());

                /*
                 * If the leader in a parallel query earlier stashed a partially
                 * released SERIALIZABLEXACT for final clean-up at end of transaction
                 * (because workers might still have been accessing it), then it's
                 * time to restore it.
                 */
                if (SavedSerializableXact != InvalidSerializableXact)
                {
                        Assert(MySerializableXact == InvalidSerializableXact);
                        MySerializableXact = SavedSerializableXact;
                        SavedSerializableXact = InvalidSerializableXact;
                        Assert(SxactIsPartiallyReleased(MySerializableXact));
                }
        }

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

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

        /*
         * If the transaction is committing, but it has been partially released
         * already, then treat this as a roll back.  It was marked as rolled back.
         */
        if (isCommit && SxactIsPartiallyReleased(MySerializableXact))
                isCommit = false;

        /*
         * If we're called in the middle of a transaction because we discovered
         * that the SXACT_FLAG_RO_SAFE flag was set, then we'll partially release
         * it (that is, release the predicate locks and conflicts, but not the
         * SERIALIZABLEXACT itself) if we're the first backend to have noticed.
         */
        if (isReadOnlySafe && IsInParallelMode())
        {
                /*
                 * The leader needs to stash a pointer to it, so that it can
                 * completely release it at end-of-transaction.
                 */
                if (!IsParallelWorker())
                        SavedSerializableXact = MySerializableXact;

                /*
                 * The first backend to reach this condition will partially release
                 * the SERIALIZABLEXACT.  All others will just clear their
                 * backend-local state so that they stop doing SSI checks for the rest
                 * of the transaction.
                 */
                if (SxactIsPartiallyReleased(MySerializableXact))
                {
                        LWLockRelease(SerializableXactHashLock);
                        ReleasePredicateLocksLocal();
                        return;
                }
                else
                {
                        MySerializableXact->flags |= SXACT_FLAG_PARTIALLY_RELEASED;
                        partiallyReleasing = true;
                        /* ... and proceed to perform the partial release below. */
                }
        }
        Assert(!isCommit || SxactIsPrepared(MySerializableXact));
        Assert(!isCommit || !SxactIsDoomed(MySerializableXact));
        Assert(!SxactIsCommitted(MySerializableXact));
        Assert(SxactIsPartiallyReleased(MySerializableXact)
                   || !SxactIsRolledBack(MySerializableXact));

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

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

        topLevelIsDeclaredReadOnly = SxactIsReadOnly(MySerializableXact);

        /*
         * 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
         * SxactGlobalXmin 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 = XidFromFullTransactionId(TransamVariables->nextXid);

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

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

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

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

                        ReleaseRWConflict(possibleUnsafeConflict);
                }
        }

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

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

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

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

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

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

        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.
                 */
                dlist_foreach_modify(iter, &MySerializableXact->possibleUnsafeConflicts)
                {
                        RWConflict      possibleUnsafeConflict =
                                dlist_container(RWConflictData, outLink, iter.cur);

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

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

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

        /*
         * 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.
         *
         * For parallel queries in read-only transactions, it might run twice. We
         * only release the reference on the first call.
         */
        needToClear = false;
        if ((partiallyReleasing ||
                 !SxactIsPartiallyReleased(MySerializableXact)) &&
                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)
                dlist_push_tail(FinishedSerializableTransactions,
                                                &MySerializableXact->finishedLink);

        /*
         * If we're releasing a RO_SAFE transaction in parallel mode, we'll only
         * partially release it.  That's necessary because other backends may have
         * a reference to it.  The leader will release the SERIALIZABLEXACT itself
         * at the end of the transaction after workers have stopped running.
         */
        if (!isCommit)
                ReleaseOneSerializableXact(MySerializableXact,
                                                                   isReadOnlySafe && IsInParallelMode(),
                                                                   false);

        LWLockRelease(SerializableFinishedListLock);

        if (needToClear)
                ClearOldPredicateLocks();

        ReleasePredicateLocksLocal();
}

static void
ReleasePredicateLocksLocal(void)
{
        MySerializableXact = InvalidSerializableXact;
        MyXactDidWrite = false;

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

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

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

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

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

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

        /*
         * Loop through predicate locks on dummy transaction for summarized data.
         */
        LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
        dlist_foreach_modify(iter, &OldCommittedSxact->predicateLocks)
        {
                PREDICATELOCK *predlock =
                        dlist_container(PREDICATELOCK, xactLink, iter.cur);
                bool            canDoPartialCleanup;

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

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

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

                        LWLockAcquire(partitionLock, LW_EXCLUSIVE);

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

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

                        LWLockRelease(partitionLock);
                }
        }

        LWLockRelease(SerializablePredicateListLock);
        LWLockRelease(SerializableFinishedListLock);
}

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

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

        /*
         * First release all the predicate locks held by this xact (or transfer
         * them to OldCommittedSxact if summarize is true)
         */
        LWLockAcquire(SerializablePredicateListLock, LW_SHARED);
        if (IsInParallelMode())
                LWLockAcquire(&sxact->perXactPredicateListLock, LW_EXCLUSIVE);
        dlist_foreach_modify(iter, &sxact->predicateLocks)
        {
                PREDICATELOCK *predlock =
                        dlist_container(PREDICATELOCK, xactLink, iter.cur);
                PREDICATELOCKTAG tag;
                PREDICATELOCKTARGET *target;
                PREDICATELOCKTARGETTAG targettag;
                uint32          targettaghash;
                LWLock     *partitionLock;

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

                LWLockAcquire(partitionLock, LW_EXCLUSIVE);

                dlist_delete(&predlock->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 %s.", "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
                        {
                                dlist_push_tail(&target->predicateLocks,
                                                                &predlock->targetLink);
                                dlist_push_tail(&OldCommittedSxact->predicateLocks,
                                                                &predlock->xactLink);
                                predlock->commitSeqNo = sxact->commitSeqNo;
                        }
                }
                else
                        RemoveTargetIfNoLongerUsed(target, targettaghash);

                LWLockRelease(partitionLock);
        }

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

        if (IsInParallelMode())
                LWLockRelease(&sxact->perXactPredicateListLock);
        LWLockRelease(SerializablePredicateListLock);

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

        /* Release all outConflicts (unless 'partial' is true) */
        if (!partial)
        {
                dlist_foreach_modify(iter, &sxact->outConflicts)
                {
                        RWConflict      conflict =
                                dlist_container(RWConflictData, outLink, iter.cur);

                        if (summarize)
                                conflict->sxactIn->flags |= SXACT_FLAG_SUMMARY_CONFLICT_IN;
                        ReleaseRWConflict(conflict);
                }
        }

        /* Release all inConflicts. */
        dlist_foreach_modify(iter, &sxact->inConflicts)
        {
                RWConflict      conflict =
                        dlist_container(RWConflictData, inLink, iter.cur);

                if (summarize)
                        conflict->sxactOut->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
                ReleaseRWConflict(conflict);
        }

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

        LWLockRelease(SerializableXactHashLock);
}

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

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

        snap = GetTransactionSnapshot();

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

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

        return pg_lfind32(xid, snap->xip, snap->xcnt);
}

bool
CheckForSerializableConflictOutNeeded(Relation relation, Snapshot snapshot)
{
        if (!SerializationNeededForRead(relation, snapshot))
                return false;

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

        return true;
}

/*
 * CheckForSerializableConflictOut
 *              A table AM is reading a tuple that has been modified.  If it determines
 *              that the tuple version it is reading is not visible to us, it should
 *              pass in the top level xid of the transaction that created it.
 *              Otherwise, if it determines that it is visible to us but it has been
 *              deleted or there is a newer version available due to an update, it
 *              should pass in the top level xid of the modifying transaction.
 *
 * This function will check for overlap with our own transaction.  If the given
 * xid is also serializable and the transactions overlap (i.e., they cannot see
 * each other's writes), then we have a conflict out.
 */
void
CheckForSerializableConflictOut(Relation relation, TransactionId xid, Snapshot snapshot)
{
        SERIALIZABLEXIDTAG sxidtag;
        SERIALIZABLEXID *sxid;
        SERIALIZABLEXACT *sxact;

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

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

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

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

                        MySerializableXact->flags |= SXACT_FLAG_SUMMARY_CONFLICT_OUT;
                }

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

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

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

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

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

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

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

        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.
         */
        LWLockAcquire(SerializableXactHashLock, LW_SHARED);

        dlist_foreach_modify(iter, &target->predicateLocks)
        {
                PREDICATELOCK *predlock =
                        dlist_container(PREDICATELOCK, targetLink, iter.cur);
                SERIALIZABLEXACT *sxact = predlock->tag.myXact;

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

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

                        LWLockRelease(SerializableXactHashLock);
                        LWLockAcquire(SerializableXactHashLock, LW_SHARED);
                }
        }
        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 a RowExclusiveLock 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(SerializablePredicateListLock, LW_SHARED);
                if (IsInParallelMode())
                        LWLockAcquire(&MySerializableXact->perXactPredicateListLock, LW_EXCLUSIVE);
                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);

                        dlist_delete(&(mypredlock->targetLink));
                        dlist_delete(&(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);
                if (IsInParallelMode())
                        LWLockRelease(&MySerializableXact->perXactPredicateListLock);
                LWLockRelease(SerializablePredicateListLock);

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

                        DecrementParentLocks(targettag);
                }
        }
}

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

        if (!SerializationNeededForWrite(relation))
                return;

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

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

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

        if (blkno != InvalidBlockNumber)
        {
                SET_PREDICATELOCKTARGETTAG_PAGE(targettag,
                                                                                relation->rd_locator.dbOid,
                                                                                relation->rd_id,
                                                                                blkno);
                CheckTargetForConflictsIn(&targettag);
        }

        SET_PREDICATELOCKTARGETTAG_RELATION(targettag,
                                                                                relation->rd_locator.dbOid,
                                                                                relation->rd_id);
        CheckTargetForConflictsIn(&targettag);
}

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

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

        if (!SerializationNeededForWrite(relation))
                return;

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

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

        dbId = relation->rd_locator.dbOid;
        heapId = relation->rd_id;

        LWLockAcquire(SerializablePredicateListLock, LW_EXCLUSIVE);
        for (i = 0; i < NUM_PREDICATELOCK_PARTITIONS; i++)
                LWLockAcquire(PredicateLockHashPartitionLockByIndex(i), LW_SHARED);
        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

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

        while ((target = (PREDICATELOCKTARGET *) hash_seq_search(&seqstat)))
        {
                dlist_mutable_iter iter;

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

                /*
                 * Loop through locks for this target and flag conflicts.
                 */
                dlist_foreach_modify(iter, &target->predicateLocks)
                {
                        PREDICATELOCK *predlock =
                                dlist_container(PREDICATELOCK, targetLink, iter.cur);

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

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


/*
 * 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;

        Assert(LWLockHeldByMe(SerializableXactHashLock));

        failure = false;

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

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

                dlist_foreach(iter, &writer->outConflicts)
                {
                        RWConflict      conflict =
                                dlist_container(RWConflictData, outLink, iter.cur);
                        SERIALIZABLEXACT *t2 = conflict->sxactIn;

                        if (SxactIsPrepared(t2)
                                && (!SxactIsCommitted(reader)
                                        || t2->prepareSeqNo <= reader->commitSeqNo)
                                && (!SxactIsCommitted(writer)
                                        || t2->prepareSeqNo <= writer->commitSeqNo)
                                && (!SxactIsReadOnly(reader)
                                        || t2->prepareSeqNo <= reader->SeqNo.lastCommitBeforeSnapshot))
                        {
                                failure = true;
                                break;
                        }
                }
        }

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

                        /*
                         * The unconstify is needed as we have no const version of
                         * dlist_foreach().
                         */
                        dlist_foreach(iter, &unconstify(SERIALIZABLEXACT *, reader)->inConflicts)
                        {
                                const RWConflict conflict =
                                        dlist_container(RWConflictData, inLink, iter.cur);
                                const SERIALIZABLEXACT *t0 = conflict->sxactOut;

                                if (!SxactIsDoomed(t0)
                                        && (!SxactIsCommitted(t0)
                                                || t0->commitSeqNo >= writer->prepareSeqNo)
                                        && (!SxactIsReadOnly(t0)
                                                || t0->SeqNo.lastCommitBeforeSnapshot >= writer->prepareSeqNo))
                                {
                                        failure = true;
                                        break;
                                }
                        }
                }
        }

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

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

/*
 * PreCommit_CheckForSerializationFailure
 *              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 fail without ever making progress.  This transaction is
 * committing writes, so letting it commit ensures progress.  If we
 * canceled the far conflict, it might immediately fail again on retry.
 */
void
PreCommit_CheckForSerializationFailure(void)
{
        dlist_iter      near_iter;

        if (MySerializableXact == InvalidSerializableXact)
                return;

        Assert(IsolationIsSerializable());

        LWLockAcquire(SerializableXactHashLock, LW_EXCLUSIVE);

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

        dlist_foreach(near_iter, &MySerializableXact->inConflicts)
        {
                RWConflict      nearConflict =
                        dlist_container(RWConflictData, inLink, near_iter.cur);

                if (!SxactIsCommitted(nearConflict->sxactOut)
                        && !SxactIsDoomed(nearConflict->sxactOut))
                {
                        dlist_iter      far_iter;

                        dlist_foreach(far_iter, &nearConflict->sxactOut->inConflicts)
                        {
                                RWConflict      farConflict =
                                        dlist_container(RWConflictData, inLink, far_iter.cur);

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

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

        LWLockRelease(SerializableXactHashLock);
}

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

/*
 * Two-phase commit support
 */

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

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

        if (MySerializableXact == InvalidSerializableXact)
                return;

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

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

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

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

        /*
         * No need to take sxact->perXactPredicateListLock in parallel mode
         * because there cannot be any parallel workers running while we are
         * preparing a transaction.
         */
        Assert(!IsParallelWorker() && !ParallelContextActive());

        dlist_foreach(iter, &sxact->predicateLocks)
        {
                PREDICATELOCK *predlock =
                        dlist_container(PREDICATELOCK, xactLink, iter.cur);

                record.type = TWOPHASEPREDICATERECORD_LOCK;
                lockRecord->target = predlock->tag.myTarget->tag;

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

        LWLockRelease(SerializablePredicateListLock);
}

/*
 * 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;
        MySerializableXact->pgprocno = INVALID_PROC_NUMBER;

        hash_destroy(LocalPredicateLockHash);
        LocalPredicateLockHash = NULL;

        MySerializableXact = InvalidSerializableXact;
        MyXactDidWrite = false;
}

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

        sxidtag.xid = xid;

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

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

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

/*
 * 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 INVALID_PROC_NUMBER/xid; no pid */
                sxact->vxid.procNumber = INVALID_PROC_NUMBER;
                sxact->vxid.localTransactionId = (LocalTransactionId) xid;
                sxact->pid = 0;
                sxact->pgprocno = INVALID_PROC_NUMBER;

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

                sxact->SeqNo.lastCommitBeforeSnapshot = RecoverySerCommitSeqNo;

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

                dlist_init(&(sxact->predicateLocks));
                dlist_node_init(&sxact->finishedLink);

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

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

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

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

/*
 * Prepare to share the current SERIALIZABLEXACT with parallel workers.
 * Return a handle object that can be used by AttachSerializableXact() in a
 * parallel worker.
 */
SerializableXactHandle
ShareSerializableXact(void)
{
        return MySerializableXact;
}

/*
 * Allow parallel workers to import the leader's SERIALIZABLEXACT.
 */
void
AttachSerializableXact(SerializableXactHandle handle)
{

        Assert(MySerializableXact == InvalidSerializableXact);

        MySerializableXact = (SERIALIZABLEXACT *) handle;
        if (MySerializableXact != InvalidSerializableXact)
                CreateLocalPredicateLockHash();
}