/src/postgres/src/backend/storage/lmgr/predicate.c

Source
/*-------------------------------------------------------------------------
 *
 * 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
 *
 *  SLRU per-bank locks
 *    - Protects SerialSlruCtl
 *
 * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *    src/backend/storage/lmgr/predicate.c
 *
 *-------------------------------------------------------------------------
 */
/*
 * INTERFACE ROUTINES
 *
 * 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, const ItemPointerData *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, const ItemPointerData *tid,
 *                     BlockNumber blkno)
 *    CheckTableForSerializableConflictIn(Relation relation)
 *
 * final rollback checking
 *    PreCommit_CheckForSerializationFailure(void)
 *
 * two-phase commit support
 *    AtPrepare_PredicateLocks(void);
 *    PostPrepare_PredicateLocks(TransactionId xid);
 *    PredicateLockTwoPhaseFinish(FullTransactionId fxid, bool isCommit);
 *    predicatelock_twophase_recover(FullTransactionId fxid, 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 "storage/shmem.h"
#include "storage/subsystems.h"
#include "utils/guc_hooks.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"
#include "utils/wait_event.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 bool SerialPagePrecedesLogically(int64 page1, int64 page2);
static int  serial_errdetail_for_io_error(const void *opaque_data);

static SlruDesc SerialSlruDesc;

#define SerialSlruCtl     (&SerialSlruDesc)

#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
{
  int64   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;

static void PredicateLockShmemRequest(void *arg);
static void PredicateLockShmemInit(void *arg);
static void PredicateLockShmemAttach(void *arg);

const ShmemCallbacks PredicateLockShmemCallbacks = {
  .request_fn = PredicateLockShmemRequest,
  .init_fn = PredicateLockShmemInit,
  .attach_fn = PredicateLockShmemAttach,
};


/*
 * 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;

static int64 max_serializable_xacts;

/* 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 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));
}

static int
serial_errdetail_for_io_error(const void *opaque_data)
{
  TransactionId xid = *(const TransactionId *) opaque_data;

  return errdetail("Could not access serializable CSN of transaction %u.", xid);
}

#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

/*
 * 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 'xid' is older than the global xmin (== tailXid), there's no need to
   * store it, after all. This can happen if the oldest transaction holding
   * back the global xmin just finished, making 'xid' uninteresting, but
   * ClearOldPredicateLocks() has not yet run.
   */
  tailXid = serialControl->tailXid;
  if (!TransactionIdIsValid(tailXid) || TransactionIdPrecedes(xid, tailXid))
  {
    LWLockRelease(SerialControlLock);
    return;
  }

  /*
   * 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;

  if (isNewPage)
  {
    /* Initialize intervening pages; might involve trading locks */
    for (;;)
    {
      lock = SimpleLruGetBankLock(SerialSlruCtl, firstZeroPage);
      LWLockAcquire(lock, LW_EXCLUSIVE);
      slotno = SimpleLruZeroPage(SerialSlruCtl, firstZeroPage);
      if (firstZeroPage == targetPage)
        break;
      firstZeroPage = SerialNextPage(firstZeroPage);
      LWLockRelease(lock);
    }
  }
  else
  {
    LWLockAcquire(lock, LW_EXCLUSIVE);
    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)
{
  int64   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))
  {
    int64   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);
}

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

/*
 * PredicateLockShmemRequest -- Register the predicate locking data structures.
 */
static void
PredicateLockShmemRequest(void *arg)
{
  int64   max_predicate_lock_targets;
  int64   max_predicate_locks;
  int64   max_rw_conflicts;

  /*
   * Register hash table for PREDICATELOCKTARGET structs.  This stores
   * per-predicate-lock-target information.
   */
  max_predicate_lock_targets = NPREDICATELOCKTARGETENTS();

  ShmemRequestHash(.name = "PREDICATELOCKTARGET hash",
           .nelems = max_predicate_lock_targets,
           .ptr = &PredicateLockTargetHash,
           .hash_info.keysize = sizeof(PREDICATELOCKTARGETTAG),
           .hash_info.entrysize = sizeof(PREDICATELOCKTARGET),
           .hash_info.num_partitions = NUM_PREDICATELOCK_PARTITIONS,
           .hash_flags = HASH_ELEM | HASH_BLOBS | HASH_PARTITION | HASH_FIXED_SIZE,
    );

  /*
   * Allocate hash table for PREDICATELOCK structs.  This stores per
   * xact-lock-of-a-target information.
   *
   * Assume an average of 2 xacts per target.
   */
  max_predicate_locks = max_predicate_lock_targets * 2;

  ShmemRequestHash(.name = "PREDICATELOCK hash",
           .nelems = max_predicate_locks,
           .ptr = &PredicateLockHash,
           .hash_info.keysize = sizeof(PREDICATELOCKTAG),
           .hash_info.entrysize = sizeof(PREDICATELOCK),
           .hash_info.hash = predicatelock_hash,
           .hash_info.num_partitions = NUM_PREDICATELOCK_PARTITIONS,
           .hash_flags = HASH_ELEM | HASH_FUNCTION | HASH_PARTITION | HASH_FIXED_SIZE,
    );

  /*
   * Compute size for serializable transaction hashtable.
   *
   * 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_serializable_xacts = (MaxBackends + max_prepared_xacts) * 10;

  /*
   * Register a list to hold information on transactions participating in
   * predicate locking.
   */
  ShmemRequestStruct(.name = "PredXactList",
             .size = add_size(PredXactListDataSize,
                    (mul_size((Size) max_serializable_xacts,
                          sizeof(SERIALIZABLEXACT)))),
             .ptr = (void **) &PredXact,
    );

  /*
   * Register hash table for SERIALIZABLEXID structs.  This stores per-xid
   * information for serializable transactions which have accessed data.
   */
  ShmemRequestHash(.name = "SERIALIZABLEXID hash",
           .nelems = max_serializable_xacts,
           .ptr = &SerializableXidHash,
           .hash_info.keysize = sizeof(SERIALIZABLEXIDTAG),
           .hash_info.entrysize = sizeof(SERIALIZABLEXID),
           .hash_flags = 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_rw_conflicts = max_serializable_xacts * 5;

  ShmemRequestStruct(.name = "RWConflictPool",
             .size = RWConflictPoolHeaderDataSize + mul_size((Size) max_rw_conflicts,
                                     RWConflictDataSize),
             .ptr = (void **) &RWConflictPool,
    );

  ShmemRequestStruct(.name = "FinishedSerializableTransactions",
             .size = sizeof(dlist_head),
             .ptr = (void **) &FinishedSerializableTransactions,
    );

  /*
   * Initialize the SLRU storage for old committed serializable
   * transactions.
   */
  SimpleLruRequest(.desc = &SerialSlruDesc,
           .name = "serializable",
           .Dir = "pg_serial",
           .long_segment_names = false,

           .nslots = serializable_buffers,

           .sync_handler = SYNC_HANDLER_NONE,
           .PagePrecedes = SerialPagePrecedesLogically,
           .errdetail_for_io_error = serial_errdetail_for_io_error,

           .buffer_tranche_id = LWTRANCHE_SERIAL_BUFFER,
           .bank_tranche_id = LWTRANCHE_SERIAL_SLRU,
    );
#ifdef USE_ASSERT_CHECKING
  SerialPagePrecedesLogicallyUnitTests();
#endif

  ShmemRequestStruct(.name = "SerialControlData",
             .size = sizeof(SerialControlData),
             .ptr = (void **) &serialControl,
    );
}

static void
PredicateLockShmemInit(void *arg)
{
  int     max_rw_conflicts;
  bool    found;

  /*
   * 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.
   */
  (void) hash_search(PredicateLockTargetHash, &ScratchTargetTag,
             HASH_ENTER, &found);
  Assert(!found);

  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;
  PredXact->element
    = (SERIALIZABLEXACT *) ((char *) PredXact + PredXactListDataSize);
  /* Add all elements to available list, clean. */
  for (int i = 0; i < max_serializable_xacts; 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;

  /* Initialize the rw-conflict pool */
  dlist_init(&RWConflictPool->availableList);
  RWConflictPool->element = (RWConflict) ((char *) RWConflictPool +
                      RWConflictPoolHeaderDataSize);

  max_rw_conflicts = max_serializable_xacts * 5;

  /* Add all elements to available list, clean. */
  for (int i = 0; i < max_rw_conflicts; i++)
  {
    dlist_push_tail(&RWConflictPool->availableList,
            &RWConflictPool->element[i].outLink);
  }

  /* Initialize the list of finished serializable transactions */
  dlist_init(FinishedSerializableTransactions);

  /* Initialize SerialControl to reflect empty SLRU. */
  LWLockAcquire(SerialControlLock, LW_EXCLUSIVE);
  serialControl->headPage = -1;
  serialControl->headXid = InvalidTransactionId;
  serialControl->tailXid = InvalidTransactionId;
  LWLockRelease(SerialControlLock);

  SlruPagePrecedesUnitTests(SerialSlruCtl, SERIAL_ENTRIESPERPAGE);

  /* This never changes, so let's keep a local copy. */
  OldCommittedSxact = PredXact->OldCommittedSxact;

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

static void
PredicateLockShmemAttach(void *arg)
{
  /* This never changes, so let's keep a local copy. */
  OldCommittedSxact = PredXact->OldCommittedSxact;

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

/*
 * 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 = palloc_object(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 = palloc_array(PREDICATELOCKTARGETTAG, els);
  data->xacts = palloc_array(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, const ItemPointerData *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, const ItemPointerData *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(FullTransactionId fxid)
{
  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(FullTransactionId fxid, bool isCommit)
{
  SERIALIZABLEXID *sxid;
  SERIALIZABLEXIDTAG sxidtag;

  sxidtag.xid = XidFromFullTransactionId(fxid);

  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(FullTransactionId fxid, uint16 info,
                 void *recdata, uint32 len)
{
  TwoPhasePredicateRecord *record;
  TransactionId xid = XidFromFullTransactionId(fxid);

  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 = 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();
}

Coverage Report

Created: 2026-10-16 06:48