[postgresql] / src / backend / storage / ipc / procarray.c

/*-------------------------------------------------------------------------
 *
 * procarray.c
 *        POSTGRES process array code.
 *
 *
 * This module maintains arrays of the PGPROC and PGXACT structures for all
 * active backends.  Although there are several uses for this, the principal
 * one is as a means of determining the set of currently running transactions.
 *
 * Because of various subtle race conditions it is critical that a backend
 * hold the correct locks while setting or clearing its MyPgXact->xid field.
 * See notes in src/backend/access/transam/README.
 *
 * The process arrays now also include structures representing prepared
 * transactions.  The xid and subxids fields of these are valid, as are the
 * myProcLocks lists.  They can be distinguished from regular backend PGPROCs
 * at need by checking for pid == 0.
 *
 * During hot standby, we also keep a list of XIDs representing transactions
 * that are known to be running in the master (or more precisely, were running
 * as of the current point in the WAL stream).  This list is kept in the
 * KnownAssignedXids array, and is updated by watching the sequence of
 * arriving XIDs.  This is necessary because if we leave those XIDs out of
 * snapshots taken for standby queries, then they will appear to be already
 * complete, leading to MVCC failures.  Note that in hot standby, the PGPROC
 * array represents standby processes, which by definition are not running
 * transactions that have XIDs.
 *
 * It is perhaps possible for a backend on the master to terminate without
 * writing an abort record for its transaction.  While that shouldn't really
 * happen, it would tie up KnownAssignedXids indefinitely, so we protect
 * ourselves by pruning the array when a valid list of running XIDs arrives.
 *
 * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *        src/backend/storage/ipc/procarray.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <signal.h>

#include "access/clog.h"
#include "access/subtrans.h"
#include "access/transam.h"
#include "access/twophase.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "catalog/catalog.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"


/* Our shared memory area */
typedef struct ProcArrayStruct
{
        int                     numProcs;               /* number of valid procs entries */
        int                     maxProcs;               /* allocated size of procs array */

        /*
         * Known assigned XIDs handling
         */
        int                     maxKnownAssignedXids;   /* allocated size of array */
        int                     numKnownAssignedXids;   /* current # of valid entries */
        int                     tailKnownAssignedXids;  /* index of oldest valid element */
        int                     headKnownAssignedXids;  /* index of newest element, + 1 */
        slock_t         known_assigned_xids_lck;        /* protects head/tail pointers */

        /*
         * Highest subxid that has been removed from KnownAssignedXids array to
         * prevent overflow; or InvalidTransactionId if none.  We track this for
         * similar reasons to tracking overflowing cached subxids in PGXACT
         * entries.  Must hold exclusive ProcArrayLock to change this, and shared
         * lock to read it.
         */
        TransactionId lastOverflowedXid;

        /* oldest xmin of any replication slot */
        TransactionId replication_slot_xmin;
        /* oldest catalog xmin of any replication slot */
        TransactionId replication_slot_catalog_xmin;

        /* indexes into allPgXact[], has PROCARRAY_MAXPROCS entries */
        int                     pgprocnos[FLEXIBLE_ARRAY_MEMBER];
} ProcArrayStruct;

static ProcArrayStruct *procArray;

static PGPROC *allProcs;
static PGXACT *allPgXact;

/*
 * Bookkeeping for tracking emulated transactions in recovery
 */
static TransactionId *KnownAssignedXids;
static bool *KnownAssignedXidsValid;
static TransactionId latestObservedXid = InvalidTransactionId;

/*
 * If we're in STANDBY_SNAPSHOT_PENDING state, standbySnapshotPendingXmin is
 * the highest xid that might still be running that we don't have in
 * KnownAssignedXids.
 */
static TransactionId standbySnapshotPendingXmin;

#ifdef XIDCACHE_DEBUG

/* counters for XidCache measurement */
static long xc_by_recent_xmin = 0;
static long xc_by_known_xact = 0;
static long xc_by_my_xact = 0;
static long xc_by_latest_xid = 0;
static long xc_by_main_xid = 0;
static long xc_by_child_xid = 0;
static long xc_by_known_assigned = 0;
static long xc_no_overflow = 0;
static long xc_slow_answer = 0;

#define xc_by_recent_xmin_inc()         (xc_by_recent_xmin++)
#define xc_by_known_xact_inc()          (xc_by_known_xact++)
#define xc_by_my_xact_inc()                     (xc_by_my_xact++)
#define xc_by_latest_xid_inc()          (xc_by_latest_xid++)
#define xc_by_main_xid_inc()            (xc_by_main_xid++)
#define xc_by_child_xid_inc()           (xc_by_child_xid++)
#define xc_by_known_assigned_inc()      (xc_by_known_assigned++)
#define xc_no_overflow_inc()            (xc_no_overflow++)
#define xc_slow_answer_inc()            (xc_slow_answer++)

static void DisplayXidCache(void);
#else                                                   /* !XIDCACHE_DEBUG */

#define xc_by_recent_xmin_inc()         ((void) 0)
#define xc_by_known_xact_inc()          ((void) 0)
#define xc_by_my_xact_inc()                     ((void) 0)
#define xc_by_latest_xid_inc()          ((void) 0)
#define xc_by_main_xid_inc()            ((void) 0)
#define xc_by_child_xid_inc()           ((void) 0)
#define xc_by_known_assigned_inc()      ((void) 0)
#define xc_no_overflow_inc()            ((void) 0)
#define xc_slow_answer_inc()            ((void) 0)
#endif                                                  /* XIDCACHE_DEBUG */

/* Primitives for KnownAssignedXids array handling for standby */
static void KnownAssignedXidsCompress(bool force);
static void KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
                                         bool exclusive_lock);
static bool KnownAssignedXidsSearch(TransactionId xid, bool remove);
static bool KnownAssignedXidExists(TransactionId xid);
static void KnownAssignedXidsRemove(TransactionId xid);
static void KnownAssignedXidsRemoveTree(TransactionId xid, int nsubxids,
                                                        TransactionId *subxids);
static void KnownAssignedXidsRemovePreceding(TransactionId xid);
static int      KnownAssignedXidsGet(TransactionId *xarray, TransactionId xmax);
static int KnownAssignedXidsGetAndSetXmin(TransactionId *xarray,
                                                           TransactionId *xmin,
                                                           TransactionId xmax);
static TransactionId KnownAssignedXidsGetOldestXmin(void);
static void KnownAssignedXidsDisplay(int trace_level);
static void KnownAssignedXidsReset(void);
static inline void ProcArrayEndTransactionInternal(PGPROC *proc,
                                                                PGXACT *pgxact, TransactionId latestXid);
static void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);

/*
 * Report shared-memory space needed by CreateSharedProcArray.
 */
Size
ProcArrayShmemSize(void)
{
        Size            size;

        /* Size of the ProcArray structure itself */
#define PROCARRAY_MAXPROCS      (MaxBackends + max_prepared_xacts)

        size = offsetof(ProcArrayStruct, pgprocnos);
        size = add_size(size, mul_size(sizeof(int), PROCARRAY_MAXPROCS));

        /*
         * During Hot Standby processing we have a data structure called
         * KnownAssignedXids, created in shared memory. Local data structures are
         * also created in various backends during GetSnapshotData(),
         * TransactionIdIsInProgress() and GetRunningTransactionData(). All of the
         * main structures created in those functions must be identically sized,
         * since we may at times copy the whole of the data structures around. We
         * refer to this size as TOTAL_MAX_CACHED_SUBXIDS.
         *
         * Ideally we'd only create this structure if we were actually doing hot
         * standby in the current run, but we don't know that yet at the time
         * shared memory is being set up.
         */
#define TOTAL_MAX_CACHED_SUBXIDS \
        ((PGPROC_MAX_CACHED_SUBXIDS + 1) * PROCARRAY_MAXPROCS)

        if (EnableHotStandby)
        {
                size = add_size(size,
                                                mul_size(sizeof(TransactionId),
                                                                 TOTAL_MAX_CACHED_SUBXIDS));
                size = add_size(size,
                                                mul_size(sizeof(bool), TOTAL_MAX_CACHED_SUBXIDS));
        }

        return size;
}

/*
 * Initialize the shared PGPROC array during postmaster startup.
 */
void
CreateSharedProcArray(void)
{
        bool            found;

        /* Create or attach to the ProcArray shared structure */
        procArray = (ProcArrayStruct *)
                ShmemInitStruct("Proc Array",
                                                add_size(offsetof(ProcArrayStruct, pgprocnos),
                                                                 mul_size(sizeof(int),
                                                                                  PROCARRAY_MAXPROCS)),
                                                &found);

        if (!found)
        {
                /*
                 * We're the first - initialize.
                 */
                procArray->numProcs = 0;
                procArray->maxProcs = PROCARRAY_MAXPROCS;
                procArray->replication_slot_xmin = InvalidTransactionId;
                procArray->maxKnownAssignedXids = TOTAL_MAX_CACHED_SUBXIDS;
                procArray->numKnownAssignedXids = 0;
                procArray->tailKnownAssignedXids = 0;
                procArray->headKnownAssignedXids = 0;
                SpinLockInit(&procArray->known_assigned_xids_lck);
                procArray->lastOverflowedXid = InvalidTransactionId;
        }

        allProcs = ProcGlobal->allProcs;
        allPgXact = ProcGlobal->allPgXact;

        /* Create or attach to the KnownAssignedXids arrays too, if needed */
        if (EnableHotStandby)
        {
                KnownAssignedXids = (TransactionId *)
                        ShmemInitStruct("KnownAssignedXids",
                                                        mul_size(sizeof(TransactionId),
                                                                         TOTAL_MAX_CACHED_SUBXIDS),
                                                        &found);
                KnownAssignedXidsValid = (bool *)
                        ShmemInitStruct("KnownAssignedXidsValid",
                                                        mul_size(sizeof(bool), TOTAL_MAX_CACHED_SUBXIDS),
                                                        &found);
        }

        /* Register and initialize fields of ProcLWLockTranche */
        LWLockRegisterTranche(LWTRANCHE_PROC, "proc");
}

/*
 * Add the specified PGPROC to the shared array.
 */
void
ProcArrayAdd(PGPROC *proc)
{
        ProcArrayStruct *arrayP = procArray;
        int                     index;

        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        if (arrayP->numProcs >= arrayP->maxProcs)
        {
                /*
                 * Oops, no room.  (This really shouldn't happen, since there is a
                 * fixed supply of PGPROC structs too, and so we should have failed
                 * earlier.)
                 */
                LWLockRelease(ProcArrayLock);
                ereport(FATAL,
                                (errcode(ERRCODE_TOO_MANY_CONNECTIONS),
                                 errmsg("sorry, too many clients already")));
        }

        /*
         * Keep the procs array sorted by (PGPROC *) so that we can utilize
         * locality of references much better. This is useful while traversing the
         * ProcArray because there is an increased likelihood of finding the next
         * PGPROC structure in the cache.
         *
         * Since the occurrence of adding/removing a proc is much lower than the
         * access to the ProcArray itself, the overhead should be marginal
         */
        for (index = 0; index < arrayP->numProcs; index++)
        {
                /*
                 * If we are the first PGPROC or if we have found our right position
                 * in the array, break
                 */
                if ((arrayP->pgprocnos[index] == -1) || (arrayP->pgprocnos[index] > proc->pgprocno))
                        break;
        }

        memmove(&arrayP->pgprocnos[index + 1], &arrayP->pgprocnos[index],
                        (arrayP->numProcs - index) * sizeof(int));
        arrayP->pgprocnos[index] = proc->pgprocno;
        arrayP->numProcs++;

        LWLockRelease(ProcArrayLock);
}

/*
 * Remove the specified PGPROC from the shared array.
 *
 * When latestXid is a valid XID, we are removing a live 2PC gxact from the
 * array, and thus causing it to appear as "not running" anymore.  In this
 * case we must advance latestCompletedXid.  (This is essentially the same
 * as ProcArrayEndTransaction followed by removal of the PGPROC, but we take
 * the ProcArrayLock only once, and don't damage the content of the PGPROC;
 * twophase.c depends on the latter.)
 */
void
ProcArrayRemove(PGPROC *proc, TransactionId latestXid)
{
        ProcArrayStruct *arrayP = procArray;
        int                     index;

#ifdef XIDCACHE_DEBUG
        /* dump stats at backend shutdown, but not prepared-xact end */
        if (proc->pid != 0)
                DisplayXidCache();
#endif

        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        if (TransactionIdIsValid(latestXid))
        {
                Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));

                /* Advance global latestCompletedXid while holding the lock */
                if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
                                                                  latestXid))
                        ShmemVariableCache->latestCompletedXid = latestXid;
        }
        else
        {
                /* Shouldn't be trying to remove a live transaction here */
                Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid));
        }

        for (index = 0; index < arrayP->numProcs; index++)
        {
                if (arrayP->pgprocnos[index] == proc->pgprocno)
                {
                        /* Keep the PGPROC array sorted. See notes above */
                        memmove(&arrayP->pgprocnos[index], &arrayP->pgprocnos[index + 1],
                                        (arrayP->numProcs - index - 1) * sizeof(int));
                        arrayP->pgprocnos[arrayP->numProcs - 1] = -1;   /* for debugging */
                        arrayP->numProcs--;
                        LWLockRelease(ProcArrayLock);
                        return;
                }
        }

        /* Oops */
        LWLockRelease(ProcArrayLock);

        elog(LOG, "failed to find proc %p in ProcArray", proc);
}


/*
 * ProcArrayEndTransaction -- mark a transaction as no longer running
 *
 * This is used interchangeably for commit and abort cases.  The transaction
 * commit/abort must already be reported to WAL and pg_xact.
 *
 * proc is currently always MyProc, but we pass it explicitly for flexibility.
 * latestXid is the latest Xid among the transaction's main XID and
 * subtransactions, or InvalidTransactionId if it has no XID.  (We must ask
 * the caller to pass latestXid, instead of computing it from the PGPROC's
 * contents, because the subxid information in the PGPROC might be
 * incomplete.)
 */
void
ProcArrayEndTransaction(PGPROC *proc, TransactionId latestXid)
{
        PGXACT     *pgxact = &allPgXact[proc->pgprocno];

        if (TransactionIdIsValid(latestXid))
        {
                /*
                 * We must lock ProcArrayLock while clearing our advertised XID, so
                 * that we do not exit the set of "running" transactions while someone
                 * else is taking a snapshot.  See discussion in
                 * src/backend/access/transam/README.
                 */
                Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));

                /*
                 * If we can immediately acquire ProcArrayLock, we clear our own XID
                 * and release the lock.  If not, use group XID clearing to improve
                 * efficiency.
                 */
                if (LWLockConditionalAcquire(ProcArrayLock, LW_EXCLUSIVE))
                {
                        ProcArrayEndTransactionInternal(proc, pgxact, latestXid);
                        LWLockRelease(ProcArrayLock);
                }
                else
                        ProcArrayGroupClearXid(proc, latestXid);
        }
        else
        {
                /*
                 * If we have no XID, we don't need to lock, since we won't affect
                 * anyone else's calculation of a snapshot.  We might change their
                 * estimate of global xmin, but that's OK.
                 */
                Assert(!TransactionIdIsValid(allPgXact[proc->pgprocno].xid));

                proc->lxid = InvalidLocalTransactionId;
                pgxact->xmin = InvalidTransactionId;
                /* must be cleared with xid/xmin: */
                pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
                pgxact->delayChkpt = false; /* be sure this is cleared in abort */
                proc->recoveryConflictPending = false;

                Assert(pgxact->nxids == 0);
                Assert(pgxact->overflowed == false);
        }
}

/*
 * Mark a write transaction as no longer running.
 *
 * We don't do any locking here; caller must handle that.
 */
static inline void
ProcArrayEndTransactionInternal(PGPROC *proc, PGXACT *pgxact,
                                                                TransactionId latestXid)
{
        pgxact->xid = InvalidTransactionId;
        proc->lxid = InvalidLocalTransactionId;
        pgxact->xmin = InvalidTransactionId;
        /* must be cleared with xid/xmin: */
        pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
        pgxact->delayChkpt = false; /* be sure this is cleared in abort */
        proc->recoveryConflictPending = false;

        /* Clear the subtransaction-XID cache too while holding the lock */
        pgxact->nxids = 0;
        pgxact->overflowed = false;

        /* Also advance global latestCompletedXid while holding the lock */
        if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
                                                          latestXid))
                ShmemVariableCache->latestCompletedXid = latestXid;
}

/*
 * ProcArrayGroupClearXid -- group XID clearing
 *
 * When we cannot immediately acquire ProcArrayLock in exclusive mode at
 * commit time, add ourselves to a list of processes that need their XIDs
 * cleared.  The first process to add itself to the list will acquire
 * ProcArrayLock in exclusive mode and perform ProcArrayEndTransactionInternal
 * on behalf of all group members.  This avoids a great deal of contention
 * around ProcArrayLock when many processes are trying to commit at once,
 * since the lock need not be repeatedly handed off from one committing
 * process to the next.
 */
static void
ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid)
{
        volatile PROC_HDR *procglobal = ProcGlobal;
        uint32          nextidx;
        uint32          wakeidx;

        /* We should definitely have an XID to clear. */
        Assert(TransactionIdIsValid(allPgXact[proc->pgprocno].xid));

        /* Add ourselves to the list of processes needing a group XID clear. */
        proc->procArrayGroupMember = true;
        proc->procArrayGroupMemberXid = latestXid;
        while (true)
        {
                nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
                pg_atomic_write_u32(&proc->procArrayGroupNext, nextidx);

                if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
                                                                                   &nextidx,
                                                                                   (uint32) proc->pgprocno))
                        break;
        }

        /*
         * If the list was not empty, the leader will clear our XID.  It is
         * impossible to have followers without a leader because the first process
         * that has added itself to the list will always have nextidx as
         * INVALID_PGPROCNO.
         */
        if (nextidx != INVALID_PGPROCNO)
        {
                int                     extraWaits = 0;

                /* Sleep until the leader clears our XID. */
                pgstat_report_wait_start(WAIT_EVENT_PROCARRAY_GROUP_UPDATE);
                for (;;)
                {
                        /* acts as a read barrier */
                        PGSemaphoreLock(proc->sem);
                        if (!proc->procArrayGroupMember)
                                break;
                        extraWaits++;
                }
                pgstat_report_wait_end();

                Assert(pg_atomic_read_u32(&proc->procArrayGroupNext) == INVALID_PGPROCNO);

                /* Fix semaphore count for any absorbed wakeups */
                while (extraWaits-- > 0)
                        PGSemaphoreUnlock(proc->sem);
                return;
        }

        /* We are the leader.  Acquire the lock on behalf of everyone. */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        /*
         * Now that we've got the lock, clear the list of processes waiting for
         * group XID clearing, saving a pointer to the head of the list.  Trying
         * to pop elements one at a time could lead to an ABA problem.
         */
        while (true)
        {
                nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
                if (pg_atomic_compare_exchange_u32(&procglobal->procArrayGroupFirst,
                                                                                   &nextidx,
                                                                                   INVALID_PGPROCNO))
                        break;
        }

        /* Remember head of list so we can perform wakeups after dropping lock. */
        wakeidx = nextidx;

        /* Walk the list and clear all XIDs. */
        while (nextidx != INVALID_PGPROCNO)
        {
                PGPROC     *proc = &allProcs[nextidx];
                PGXACT     *pgxact = &allPgXact[nextidx];

                ProcArrayEndTransactionInternal(proc, pgxact, proc->procArrayGroupMemberXid);

                /* Move to next proc in list. */
                nextidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
        }

        /* We're done with the lock now. */
        LWLockRelease(ProcArrayLock);

        /*
         * Now that we've released the lock, go back and wake everybody up.  We
         * don't do this under the lock so as to keep lock hold times to a
         * minimum.  The system calls we need to perform to wake other processes
         * up are probably much slower than the simple memory writes we did while
         * holding the lock.
         */
        while (wakeidx != INVALID_PGPROCNO)
        {
                PGPROC     *proc = &allProcs[wakeidx];

                wakeidx = pg_atomic_read_u32(&proc->procArrayGroupNext);
                pg_atomic_write_u32(&proc->procArrayGroupNext, INVALID_PGPROCNO);

                /* ensure all previous writes are visible before follower continues. */
                pg_write_barrier();

                proc->procArrayGroupMember = false;

                if (proc != MyProc)
                        PGSemaphoreUnlock(proc->sem);
        }
}

/*
 * ProcArrayClearTransaction -- clear the transaction fields
 *
 * This is used after successfully preparing a 2-phase transaction.  We are
 * not actually reporting the transaction's XID as no longer running --- it
 * will still appear as running because the 2PC's gxact is in the ProcArray
 * too.  We just have to clear out our own PGXACT.
 */
void
ProcArrayClearTransaction(PGPROC *proc)
{
        PGXACT     *pgxact = &allPgXact[proc->pgprocno];

        /*
         * We can skip locking ProcArrayLock here, because this action does not
         * actually change anyone's view of the set of running XIDs: our entry is
         * duplicate with the gxact that has already been inserted into the
         * ProcArray.
         */
        pgxact->xid = InvalidTransactionId;
        proc->lxid = InvalidLocalTransactionId;
        pgxact->xmin = InvalidTransactionId;
        proc->recoveryConflictPending = false;

        /* redundant, but just in case */
        pgxact->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
        pgxact->delayChkpt = false;

        /* Clear the subtransaction-XID cache too */
        pgxact->nxids = 0;
        pgxact->overflowed = false;
}

/*
 * ProcArrayInitRecovery -- initialize recovery xid mgmt environment
 *
 * Remember up to where the startup process initialized the CLOG and subtrans
 * so we can ensure it's initialized gaplessly up to the point where necessary
 * while in recovery.
 */
void
ProcArrayInitRecovery(TransactionId initializedUptoXID)
{
        Assert(standbyState == STANDBY_INITIALIZED);
        Assert(TransactionIdIsNormal(initializedUptoXID));

        /*
         * we set latestObservedXid to the xid SUBTRANS has been initialized up
         * to, so we can extend it from that point onwards in
         * RecordKnownAssignedTransactionIds, and when we get consistent in
         * ProcArrayApplyRecoveryInfo().
         */
        latestObservedXid = initializedUptoXID;
        TransactionIdRetreat(latestObservedXid);
}

/*
 * ProcArrayApplyRecoveryInfo -- apply recovery info about xids
 *
 * Takes us through 3 states: Initialized, Pending and Ready.
 * Normal case is to go all the way to Ready straight away, though there
 * are atypical cases where we need to take it in steps.
 *
 * Use the data about running transactions on master to create the initial
 * state of KnownAssignedXids. We also use these records to regularly prune
 * KnownAssignedXids because we know it is possible that some transactions
 * with FATAL errors fail to write abort records, which could cause eventual
 * overflow.
 *
 * See comments for LogStandbySnapshot().
 */
void
ProcArrayApplyRecoveryInfo(RunningTransactions running)
{
        TransactionId *xids;
        int                     nxids;
        TransactionId nextXid;
        int                     i;

        Assert(standbyState >= STANDBY_INITIALIZED);
        Assert(TransactionIdIsValid(running->nextXid));
        Assert(TransactionIdIsValid(running->oldestRunningXid));
        Assert(TransactionIdIsNormal(running->latestCompletedXid));

        /*
         * Remove stale transactions, if any.
         */
        ExpireOldKnownAssignedTransactionIds(running->oldestRunningXid);

        /*
         * Remove stale locks, if any.
         *
         * Locks are always assigned to the toplevel xid so we don't need to care
         * about subxcnt/subxids (and by extension not about ->suboverflowed).
         */
        StandbyReleaseOldLocks(running->xcnt, running->xids);

        /*
         * If our snapshot is already valid, nothing else to do...
         */
        if (standbyState == STANDBY_SNAPSHOT_READY)
                return;

        /*
         * If our initial RunningTransactionsData had an overflowed snapshot then
         * we knew we were missing some subxids from our snapshot. If we continue
         * to see overflowed snapshots then we might never be able to start up, so
         * we make another test to see if our snapshot is now valid. We know that
         * the missing subxids are equal to or earlier than nextXid. After we
         * initialise we continue to apply changes during recovery, so once the
         * oldestRunningXid is later than the nextXid from the initial snapshot we
         * know that we no longer have missing information and can mark the
         * snapshot as valid.
         */
        if (standbyState == STANDBY_SNAPSHOT_PENDING)
        {
                /*
                 * If the snapshot isn't overflowed or if its empty we can reset our
                 * pending state and use this snapshot instead.
                 */
                if (!running->subxid_overflow || running->xcnt == 0)
                {
                        /*
                         * If we have already collected known assigned xids, we need to
                         * throw them away before we apply the recovery snapshot.
                         */
                        KnownAssignedXidsReset();
                        standbyState = STANDBY_INITIALIZED;
                }
                else
                {
                        if (TransactionIdPrecedes(standbySnapshotPendingXmin,
                                                                          running->oldestRunningXid))
                        {
                                standbyState = STANDBY_SNAPSHOT_READY;
                                elog(trace_recovery(DEBUG1),
                                         "recovery snapshots are now enabled");
                        }
                        else
                                elog(trace_recovery(DEBUG1),
                                         "recovery snapshot waiting for non-overflowed snapshot or "
                                         "until oldest active xid on standby is at least %u (now %u)",
                                         standbySnapshotPendingXmin,
                                         running->oldestRunningXid);
                        return;
                }
        }

        Assert(standbyState == STANDBY_INITIALIZED);

        /*
         * OK, we need to initialise from the RunningTransactionsData record.
         *
         * NB: this can be reached at least twice, so make sure new code can deal
         * with that.
         */

        /*
         * Nobody else is running yet, but take locks anyhow
         */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        /*
         * KnownAssignedXids is sorted so we cannot just add the xids, we have to
         * sort them first.
         *
         * Some of the new xids are top-level xids and some are subtransactions.
         * We don't call SubtransSetParent because it doesn't matter yet. If we
         * aren't overflowed then all xids will fit in snapshot and so we don't
         * need subtrans. If we later overflow, an xid assignment record will add
         * xids to subtrans. If RunningXacts is overflowed then we don't have
         * enough information to correctly update subtrans anyway.
         */

        /*
         * Allocate a temporary array to avoid modifying the array passed as
         * argument.
         */
        xids = palloc(sizeof(TransactionId) * (running->xcnt + running->subxcnt));

        /*
         * Add to the temp array any xids which have not already completed.
         */
        nxids = 0;
        for (i = 0; i < running->xcnt + running->subxcnt; i++)
        {
                TransactionId xid = running->xids[i];

                /*
                 * The running-xacts snapshot can contain xids that were still visible
                 * in the procarray when the snapshot was taken, but were already
                 * WAL-logged as completed. They're not running anymore, so ignore
                 * them.
                 */
                if (TransactionIdDidCommit(xid) || TransactionIdDidAbort(xid))
                        continue;

                xids[nxids++] = xid;
        }

        if (nxids > 0)
        {
                if (procArray->numKnownAssignedXids != 0)
                {
                        LWLockRelease(ProcArrayLock);
                        elog(ERROR, "KnownAssignedXids is not empty");
                }

                /*
                 * Sort the array so that we can add them safely into
                 * KnownAssignedXids.
                 */
                qsort(xids, nxids, sizeof(TransactionId), xidComparator);

                /*
                 * Add the sorted snapshot into KnownAssignedXids
                 */
                for (i = 0; i < nxids; i++)
                        KnownAssignedXidsAdd(xids[i], xids[i], true);

                KnownAssignedXidsDisplay(trace_recovery(DEBUG3));
        }

        pfree(xids);

        /*
         * latestObservedXid is at least set to the point where SUBTRANS was
         * started up to (c.f. ProcArrayInitRecovery()) or to the biggest xid
         * RecordKnownAssignedTransactionIds() was called for.  Initialize
         * subtrans from thereon, up to nextXid - 1.
         *
         * We need to duplicate parts of RecordKnownAssignedTransactionId() here,
         * because we've just added xids to the known assigned xids machinery that
         * haven't gone through RecordKnownAssignedTransactionId().
         */
        Assert(TransactionIdIsNormal(latestObservedXid));
        TransactionIdAdvance(latestObservedXid);
        while (TransactionIdPrecedes(latestObservedXid, running->nextXid))
        {
                ExtendSUBTRANS(latestObservedXid);
                TransactionIdAdvance(latestObservedXid);
        }
        TransactionIdRetreat(latestObservedXid);        /* = running->nextXid - 1 */

        /* ----------
         * Now we've got the running xids we need to set the global values that
         * are used to track snapshots as they evolve further.
         *
         * - latestCompletedXid which will be the xmax for snapshots
         * - lastOverflowedXid which shows whether snapshots overflow
         * - nextXid
         *
         * If the snapshot overflowed, then we still initialise with what we know,
         * but the recovery snapshot isn't fully valid yet because we know there
         * are some subxids missing. We don't know the specific subxids that are
         * missing, so conservatively assume the last one is latestObservedXid.
         * ----------
         */
        if (running->subxid_overflow)
        {
                standbyState = STANDBY_SNAPSHOT_PENDING;

                standbySnapshotPendingXmin = latestObservedXid;
                procArray->lastOverflowedXid = latestObservedXid;
        }
        else
        {
                standbyState = STANDBY_SNAPSHOT_READY;

                standbySnapshotPendingXmin = InvalidTransactionId;
        }

        /*
         * If a transaction wrote a commit record in the gap between taking and
         * logging the snapshot then latestCompletedXid may already be higher than
         * the value from the snapshot, so check before we use the incoming value.
         */
        if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
                                                          running->latestCompletedXid))
                ShmemVariableCache->latestCompletedXid = running->latestCompletedXid;

        Assert(TransactionIdIsNormal(ShmemVariableCache->latestCompletedXid));

        LWLockRelease(ProcArrayLock);

        /*
         * ShmemVariableCache->nextXid must be beyond any observed xid.
         *
         * We don't expect anyone else to modify nextXid, hence we don't need to
         * hold a lock while examining it.  We still acquire the lock to modify
         * it, though.
         */
        nextXid = latestObservedXid;
        TransactionIdAdvance(nextXid);
        if (TransactionIdFollows(nextXid, ShmemVariableCache->nextXid))
        {
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextXid = nextXid;
                LWLockRelease(XidGenLock);
        }

        Assert(TransactionIdIsValid(ShmemVariableCache->nextXid));

        KnownAssignedXidsDisplay(trace_recovery(DEBUG3));
        if (standbyState == STANDBY_SNAPSHOT_READY)
                elog(trace_recovery(DEBUG1), "recovery snapshots are now enabled");
        else
                elog(trace_recovery(DEBUG1),
                         "recovery snapshot waiting for non-overflowed snapshot or "
                         "until oldest active xid on standby is at least %u (now %u)",
                         standbySnapshotPendingXmin,
                         running->oldestRunningXid);
}

/*
 * ProcArrayApplyXidAssignment
 *              Process an XLOG_XACT_ASSIGNMENT WAL record
 */
void
ProcArrayApplyXidAssignment(TransactionId topxid,
                                                        int nsubxids, TransactionId *subxids)
{
        TransactionId max_xid;
        int                     i;

        Assert(standbyState >= STANDBY_INITIALIZED);

        max_xid = TransactionIdLatest(topxid, nsubxids, subxids);

        /*
         * Mark all the subtransactions as observed.
         *
         * NOTE: This will fail if the subxid contains too many previously
         * unobserved xids to fit into known-assigned-xids. That shouldn't happen
         * as the code stands, because xid-assignment records should never contain
         * more than PGPROC_MAX_CACHED_SUBXIDS entries.
         */
        RecordKnownAssignedTransactionIds(max_xid);

        /*
         * Notice that we update pg_subtrans with the top-level xid, rather than
         * the parent xid. This is a difference between normal processing and
         * recovery, yet is still correct in all cases. The reason is that
         * subtransaction commit is not marked in clog until commit processing, so
         * all aborted subtransactions have already been clearly marked in clog.
         * As a result we are able to refer directly to the top-level
         * transaction's state rather than skipping through all the intermediate
         * states in the subtransaction tree. This should be the first time we
         * have attempted to SubTransSetParent().
         */
        for (i = 0; i < nsubxids; i++)
                SubTransSetParent(subxids[i], topxid);

        /* KnownAssignedXids isn't maintained yet, so we're done for now */
        if (standbyState == STANDBY_INITIALIZED)
                return;

        /*
         * Uses same locking as transaction commit
         */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        /*
         * Remove subxids from known-assigned-xacts.
         */
        KnownAssignedXidsRemoveTree(InvalidTransactionId, nsubxids, subxids);

        /*
         * Advance lastOverflowedXid to be at least the last of these subxids.
         */
        if (TransactionIdPrecedes(procArray->lastOverflowedXid, max_xid))
                procArray->lastOverflowedXid = max_xid;

        LWLockRelease(ProcArrayLock);
}

/*
 * TransactionIdIsInProgress -- is given transaction running in some backend
 *
 * Aside from some shortcuts such as checking RecentXmin and our own Xid,
 * there are four possibilities for finding a running transaction:
 *
 * 1. The given Xid is a main transaction Id.  We will find this out cheaply
 * by looking at the PGXACT struct for each backend.
 *
 * 2. The given Xid is one of the cached subxact Xids in the PGPROC array.
 * We can find this out cheaply too.
 *
 * 3. In Hot Standby mode, we must search the KnownAssignedXids list to see
 * if the Xid is running on the master.
 *
 * 4. Search the SubTrans tree to find the Xid's topmost parent, and then see
 * if that is running according to PGXACT or KnownAssignedXids.  This is the
 * slowest way, but sadly it has to be done always if the others failed,
 * unless we see that the cached subxact sets are complete (none have
 * overflowed).
 *
 * ProcArrayLock has to be held while we do 1, 2, 3.  If we save the top Xids
 * while doing 1 and 3, we can release the ProcArrayLock while we do 4.
 * This buys back some concurrency (and we can't retrieve the main Xids from
 * PGXACT again anyway; see GetNewTransactionId).
 */
bool
TransactionIdIsInProgress(TransactionId xid)
{
        static TransactionId *xids = NULL;
        int                     nxids = 0;
        ProcArrayStruct *arrayP = procArray;
        TransactionId topxid;
        int                     i,
                                j;

        /*
         * Don't bother checking a transaction older than RecentXmin; it could not
         * possibly still be running.  (Note: in particular, this guarantees that
         * we reject InvalidTransactionId, FrozenTransactionId, etc as not
         * running.)
         */
        if (TransactionIdPrecedes(xid, RecentXmin))
        {
                xc_by_recent_xmin_inc();
                return false;
        }

        /*
         * We may have just checked the status of this transaction, so if it is
         * already known to be completed, we can fall out without any access to
         * shared memory.
         */
        if (TransactionIdIsKnownCompleted(xid))
        {
                xc_by_known_xact_inc();
                return false;
        }

        /*
         * Also, we can handle our own transaction (and subtransactions) without
         * any access to shared memory.
         */
        if (TransactionIdIsCurrentTransactionId(xid))
        {
                xc_by_my_xact_inc();
                return true;
        }

        /*
         * If first time through, get workspace to remember main XIDs in. We
         * malloc it permanently to avoid repeated palloc/pfree overhead.
         */
        if (xids == NULL)
        {
                /*
                 * In hot standby mode, reserve enough space to hold all xids in the
                 * known-assigned list. If we later finish recovery, we no longer need
                 * the bigger array, but we don't bother to shrink it.
                 */
                int                     maxxids = RecoveryInProgress() ? TOTAL_MAX_CACHED_SUBXIDS : arrayP->maxProcs;

                xids = (TransactionId *) malloc(maxxids * sizeof(TransactionId));
                if (xids == NULL)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory")));
        }

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        /*
         * Now that we have the lock, we can check latestCompletedXid; if the
         * target Xid is after that, it's surely still running.
         */
        if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid, xid))
        {
                LWLockRelease(ProcArrayLock);
                xc_by_latest_xid_inc();
                return true;
        }

        /* No shortcuts, gotta grovel through the array */
        for (i = 0; i < arrayP->numProcs; i++)
        {
                int                     pgprocno = arrayP->pgprocnos[i];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                TransactionId pxid;

                /* Ignore my own proc --- dealt with it above */
                if (proc == MyProc)
                        continue;

                /* Fetch xid just once - see GetNewTransactionId */
                pxid = pgxact->xid;

                if (!TransactionIdIsValid(pxid))
                        continue;

                /*
                 * Step 1: check the main Xid
                 */
                if (TransactionIdEquals(pxid, xid))
                {
                        LWLockRelease(ProcArrayLock);
                        xc_by_main_xid_inc();
                        return true;
                }

                /*
                 * We can ignore main Xids that are younger than the target Xid, since
                 * the target could not possibly be their child.
                 */
                if (TransactionIdPrecedes(xid, pxid))
                        continue;

                /*
                 * Step 2: check the cached child-Xids arrays
                 */
                for (j = pgxact->nxids - 1; j >= 0; j--)
                {
                        /* Fetch xid just once - see GetNewTransactionId */
                        TransactionId cxid = proc->subxids.xids[j];

                        if (TransactionIdEquals(cxid, xid))
                        {
                                LWLockRelease(ProcArrayLock);
                                xc_by_child_xid_inc();
                                return true;
                        }
                }

                /*
                 * Save the main Xid for step 4.  We only need to remember main Xids
                 * that have uncached children.  (Note: there is no race condition
                 * here because the overflowed flag cannot be cleared, only set, while
                 * we hold ProcArrayLock.  So we can't miss an Xid that we need to
                 * worry about.)
                 */
                if (pgxact->overflowed)
                        xids[nxids++] = pxid;
        }

        /*
         * Step 3: in hot standby mode, check the known-assigned-xids list.  XIDs
         * in the list must be treated as running.
         */
        if (RecoveryInProgress())
        {
                /* none of the PGXACT entries should have XIDs in hot standby mode */
                Assert(nxids == 0);

                if (KnownAssignedXidExists(xid))
                {
                        LWLockRelease(ProcArrayLock);
                        xc_by_known_assigned_inc();
                        return true;
                }

                /*
                 * If the KnownAssignedXids overflowed, we have to check pg_subtrans
                 * too.  Fetch all xids from KnownAssignedXids that are lower than
                 * xid, since if xid is a subtransaction its parent will always have a
                 * lower value.  Note we will collect both main and subXIDs here, but
                 * there's no help for it.
                 */
                if (TransactionIdPrecedesOrEquals(xid, procArray->lastOverflowedXid))
                        nxids = KnownAssignedXidsGet(xids, xid);
        }

        LWLockRelease(ProcArrayLock);

        /*
         * If none of the relevant caches overflowed, we know the Xid is not
         * running without even looking at pg_subtrans.
         */
        if (nxids == 0)
        {
                xc_no_overflow_inc();
                return false;
        }

        /*
         * Step 4: have to check pg_subtrans.
         *
         * At this point, we know it's either a subtransaction of one of the Xids
         * in xids[], or it's not running.  If it's an already-failed
         * subtransaction, we want to say "not running" even though its parent may
         * still be running.  So first, check pg_xact to see if it's been aborted.
         */
        xc_slow_answer_inc();

        if (TransactionIdDidAbort(xid))
                return false;

        /*
         * It isn't aborted, so check whether the transaction tree it belongs to
         * is still running (or, more precisely, whether it was running when we
         * held ProcArrayLock).
         */
        topxid = SubTransGetTopmostTransaction(xid);
        Assert(TransactionIdIsValid(topxid));
        if (!TransactionIdEquals(topxid, xid))
        {
                for (i = 0; i < nxids; i++)
                {
                        if (TransactionIdEquals(xids[i], topxid))
                                return true;
                }
        }

        return false;
}

/*
 * TransactionIdIsActive -- is xid the top-level XID of an active backend?
 *
 * This differs from TransactionIdIsInProgress in that it ignores prepared
 * transactions, as well as transactions running on the master if we're in
 * hot standby.  Also, we ignore subtransactions since that's not needed
 * for current uses.
 */
bool
TransactionIdIsActive(TransactionId xid)
{
        bool            result = false;
        ProcArrayStruct *arrayP = procArray;
        int                     i;

        /*
         * Don't bother checking a transaction older than RecentXmin; it could not
         * possibly still be running.
         */
        if (TransactionIdPrecedes(xid, RecentXmin))
                return false;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (i = 0; i < arrayP->numProcs; i++)
        {
                int                     pgprocno = arrayP->pgprocnos[i];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                TransactionId pxid;

                /* Fetch xid just once - see GetNewTransactionId */
                pxid = pgxact->xid;

                if (!TransactionIdIsValid(pxid))
                        continue;

                if (proc->pid == 0)
                        continue;                       /* ignore prepared transactions */

                if (TransactionIdEquals(pxid, xid))
                {
                        result = true;
                        break;
                }
        }

        LWLockRelease(ProcArrayLock);

        return result;
}


/*
 * GetOldestXmin -- returns oldest transaction that was running
 *                                      when any current transaction was started.
 *
 * If rel is NULL or a shared relation, all backends are considered, otherwise
 * only backends running in this database are considered.
 *
 * The flags are used to ignore the backends in calculation when any of the
 * corresponding flags is set. Typically, if you want to ignore ones with
 * PROC_IN_VACUUM flag, you can use PROCARRAY_FLAGS_VACUUM.
 *
 * PROCARRAY_SLOTS_XMIN causes GetOldestXmin to ignore the xmin and
 * catalog_xmin of any replication slots that exist in the system when
 * calculating the oldest xmin.
 *
 * This is used by VACUUM to decide which deleted tuples must be preserved in
 * the passed in table. For shared relations backends in all databases must be
 * considered, but for non-shared relations that's not required, since only
 * backends in my own database could ever see the tuples in them. Also, we can
 * ignore concurrently running lazy VACUUMs because (a) they must be working
 * on other tables, and (b) they don't need to do snapshot-based lookups.
 *
 * This is also used to determine where to truncate pg_subtrans.  For that
 * backends in all databases have to be considered, so rel = NULL has to be
 * passed in.
 *
 * Note: we include all currently running xids in the set of considered xids.
 * This ensures that if a just-started xact has not yet set its snapshot,
 * when it does set the snapshot it cannot set xmin less than what we compute.
 * See notes in src/backend/access/transam/README.
 *
 * Note: despite the above, it's possible for the calculated value to move
 * backwards on repeated calls. The calculated value is conservative, so that
 * anything older is definitely not considered as running by anyone anymore,
 * but the exact value calculated depends on a number of things. For example,
 * if rel = NULL and there are no transactions running in the current
 * database, GetOldestXmin() returns latestCompletedXid. If a transaction
 * begins after that, its xmin will include in-progress transactions in other
 * databases that started earlier, so another call will return a lower value.
 * Nonetheless it is safe to vacuum a table in the current database with the
 * first result.  There are also replication-related effects: a walsender
 * process can set its xmin based on transactions that are no longer running
 * in the master but are still being replayed on the standby, thus possibly
 * making the GetOldestXmin reading go backwards.  In this case there is a
 * possibility that we lose data that the standby would like to have, but
 * unless the standby uses a replication slot to make its xmin persistent
 * there is little we can do about that --- data is only protected if the
 * walsender runs continuously while queries are executed on the standby.
 * (The Hot Standby code deals with such cases by failing standby queries
 * that needed to access already-removed data, so there's no integrity bug.)
 * The return value is also adjusted with vacuum_defer_cleanup_age, so
 * increasing that setting on the fly is another easy way to make
 * GetOldestXmin() move backwards, with no consequences for data integrity.
 */
TransactionId
GetOldestXmin(Relation rel, int flags)
{
        ProcArrayStruct *arrayP = procArray;
        TransactionId result;
        int                     index;
        bool            allDbs;

        volatile TransactionId replication_slot_xmin = InvalidTransactionId;
        volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId;

        /*
         * If we're not computing a relation specific limit, or if a shared
         * relation has been passed in, backends in all databases have to be
         * considered.
         */
        allDbs = rel == NULL || rel->rd_rel->relisshared;

        /* Cannot look for individual databases during recovery */
        Assert(allDbs || !RecoveryInProgress());

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        /*
         * We initialize the MIN() calculation with latestCompletedXid + 1. This
         * is a lower bound for the XIDs that might appear in the ProcArray later,
         * and so protects us against overestimating the result due to future
         * additions.
         */
        result = ShmemVariableCache->latestCompletedXid;
        Assert(TransactionIdIsNormal(result));
        TransactionIdAdvance(result);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                if (pgxact->vacuumFlags & (flags & PROCARRAY_PROC_FLAGS_MASK))
                        continue;

                if (allDbs ||
                        proc->databaseId == MyDatabaseId ||
                        proc->databaseId == 0)  /* always include WalSender */
                {
                        /* Fetch xid just once - see GetNewTransactionId */
                        TransactionId xid = pgxact->xid;

                        /* First consider the transaction's own Xid, if any */
                        if (TransactionIdIsNormal(xid) &&
                                TransactionIdPrecedes(xid, result))
                                result = xid;

                        /*
                         * Also consider the transaction's Xmin, if set.
                         *
                         * We must check both Xid and Xmin because a transaction might
                         * have an Xmin but not (yet) an Xid; conversely, if it has an
                         * Xid, that could determine some not-yet-set Xmin.
                         */
                        xid = pgxact->xmin; /* Fetch just once */
                        if (TransactionIdIsNormal(xid) &&
                                TransactionIdPrecedes(xid, result))
                                result = xid;
                }
        }

        /* fetch into volatile var while ProcArrayLock is held */
        replication_slot_xmin = procArray->replication_slot_xmin;
        replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;

        if (RecoveryInProgress())
        {
                /*
                 * Check to see whether KnownAssignedXids contains an xid value older
                 * than the main procarray.
                 */
                TransactionId kaxmin = KnownAssignedXidsGetOldestXmin();

                LWLockRelease(ProcArrayLock);

                if (TransactionIdIsNormal(kaxmin) &&
                        TransactionIdPrecedes(kaxmin, result))
                        result = kaxmin;
        }
        else
        {
                /*
                 * No other information needed, so release the lock immediately.
                 */
                LWLockRelease(ProcArrayLock);

                /*
                 * Compute the cutoff XID by subtracting vacuum_defer_cleanup_age,
                 * being careful not to generate a "permanent" XID.
                 *
                 * vacuum_defer_cleanup_age provides some additional "slop" for the
                 * benefit of hot standby queries on standby servers.  This is quick
                 * and dirty, and perhaps not all that useful unless the master has a
                 * predictable transaction rate, but it offers some protection when
                 * there's no walsender connection.  Note that we are assuming
                 * vacuum_defer_cleanup_age isn't large enough to cause wraparound ---
                 * so guc.c should limit it to no more than the xidStopLimit threshold
                 * in varsup.c.  Also note that we intentionally don't apply
                 * vacuum_defer_cleanup_age on standby servers.
                 */
                result -= vacuum_defer_cleanup_age;
                if (!TransactionIdIsNormal(result))
                        result = FirstNormalTransactionId;
        }

        /*
         * Check whether there are replication slots requiring an older xmin.
         */
        if (!(flags & PROCARRAY_SLOTS_XMIN) &&
                TransactionIdIsValid(replication_slot_xmin) &&
                NormalTransactionIdPrecedes(replication_slot_xmin, result))
                result = replication_slot_xmin;

        /*
         * After locks have been released and defer_cleanup_age has been applied,
         * check whether we need to back up further to make logical decoding
         * possible. We need to do so if we're computing the global limit (rel =
         * NULL) or if the passed relation is a catalog relation of some kind.
         */
        if (!(flags & PROCARRAY_SLOTS_XMIN) &&
                (rel == NULL ||
                 RelationIsAccessibleInLogicalDecoding(rel)) &&
                TransactionIdIsValid(replication_slot_catalog_xmin) &&
                NormalTransactionIdPrecedes(replication_slot_catalog_xmin, result))
                result = replication_slot_catalog_xmin;

        return result;
}

/*
 * GetMaxSnapshotXidCount -- get max size for snapshot XID array
 *
 * We have to export this for use by snapmgr.c.
 */
int
GetMaxSnapshotXidCount(void)
{
        return procArray->maxProcs;
}

/*
 * GetMaxSnapshotSubxidCount -- get max size for snapshot sub-XID array
 *
 * We have to export this for use by snapmgr.c.
 */
int
GetMaxSnapshotSubxidCount(void)
{
        return TOTAL_MAX_CACHED_SUBXIDS;
}

/*
 * GetSnapshotData -- returns information about running transactions.
 *
 * The returned snapshot includes xmin (lowest still-running xact ID),
 * xmax (highest completed xact ID + 1), and a list of running xact IDs
 * in the range xmin <= xid < xmax.  It is used as follows:
 *              All xact IDs < xmin are considered finished.
 *              All xact IDs >= xmax are considered still running.
 *              For an xact ID xmin <= xid < xmax, consult list to see whether
 *              it is considered running or not.
 * This ensures that the set of transactions seen as "running" by the
 * current xact will not change after it takes the snapshot.
 *
 * All running top-level XIDs are included in the snapshot, except for lazy
 * VACUUM processes.  We also try to include running subtransaction XIDs,
 * but since PGPROC has only a limited cache area for subxact XIDs, full
 * information may not be available.  If we find any overflowed subxid arrays,
 * we have to mark the snapshot's subxid data as overflowed, and extra work
 * *may* need to be done to determine what's running (see XidInMVCCSnapshot()
 * in tqual.c).
 *
 * We also update the following backend-global variables:
 *              TransactionXmin: the oldest xmin of any snapshot in use in the
 *                      current transaction (this is the same as MyPgXact->xmin).
 *              RecentXmin: the xmin computed for the most recent snapshot.  XIDs
 *                      older than this are known not running any more.
 *              RecentGlobalXmin: the global xmin (oldest TransactionXmin across all
 *                      running transactions, except those running LAZY VACUUM).  This is
 *                      the same computation done by
 *                      GetOldestXmin(NULL, PROCARRAY_FLAGS_VACUUM).
 *              RecentGlobalDataXmin: the global xmin for non-catalog tables
 *                      >= RecentGlobalXmin
 *
 * Note: this function should probably not be called with an argument that's
 * not statically allocated (see xip allocation below).
 */
Snapshot
GetSnapshotData(Snapshot snapshot)
{
        ProcArrayStruct *arrayP = procArray;
        TransactionId xmin;
        TransactionId xmax;
        TransactionId globalxmin;
        int                     index;
        int                     count = 0;
        int                     subcount = 0;
        bool            suboverflowed = false;
        volatile TransactionId replication_slot_xmin = InvalidTransactionId;
        volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId;

        Assert(snapshot != NULL);

        /*
         * Allocating space for maxProcs xids is usually overkill; numProcs would
         * be sufficient.  But it seems better to do the malloc while not holding
         * the lock, so we can't look at numProcs.  Likewise, we allocate much
         * more subxip storage than is probably needed.
         *
         * This does open a possibility for avoiding repeated malloc/free: since
         * maxProcs does not change at runtime, we can simply reuse the previous
         * xip arrays if any.  (This relies on the fact that all callers pass
         * static SnapshotData structs.)
         */
        if (snapshot->xip == NULL)
        {
                /*
                 * First call for this snapshot. Snapshot is same size whether or not
                 * we are in recovery, see later comments.
                 */
                snapshot->xip = (TransactionId *)
                        malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId));
                if (snapshot->xip == NULL)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory")));
                Assert(snapshot->subxip == NULL);
                snapshot->subxip = (TransactionId *)
                        malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
                if (snapshot->subxip == NULL)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory")));
        }

        /*
         * It is sufficient to get shared lock on ProcArrayLock, even if we are
         * going to set MyPgXact->xmin.
         */
        LWLockAcquire(ProcArrayLock, LW_SHARED);

        /* xmax is always latestCompletedXid + 1 */
        xmax = ShmemVariableCache->latestCompletedXid;
        Assert(TransactionIdIsNormal(xmax));
        TransactionIdAdvance(xmax);

        /* initialize xmin calculation with xmax */
        globalxmin = xmin = xmax;

        snapshot->takenDuringRecovery = RecoveryInProgress();

        if (!snapshot->takenDuringRecovery)
        {
                int                *pgprocnos = arrayP->pgprocnos;
                int                     numProcs;

                /*
                 * Spin over procArray checking xid, xmin, and subxids.  The goal is
                 * to gather all active xids, find the lowest xmin, and try to record
                 * subxids.
                 */
                numProcs = arrayP->numProcs;
                for (index = 0; index < numProcs; index++)
                {
                        int                     pgprocno = pgprocnos[index];
                        volatile PGXACT *pgxact = &allPgXact[pgprocno];
                        TransactionId xid;

                        /*
                         * Backend is doing logical decoding which manages xmin
                         * separately, check below.
                         */
                        if (pgxact->vacuumFlags & PROC_IN_LOGICAL_DECODING)
                                continue;

                        /* Ignore procs running LAZY VACUUM */
                        if (pgxact->vacuumFlags & PROC_IN_VACUUM)
                                continue;

                        /* Update globalxmin to be the smallest valid xmin */
                        xid = pgxact->xmin; /* fetch just once */
                        if (TransactionIdIsNormal(xid) &&
                                NormalTransactionIdPrecedes(xid, globalxmin))
                                globalxmin = xid;

                        /* Fetch xid just once - see GetNewTransactionId */
                        xid = pgxact->xid;

                        /*
                         * If the transaction has no XID assigned, we can skip it; it
                         * won't have sub-XIDs either.  If the XID is >= xmax, we can also
                         * skip it; such transactions will be treated as running anyway
                         * (and any sub-XIDs will also be >= xmax).
                         */
                        if (!TransactionIdIsNormal(xid)
                                || !NormalTransactionIdPrecedes(xid, xmax))
                                continue;

                        /*
                         * We don't include our own XIDs (if any) in the snapshot, but we
                         * must include them in xmin.
                         */
                        if (NormalTransactionIdPrecedes(xid, xmin))
                                xmin = xid;
                        if (pgxact == MyPgXact)
                                continue;

                        /* Add XID to snapshot. */
                        snapshot->xip[count++] = xid;

                        /*
                         * Save subtransaction XIDs if possible (if we've already
                         * overflowed, there's no point).  Note that the subxact XIDs must
                         * be later than their parent, so no need to check them against
                         * xmin.  We could filter against xmax, but it seems better not to
                         * do that much work while holding the ProcArrayLock.
                         *
                         * The other backend can add more subxids concurrently, but cannot
                         * remove any.  Hence it's important to fetch nxids just once.
                         * Should be safe to use memcpy, though.  (We needn't worry about
                         * missing any xids added concurrently, because they must postdate
                         * xmax.)
                         *
                         * Again, our own XIDs are not included in the snapshot.
                         */
                        if (!suboverflowed)
                        {
                                if (pgxact->overflowed)
                                        suboverflowed = true;
                                else
                                {
                                        int                     nxids = pgxact->nxids;

                                        if (nxids > 0)
                                        {
                                                volatile PGPROC *proc = &allProcs[pgprocno];

                                                memcpy(snapshot->subxip + subcount,
                                                           (void *) proc->subxids.xids,
                                                           nxids * sizeof(TransactionId));
                                                subcount += nxids;
                                        }
                                }
                        }
                }
        }
        else
        {
                /*
                 * We're in hot standby, so get XIDs from KnownAssignedXids.
                 *
                 * We store all xids directly into subxip[]. Here's why:
                 *
                 * In recovery we don't know which xids are top-level and which are
                 * subxacts, a design choice that greatly simplifies xid processing.
                 *
                 * It seems like we would want to try to put xids into xip[] only, but
                 * that is fairly small. We would either need to make that bigger or
                 * to increase the rate at which we WAL-log xid assignment; neither is
                 * an appealing choice.
                 *
                 * We could try to store xids into xip[] first and then into subxip[]
                 * if there are too many xids. That only works if the snapshot doesn't
                 * overflow because we do not search subxip[] in that case. A simpler
                 * way is to just store all xids in the subxact array because this is
                 * by far the bigger array. We just leave the xip array empty.
                 *
                 * Either way we need to change the way XidInMVCCSnapshot() works
                 * depending upon when the snapshot was taken, or change normal
                 * snapshot processing so it matches.
                 *
                 * Note: It is possible for recovery to end before we finish taking
                 * the snapshot, and for newly assigned transaction ids to be added to
                 * the ProcArray.  xmax cannot change while we hold ProcArrayLock, so
                 * those newly added transaction ids would be filtered away, so we
                 * need not be concerned about them.
                 */
                subcount = KnownAssignedXidsGetAndSetXmin(snapshot->subxip, &xmin,
                                                                                                  xmax);

                if (TransactionIdPrecedesOrEquals(xmin, procArray->lastOverflowedXid))
                        suboverflowed = true;
        }


        /* fetch into volatile var while ProcArrayLock is held */
        replication_slot_xmin = procArray->replication_slot_xmin;
        replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;

        if (!TransactionIdIsValid(MyPgXact->xmin))
                MyPgXact->xmin = TransactionXmin = xmin;

        LWLockRelease(ProcArrayLock);

        /*
         * Update globalxmin to include actual process xids.  This is a slightly
         * different way of computing it than GetOldestXmin uses, but should give
         * the same result.
         */
        if (TransactionIdPrecedes(xmin, globalxmin))
                globalxmin = xmin;

        /* Update global variables too */
        RecentGlobalXmin = globalxmin - vacuum_defer_cleanup_age;
        if (!TransactionIdIsNormal(RecentGlobalXmin))
                RecentGlobalXmin = FirstNormalTransactionId;

        /* Check whether there's a replication slot requiring an older xmin. */
        if (TransactionIdIsValid(replication_slot_xmin) &&
                NormalTransactionIdPrecedes(replication_slot_xmin, RecentGlobalXmin))
                RecentGlobalXmin = replication_slot_xmin;

        /* Non-catalog tables can be vacuumed if older than this xid */
        RecentGlobalDataXmin = RecentGlobalXmin;

        /*
         * Check whether there's a replication slot requiring an older catalog
         * xmin.
         */
        if (TransactionIdIsNormal(replication_slot_catalog_xmin) &&
                NormalTransactionIdPrecedes(replication_slot_catalog_xmin, RecentGlobalXmin))
                RecentGlobalXmin = replication_slot_catalog_xmin;

        RecentXmin = xmin;

        snapshot->xmin = xmin;
        snapshot->xmax = xmax;
        snapshot->xcnt = count;
        snapshot->subxcnt = subcount;
        snapshot->suboverflowed = suboverflowed;

        snapshot->curcid = GetCurrentCommandId(false);

        /*
         * This is a new snapshot, so set both refcounts are zero, and mark it as
         * not copied in persistent memory.
         */
        snapshot->active_count = 0;
        snapshot->regd_count = 0;
        snapshot->copied = false;

        if (old_snapshot_threshold < 0)
        {
                /*
                 * If not using "snapshot too old" feature, fill related fields with
                 * dummy values that don't require any locking.
                 */
                snapshot->lsn = InvalidXLogRecPtr;
                snapshot->whenTaken = 0;
        }
        else
        {
                /*
                 * Capture the current time and WAL stream location in case this
                 * snapshot becomes old enough to need to fall back on the special
                 * "old snapshot" logic.
                 */
                snapshot->lsn = GetXLogInsertRecPtr();
                snapshot->whenTaken = GetSnapshotCurrentTimestamp();
                MaintainOldSnapshotTimeMapping(snapshot->whenTaken, xmin);
        }

        return snapshot;
}

/*
 * ProcArrayInstallImportedXmin -- install imported xmin into MyPgXact->xmin
 *
 * This is called when installing a snapshot imported from another
 * transaction.  To ensure that OldestXmin doesn't go backwards, we must
 * check that the source transaction is still running, and we'd better do
 * that atomically with installing the new xmin.
 *
 * Returns TRUE if successful, FALSE if source xact is no longer running.
 */
bool
ProcArrayInstallImportedXmin(TransactionId xmin,
                                                         VirtualTransactionId *sourcevxid)
{
        bool            result = false;
        ProcArrayStruct *arrayP = procArray;
        int                     index;

        Assert(TransactionIdIsNormal(xmin));
        if (!sourcevxid)
                return false;

        /* Get lock so source xact can't end while we're doing this */
        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                TransactionId xid;

                /* Ignore procs running LAZY VACUUM */
                if (pgxact->vacuumFlags & PROC_IN_VACUUM)
                        continue;

                /* We are only interested in the specific virtual transaction. */
                if (proc->backendId != sourcevxid->backendId)
                        continue;
                if (proc->lxid != sourcevxid->localTransactionId)
                        continue;

                /*
                 * We check the transaction's database ID for paranoia's sake: if it's
                 * in another DB then its xmin does not cover us.  Caller should have
                 * detected this already, so we just treat any funny cases as
                 * "transaction not found".
                 */
                if (proc->databaseId != MyDatabaseId)
                        continue;

                /*
                 * Likewise, let's just make real sure its xmin does cover us.
                 */
                xid = pgxact->xmin;             /* fetch just once */
                if (!TransactionIdIsNormal(xid) ||
                        !TransactionIdPrecedesOrEquals(xid, xmin))
                        continue;

                /*
                 * We're good.  Install the new xmin.  As in GetSnapshotData, set
                 * TransactionXmin too.  (Note that because snapmgr.c called
                 * GetSnapshotData first, we'll be overwriting a valid xmin here, so
                 * we don't check that.)
                 */
                MyPgXact->xmin = TransactionXmin = xmin;

                result = true;
                break;
        }

        LWLockRelease(ProcArrayLock);

        return result;
}

/*
 * ProcArrayInstallRestoredXmin -- install restored xmin into MyPgXact->xmin
 *
 * This is like ProcArrayInstallImportedXmin, but we have a pointer to the
 * PGPROC of the transaction from which we imported the snapshot, rather than
 * an XID.
 *
 * Returns TRUE if successful, FALSE if source xact is no longer running.
 */
bool
ProcArrayInstallRestoredXmin(TransactionId xmin, PGPROC *proc)
{
        bool            result = false;
        TransactionId xid;
        volatile PGXACT *pgxact;

        Assert(TransactionIdIsNormal(xmin));
        Assert(proc != NULL);

        /* Get lock so source xact can't end while we're doing this */
        LWLockAcquire(ProcArrayLock, LW_SHARED);

        pgxact = &allPgXact[proc->pgprocno];

        /*
         * Be certain that the referenced PGPROC has an advertised xmin which is
         * no later than the one we're installing, so that the system-wide xmin
         * can't go backwards.  Also, make sure it's running in the same database,
         * so that the per-database xmin cannot go backwards.
         */
        xid = pgxact->xmin;                     /* fetch just once */
        if (proc->databaseId == MyDatabaseId &&
                TransactionIdIsNormal(xid) &&
                TransactionIdPrecedesOrEquals(xid, xmin))
        {
                MyPgXact->xmin = TransactionXmin = xmin;
                result = true;
        }

        LWLockRelease(ProcArrayLock);

        return result;
}

/*
 * GetRunningTransactionData -- returns information about running transactions.
 *
 * Similar to GetSnapshotData but returns more information. We include
 * all PGXACTs with an assigned TransactionId, even VACUUM processes.
 *
 * We acquire XidGenLock and ProcArrayLock, but the caller is responsible for
 * releasing them. Acquiring XidGenLock ensures that no new XIDs enter the proc
 * array until the caller has WAL-logged this snapshot, and releases the
 * lock. Acquiring ProcArrayLock ensures that no transactions commit until the
 * lock is released.
 *
 * The returned data structure is statically allocated; caller should not
 * modify it, and must not assume it is valid past the next call.
 *
 * This is never executed during recovery so there is no need to look at
 * KnownAssignedXids.
 *
 * We don't worry about updating other counters, we want to keep this as
 * simple as possible and leave GetSnapshotData() as the primary code for
 * that bookkeeping.
 *
 * Note that if any transaction has overflowed its cached subtransactions
 * then there is no real need include any subtransactions. That isn't a
 * common enough case to worry about optimising the size of the WAL record,
 * and we may wish to see that data for diagnostic purposes anyway.
 */
RunningTransactions
GetRunningTransactionData(void)
{
        /* result workspace */
        static RunningTransactionsData CurrentRunningXactsData;

        ProcArrayStruct *arrayP = procArray;
        RunningTransactions CurrentRunningXacts = &CurrentRunningXactsData;
        TransactionId latestCompletedXid;
        TransactionId oldestRunningXid;
        TransactionId *xids;
        int                     index;
        int                     count;
        int                     subcount;
        bool            suboverflowed;

        Assert(!RecoveryInProgress());

        /*
         * Allocating space for maxProcs xids is usually overkill; numProcs would
         * be sufficient.  But it seems better to do the malloc while not holding
         * the lock, so we can't look at numProcs.  Likewise, we allocate much
         * more subxip storage than is probably needed.
         *
         * Should only be allocated in bgwriter, since only ever executed during
         * checkpoints.
         */
        if (CurrentRunningXacts->xids == NULL)
        {
                /*
                 * First call
                 */
                CurrentRunningXacts->xids = (TransactionId *)
                        malloc(TOTAL_MAX_CACHED_SUBXIDS * sizeof(TransactionId));
                if (CurrentRunningXacts->xids == NULL)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory")));
        }

        xids = CurrentRunningXacts->xids;

        count = subcount = 0;
        suboverflowed = false;

        /*
         * Ensure that no xids enter or leave the procarray while we obtain
         * snapshot.
         */
        LWLockAcquire(ProcArrayLock, LW_SHARED);
        LWLockAcquire(XidGenLock, LW_SHARED);

        latestCompletedXid = ShmemVariableCache->latestCompletedXid;

        oldestRunningXid = ShmemVariableCache->nextXid;

        /*
         * Spin over procArray collecting all xids
         */
        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                TransactionId xid;

                /* Fetch xid just once - see GetNewTransactionId */
                xid = pgxact->xid;

                /*
                 * We don't need to store transactions that don't have a TransactionId
                 * yet because they will not show as running on a standby server.
                 */
                if (!TransactionIdIsValid(xid))
                        continue;

                xids[count++] = xid;

                if (TransactionIdPrecedes(xid, oldestRunningXid))
                        oldestRunningXid = xid;

                if (pgxact->overflowed)
                        suboverflowed = true;
        }

        /*
         * Spin over procArray collecting all subxids, but only if there hasn't
         * been a suboverflow.
         */
        if (!suboverflowed)
        {
                for (index = 0; index < arrayP->numProcs; index++)
                {
                        int                     pgprocno = arrayP->pgprocnos[index];
                        volatile PGPROC *proc = &allProcs[pgprocno];
                        volatile PGXACT *pgxact = &allPgXact[pgprocno];
                        int                     nxids;

                        /*
                         * Save subtransaction XIDs. Other backends can't add or remove
                         * entries while we're holding XidGenLock.
                         */
                        nxids = pgxact->nxids;
                        if (nxids > 0)
                        {
                                memcpy(&xids[count], (void *) proc->subxids.xids,
                                           nxids * sizeof(TransactionId));
                                count += nxids;
                                subcount += nxids;

                                /*
                                 * Top-level XID of a transaction is always less than any of
                                 * its subxids, so we don't need to check if any of the
                                 * subxids are smaller than oldestRunningXid
                                 */
                        }
                }
        }

        /*
         * It's important *not* to include the limits set by slots here because
         * snapbuild.c uses oldestRunningXid to manage its xmin horizon. If those
         * were to be included here the initial value could never increase because
         * of a circular dependency where slots only increase their limits when
         * running xacts increases oldestRunningXid and running xacts only
         * increases if slots do.
         */

        CurrentRunningXacts->xcnt = count - subcount;
        CurrentRunningXacts->subxcnt = subcount;
        CurrentRunningXacts->subxid_overflow = suboverflowed;
        CurrentRunningXacts->nextXid = ShmemVariableCache->nextXid;
        CurrentRunningXacts->oldestRunningXid = oldestRunningXid;
        CurrentRunningXacts->latestCompletedXid = latestCompletedXid;

        Assert(TransactionIdIsValid(CurrentRunningXacts->nextXid));
        Assert(TransactionIdIsValid(CurrentRunningXacts->oldestRunningXid));
        Assert(TransactionIdIsNormal(CurrentRunningXacts->latestCompletedXid));

        /* We don't release the locks here, the caller is responsible for that */

        return CurrentRunningXacts;
}

/*
 * GetOldestActiveTransactionId()
 *
 * Similar to GetSnapshotData but returns just oldestActiveXid. We include
 * all PGXACTs with an assigned TransactionId, even VACUUM processes.
 * We look at all databases, though there is no need to include WALSender
 * since this has no effect on hot standby conflicts.
 *
 * This is never executed during recovery so there is no need to look at
 * KnownAssignedXids.
 *
 * We don't worry about updating other counters, we want to keep this as
 * simple as possible and leave GetSnapshotData() as the primary code for
 * that bookkeeping.
 */
TransactionId
GetOldestActiveTransactionId(void)
{
        ProcArrayStruct *arrayP = procArray;
        TransactionId oldestRunningXid;
        int                     index;

        Assert(!RecoveryInProgress());

        /*
         * Read nextXid, as the upper bound of what's still active.
         *
         * Reading a TransactionId is atomic, but we must grab the lock to make
         * sure that all XIDs < nextXid are already present in the proc array (or
         * have already completed), when we spin over it.
         */
        LWLockAcquire(XidGenLock, LW_SHARED);
        oldestRunningXid = ShmemVariableCache->nextXid;
        LWLockRelease(XidGenLock);

        /*
         * Spin over procArray collecting all xids and subxids.
         */
        LWLockAcquire(ProcArrayLock, LW_SHARED);
        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                TransactionId xid;

                /* Fetch xid just once - see GetNewTransactionId */
                xid = pgxact->xid;

                if (!TransactionIdIsNormal(xid))
                        continue;

                if (TransactionIdPrecedes(xid, oldestRunningXid))
                        oldestRunningXid = xid;

                /*
                 * Top-level XID of a transaction is always less than any of its
                 * subxids, so we don't need to check if any of the subxids are
                 * smaller than oldestRunningXid
                 */
        }
        LWLockRelease(ProcArrayLock);

        return oldestRunningXid;
}

/*
 * GetOldestSafeDecodingTransactionId -- lowest xid not affected by vacuum
 *
 * Returns the oldest xid that we can guarantee not to have been affected by
 * vacuum, i.e. no rows >= that xid have been vacuumed away unless the
 * transaction aborted. Note that the value can (and most of the time will) be
 * much more conservative than what really has been affected by vacuum, but we
 * currently don't have better data available.
 *
 * This is useful to initialize the cutoff xid after which a new changeset
 * extraction replication slot can start decoding changes.
 *
 * Must be called with ProcArrayLock held either shared or exclusively,
 * although most callers will want to use exclusive mode since it is expected
 * that the caller will immediately use the xid to peg the xmin horizon.
 */
TransactionId
GetOldestSafeDecodingTransactionId(bool catalogOnly)
{
        ProcArrayStruct *arrayP = procArray;
        TransactionId oldestSafeXid;
        int                     index;
        bool            recovery_in_progress = RecoveryInProgress();

        Assert(LWLockHeldByMe(ProcArrayLock));

        /*
         * Acquire XidGenLock, so no transactions can acquire an xid while we're
         * running. If no transaction with xid were running concurrently a new xid
         * could influence the RecentXmin et al.
         *
         * We initialize the computation to nextXid since that's guaranteed to be
         * a safe, albeit pessimal, value.
         */
        LWLockAcquire(XidGenLock, LW_SHARED);
        oldestSafeXid = ShmemVariableCache->nextXid;

        /*
         * If there's already a slot pegging the xmin horizon, we can start with
         * that value, it's guaranteed to be safe since it's computed by this
         * routine initially and has been enforced since.  We can always use the
         * slot's general xmin horizon, but the catalog horizon is only usable
         * when we only catalog data is going to be looked at.
         */
        if (TransactionIdIsValid(procArray->replication_slot_xmin) &&
                TransactionIdPrecedes(procArray->replication_slot_xmin,
                                                          oldestSafeXid))
                oldestSafeXid = procArray->replication_slot_xmin;

        if (catalogOnly &&
                TransactionIdIsValid(procArray->replication_slot_catalog_xmin) &&
                TransactionIdPrecedes(procArray->replication_slot_catalog_xmin,
                                                          oldestSafeXid))
                oldestSafeXid = procArray->replication_slot_catalog_xmin;

        /*
         * If we're not in recovery, we walk over the procarray and collect the
         * lowest xid. Since we're called with ProcArrayLock held and have
         * acquired XidGenLock, no entries can vanish concurrently, since
         * PGXACT->xid is only set with XidGenLock held and only cleared with
         * ProcArrayLock held.
         *
         * In recovery we can't lower the safe value besides what we've computed
         * above, so we'll have to wait a bit longer there. We unfortunately can
         * *not* use KnownAssignedXidsGetOldestXmin() since the KnownAssignedXids
         * machinery can miss values and return an older value than is safe.
         */
        if (!recovery_in_progress)
        {
                /*
                 * Spin over procArray collecting all min(PGXACT->xid)
                 */
                for (index = 0; index < arrayP->numProcs; index++)
                {
                        int                     pgprocno = arrayP->pgprocnos[index];
                        volatile PGXACT *pgxact = &allPgXact[pgprocno];
                        TransactionId xid;

                        /* Fetch xid just once - see GetNewTransactionId */
                        xid = pgxact->xid;

                        if (!TransactionIdIsNormal(xid))
                                continue;

                        if (TransactionIdPrecedes(xid, oldestSafeXid))
                                oldestSafeXid = xid;
                }
        }

        LWLockRelease(XidGenLock);

        return oldestSafeXid;
}

/*
 * GetVirtualXIDsDelayingChkpt -- Get the VXIDs of transactions that are
 * delaying checkpoint because they have critical actions in progress.
 *
 * Constructs an array of VXIDs of transactions that are currently in commit
 * critical sections, as shown by having delayChkpt set in their PGXACT.
 *
 * Returns a palloc'd array that should be freed by the caller.
 * *nvxids is the number of valid entries.
 *
 * Note that because backends set or clear delayChkpt without holding any lock,
 * the result is somewhat indeterminate, but we don't really care.  Even in
 * a multiprocessor with delayed writes to shared memory, it should be certain
 * that setting of delayChkpt will propagate to shared memory when the backend
 * takes a lock, so we cannot fail to see a virtual xact as delayChkpt if
 * it's already inserted its commit record.  Whether it takes a little while
 * for clearing of delayChkpt to propagate is unimportant for correctness.
 */
VirtualTransactionId *
GetVirtualXIDsDelayingChkpt(int *nvxids)
{
        VirtualTransactionId *vxids;
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        /* allocate what's certainly enough result space */
        vxids = (VirtualTransactionId *)
                palloc(sizeof(VirtualTransactionId) * arrayP->maxProcs);

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                if (pgxact->delayChkpt)
                {
                        VirtualTransactionId vxid;

                        GET_VXID_FROM_PGPROC(vxid, *proc);
                        if (VirtualTransactionIdIsValid(vxid))
                                vxids[count++] = vxid;
                }
        }

        LWLockRelease(ProcArrayLock);

        *nvxids = count;
        return vxids;
}

/*
 * HaveVirtualXIDsDelayingChkpt -- Are any of the specified VXIDs delaying?
 *
 * This is used with the results of GetVirtualXIDsDelayingChkpt to see if any
 * of the specified VXIDs are still in critical sections of code.
 *
 * Note: this is O(N^2) in the number of vxacts that are/were delaying, but
 * those numbers should be small enough for it not to be a problem.
 */
bool
HaveVirtualXIDsDelayingChkpt(VirtualTransactionId *vxids, int nvxids)
{
        bool            result = false;
        ProcArrayStruct *arrayP = procArray;
        int                     index;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];
                VirtualTransactionId vxid;

                GET_VXID_FROM_PGPROC(vxid, *proc);

                if (pgxact->delayChkpt && VirtualTransactionIdIsValid(vxid))
                {
                        int                     i;

                        for (i = 0; i < nvxids; i++)
                        {
                                if (VirtualTransactionIdEquals(vxid, vxids[i]))
                                {
                                        result = true;
                                        break;
                                }
                        }
                        if (result)
                                break;
                }
        }

        LWLockRelease(ProcArrayLock);

        return result;
}

/*
 * BackendPidGetProc -- get a backend's PGPROC given its PID
 *
 * Returns NULL if not found.  Note that it is up to the caller to be
 * sure that the question remains meaningful for long enough for the
 * answer to be used ...
 */
PGPROC *
BackendPidGetProc(int pid)
{
        PGPROC     *result;

        if (pid == 0)                           /* never match dummy PGPROCs */
                return NULL;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        result = BackendPidGetProcWithLock(pid);

        LWLockRelease(ProcArrayLock);

        return result;
}

/*
 * BackendPidGetProcWithLock -- get a backend's PGPROC given its PID
 *
 * Same as above, except caller must be holding ProcArrayLock.  The found
 * entry, if any, can be assumed to be valid as long as the lock remains held.
 */
PGPROC *
BackendPidGetProcWithLock(int pid)
{
        PGPROC     *result = NULL;
        ProcArrayStruct *arrayP = procArray;
        int                     index;

        if (pid == 0)                           /* never match dummy PGPROCs */
                return NULL;

        for (index = 0; index < arrayP->numProcs; index++)
        {
                PGPROC     *proc = &allProcs[arrayP->pgprocnos[index]];

                if (proc->pid == pid)
                {
                        result = proc;
                        break;
                }
        }

        return result;
}

/*
 * BackendXidGetPid -- get a backend's pid given its XID
 *
 * Returns 0 if not found or it's a prepared transaction.  Note that
 * it is up to the caller to be sure that the question remains
 * meaningful for long enough for the answer to be used ...
 *
 * Only main transaction Ids are considered.  This function is mainly
 * useful for determining what backend owns a lock.
 *
 * Beware that not every xact has an XID assigned.  However, as long as you
 * only call this using an XID found on disk, you're safe.
 */
int
BackendXidGetPid(TransactionId xid)
{
        int                     result = 0;
        ProcArrayStruct *arrayP = procArray;
        int                     index;

        if (xid == InvalidTransactionId)        /* never match invalid xid */
                return 0;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                if (pgxact->xid == xid)
                {
                        result = proc->pid;
                        break;
                }
        }

        LWLockRelease(ProcArrayLock);

        return result;
}

/*
 * IsBackendPid -- is a given pid a running backend
 *
 * This is not called by the backend, but is called by external modules.
 */
bool
IsBackendPid(int pid)
{
        return (BackendPidGetProc(pid) != NULL);
}


/*
 * GetCurrentVirtualXIDs -- returns an array of currently active VXIDs.
 *
 * The array is palloc'd. The number of valid entries is returned into *nvxids.
 *
 * The arguments allow filtering the set of VXIDs returned.  Our own process
 * is always skipped.  In addition:
 *      If limitXmin is not InvalidTransactionId, skip processes with
 *              xmin > limitXmin.
 *      If excludeXmin0 is true, skip processes with xmin = 0.
 *      If allDbs is false, skip processes attached to other databases.
 *      If excludeVacuum isn't zero, skip processes for which
 *              (vacuumFlags & excludeVacuum) is not zero.
 *
 * Note: the purpose of the limitXmin and excludeXmin0 parameters is to
 * allow skipping backends whose oldest live snapshot is no older than
 * some snapshot we have.  Since we examine the procarray with only shared
 * lock, there are race conditions: a backend could set its xmin just after
 * we look.  Indeed, on multiprocessors with weak memory ordering, the
 * other backend could have set its xmin *before* we look.  We know however
 * that such a backend must have held shared ProcArrayLock overlapping our
 * own hold of ProcArrayLock, else we would see its xmin update.  Therefore,
 * any snapshot the other backend is taking concurrently with our scan cannot
 * consider any transactions as still running that we think are committed
 * (since backends must hold ProcArrayLock exclusive to commit).
 */
VirtualTransactionId *
GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
                                          bool allDbs, int excludeVacuum,
                                          int *nvxids)
{
        VirtualTransactionId *vxids;
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        /* allocate what's certainly enough result space */
        vxids = (VirtualTransactionId *)
                palloc(sizeof(VirtualTransactionId) * arrayP->maxProcs);

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                if (proc == MyProc)
                        continue;

                if (excludeVacuum & pgxact->vacuumFlags)
                        continue;

                if (allDbs || proc->databaseId == MyDatabaseId)
                {
                        /* Fetch xmin just once - might change on us */
                        TransactionId pxmin = pgxact->xmin;

                        if (excludeXmin0 && !TransactionIdIsValid(pxmin))
                                continue;

                        /*
                         * InvalidTransactionId precedes all other XIDs, so a proc that
                         * hasn't set xmin yet will not be rejected by this test.
                         */
                        if (!TransactionIdIsValid(limitXmin) ||
                                TransactionIdPrecedesOrEquals(pxmin, limitXmin))
                        {
                                VirtualTransactionId vxid;

                                GET_VXID_FROM_PGPROC(vxid, *proc);
                                if (VirtualTransactionIdIsValid(vxid))
                                        vxids[count++] = vxid;
                        }
                }
        }

        LWLockRelease(ProcArrayLock);

        *nvxids = count;
        return vxids;
}

/*
 * GetConflictingVirtualXIDs -- returns an array of currently active VXIDs.
 *
 * Usage is limited to conflict resolution during recovery on standby servers.
 * limitXmin is supplied as either latestRemovedXid, or InvalidTransactionId
 * in cases where we cannot accurately determine a value for latestRemovedXid.
 *
 * If limitXmin is InvalidTransactionId then we want to kill everybody,
 * so we're not worried if they have a snapshot or not, nor does it really
 * matter what type of lock we hold.
 *
 * All callers that are checking xmins always now supply a valid and useful
 * value for limitXmin. The limitXmin is always lower than the lowest
 * numbered KnownAssignedXid that is not already a FATAL error. This is
 * because we only care about cleanup records that are cleaning up tuple
 * versions from committed transactions. In that case they will only occur
 * at the point where the record is less than the lowest running xid. That
 * allows us to say that if any backend takes a snapshot concurrently with
 * us then the conflict assessment made here would never include the snapshot
 * that is being derived. So we take LW_SHARED on the ProcArray and allow
 * concurrent snapshots when limitXmin is valid. We might think about adding
 *       Assert(limitXmin < lowest(KnownAssignedXids))
 * but that would not be true in the case of FATAL errors lagging in array,
 * but we already know those are bogus anyway, so we skip that test.
 *
 * If dbOid is valid we skip backends attached to other databases.
 *
 * Be careful to *not* pfree the result from this function. We reuse
 * this array sufficiently often that we use malloc for the result.
 */
VirtualTransactionId *
GetConflictingVirtualXIDs(TransactionId limitXmin, Oid dbOid)
{
        static VirtualTransactionId *vxids;
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        /*
         * If first time through, get workspace to remember main XIDs in. We
         * malloc it permanently to avoid repeated palloc/pfree overhead. Allow
         * result space, remembering room for a terminator.
         */
        if (vxids == NULL)
        {
                vxids = (VirtualTransactionId *)
                        malloc(sizeof(VirtualTransactionId) * (arrayP->maxProcs + 1));
                if (vxids == NULL)
                        ereport(ERROR,
                                        (errcode(ERRCODE_OUT_OF_MEMORY),
                                         errmsg("out of memory")));
        }

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                /* Exclude prepared transactions */
                if (proc->pid == 0)
                        continue;

                if (!OidIsValid(dbOid) ||
                        proc->databaseId == dbOid)
                {
                        /* Fetch xmin just once - can't change on us, but good coding */
                        TransactionId pxmin = pgxact->xmin;

                        /*
                         * We ignore an invalid pxmin because this means that backend has
                         * no snapshot currently. We hold a Share lock to avoid contention
                         * with users taking snapshots.  That is not a problem because the
                         * current xmin is always at least one higher than the latest
                         * removed xid, so any new snapshot would never conflict with the
                         * test here.
                         */
                        if (!TransactionIdIsValid(limitXmin) ||
                                (TransactionIdIsValid(pxmin) && !TransactionIdFollows(pxmin, limitXmin)))
                        {
                                VirtualTransactionId vxid;

                                GET_VXID_FROM_PGPROC(vxid, *proc);
                                if (VirtualTransactionIdIsValid(vxid))
                                        vxids[count++] = vxid;
                        }
                }
        }

        LWLockRelease(ProcArrayLock);

        /* add the terminator */
        vxids[count].backendId = InvalidBackendId;
        vxids[count].localTransactionId = InvalidLocalTransactionId;

        return vxids;
}

/*
 * CancelVirtualTransaction - used in recovery conflict processing
 *
 * Returns pid of the process signaled, or 0 if not found.
 */
pid_t
CancelVirtualTransaction(VirtualTransactionId vxid, ProcSignalReason sigmode)
{
        ProcArrayStruct *arrayP = procArray;
        int                     index;
        pid_t           pid = 0;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                VirtualTransactionId procvxid;

                GET_VXID_FROM_PGPROC(procvxid, *proc);

                if (procvxid.backendId == vxid.backendId &&
                        procvxid.localTransactionId == vxid.localTransactionId)
                {
                        proc->recoveryConflictPending = true;
                        pid = proc->pid;
                        if (pid != 0)
                        {
                                /*
                                 * Kill the pid if it's still here. If not, that's what we
                                 * wanted so ignore any errors.
                                 */
                                (void) SendProcSignal(pid, sigmode, vxid.backendId);
                        }
                        break;
                }
        }

        LWLockRelease(ProcArrayLock);

        return pid;
}

/*
 * MinimumActiveBackends --- count backends (other than myself) that are
 *              in active transactions.  Return true if the count exceeds the
 *              minimum threshold passed.  This is used as a heuristic to decide if
 *              a pre-XLOG-flush delay is worthwhile during commit.
 *
 * Do not count backends that are blocked waiting for locks, since they are
 * not going to get to run until someone else commits.
 */
bool
MinimumActiveBackends(int min)
{
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        /* Quick short-circuit if no minimum is specified */
        if (min == 0)
                return true;

        /*
         * Note: for speed, we don't acquire ProcArrayLock.  This is a little bit
         * bogus, but since we are only testing fields for zero or nonzero, it
         * should be OK.  The result is only used for heuristic purposes anyway...
         */
        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];
                volatile PGXACT *pgxact = &allPgXact[pgprocno];

                /*
                 * Since we're not holding a lock, need to be prepared to deal with
                 * garbage, as someone could have incremented numProcs but not yet
                 * filled the structure.
                 *
                 * If someone just decremented numProcs, 'proc' could also point to a
                 * PGPROC entry that's no longer in the array. It still points to a
                 * PGPROC struct, though, because freed PGPROC entries just go to the
                 * free list and are recycled. Its contents are nonsense in that case,
                 * but that's acceptable for this function.
                 */
                if (pgprocno == -1)
                        continue;                       /* do not count deleted entries */
                if (proc == MyProc)
                        continue;                       /* do not count myself */
                if (pgxact->xid == InvalidTransactionId)
                        continue;                       /* do not count if no XID assigned */
                if (proc->pid == 0)
                        continue;                       /* do not count prepared xacts */
                if (proc->waitLock != NULL)
                        continue;                       /* do not count if blocked on a lock */
                count++;
                if (count >= min)
                        break;
        }

        return count >= min;
}

/*
 * CountDBBackends --- count backends that are using specified database
 */
int
CountDBBackends(Oid databaseid)
{
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];

                if (proc->pid == 0)
                        continue;                       /* do not count prepared xacts */
                if (!OidIsValid(databaseid) ||
                        proc->databaseId == databaseid)
                        count++;
        }

        LWLockRelease(ProcArrayLock);

        return count;
}

/*
 * CountDBConnections --- counts database backends ignoring any background
 *              worker processes
 */
int
CountDBConnections(Oid databaseid)
{
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];

                if (proc->pid == 0)
                        continue;                       /* do not count prepared xacts */
                if (proc->isBackgroundWorker)
                        continue;                       /* do not count background workers */
                if (!OidIsValid(databaseid) ||
                        proc->databaseId == databaseid)
                        count++;
        }

        LWLockRelease(ProcArrayLock);

        return count;
}

/*
 * CancelDBBackends --- cancel backends that are using specified database
 */
void
CancelDBBackends(Oid databaseid, ProcSignalReason sigmode, bool conflictPending)
{
        ProcArrayStruct *arrayP = procArray;
        int                     index;
        pid_t           pid = 0;

        /* tell all backends to die */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];

                if (databaseid == InvalidOid || proc->databaseId == databaseid)
                {
                        VirtualTransactionId procvxid;

                        GET_VXID_FROM_PGPROC(procvxid, *proc);

                        proc->recoveryConflictPending = conflictPending;
                        pid = proc->pid;
                        if (pid != 0)
                        {
                                /*
                                 * Kill the pid if it's still here. If not, that's what we
                                 * wanted so ignore any errors.
                                 */
                                (void) SendProcSignal(pid, sigmode, procvxid.backendId);
                        }
                }
        }

        LWLockRelease(ProcArrayLock);
}

/*
 * CountUserBackends --- count backends that are used by specified user
 */
int
CountUserBackends(Oid roleid)
{
        ProcArrayStruct *arrayP = procArray;
        int                     count = 0;
        int                     index;

        LWLockAcquire(ProcArrayLock, LW_SHARED);

        for (index = 0; index < arrayP->numProcs; index++)
        {
                int                     pgprocno = arrayP->pgprocnos[index];
                volatile PGPROC *proc = &allProcs[pgprocno];

                if (proc->pid == 0)
                        continue;                       /* do not count prepared xacts */
                if (proc->isBackgroundWorker)
                        continue;                       /* do not count background workers */
                if (proc->roleId == roleid)
                        count++;
        }

        LWLockRelease(ProcArrayLock);

        return count;
}

/*
 * CountOtherDBBackends -- check for other backends running in the given DB
 *
 * If there are other backends in the DB, we will wait a maximum of 5 seconds
 * for them to exit.  Autovacuum backends are encouraged to exit early by
 * sending them SIGTERM, but normal user backends are just waited for.
 *
 * The current backend is always ignored; it is caller's responsibility to
 * check whether the current backend uses the given DB, if it's important.
 *
 * Returns TRUE if there are (still) other backends in the DB, FALSE if not.
 * Also, *nbackends and *nprepared are set to the number of other backends
 * and prepared transactions in the DB, respectively.
 *
 * This function is used to interlock DROP DATABASE and related commands
 * against there being any active backends in the target DB --- dropping the
 * DB while active backends remain would be a Bad Thing.  Note that we cannot
 * detect here the possibility of a newly-started backend that is trying to
 * connect to the doomed database, so additional interlocking is needed during
 * backend startup.  The caller should normally hold an exclusive lock on the
 * target DB before calling this, which is one reason we mustn't wait
 * indefinitely.
 */
bool
CountOtherDBBackends(Oid databaseId, int *nbackends, int *nprepared)
{
        ProcArrayStruct *arrayP = procArray;

#define MAXAUTOVACPIDS  10              /* max autovacs to SIGTERM per iteration */
        int                     autovac_pids[MAXAUTOVACPIDS];
        int                     tries;

        /* 50 tries with 100ms sleep between tries makes 5 sec total wait */
        for (tries = 0; tries < 50; tries++)
        {
                int                     nautovacs = 0;
                bool            found = false;
                int                     index;

                CHECK_FOR_INTERRUPTS();

                *nbackends = *nprepared = 0;

                LWLockAcquire(ProcArrayLock, LW_SHARED);

                for (index = 0; index < arrayP->numProcs; index++)
                {
                        int                     pgprocno = arrayP->pgprocnos[index];
                        volatile PGPROC *proc = &allProcs[pgprocno];
                        volatile PGXACT *pgxact = &allPgXact[pgprocno];

                        if (proc->databaseId != databaseId)
                                continue;
                        if (proc == MyProc)
                                continue;

                        found = true;

                        if (proc->pid == 0)
                                (*nprepared)++;
                        else
                        {
                                (*nbackends)++;
                                if ((pgxact->vacuumFlags & PROC_IS_AUTOVACUUM) &&
                                        nautovacs < MAXAUTOVACPIDS)
                                        autovac_pids[nautovacs++] = proc->pid;
                        }
                }

                LWLockRelease(ProcArrayLock);

                if (!found)
                        return false;           /* no conflicting backends, so done */

                /*
                 * Send SIGTERM to any conflicting autovacuums before sleeping. We
                 * postpone this step until after the loop because we don't want to
                 * hold ProcArrayLock while issuing kill(). We have no idea what might
                 * block kill() inside the kernel...
                 */
                for (index = 0; index < nautovacs; index++)
                        (void) kill(autovac_pids[index], SIGTERM);      /* ignore any error */

                /* sleep, then try again */
                pg_usleep(100 * 1000L); /* 100ms */
        }

        return true;                            /* timed out, still conflicts */
}

/*
 * ProcArraySetReplicationSlotXmin
 *
 * Install limits to future computations of the xmin horizon to prevent vacuum
 * and HOT pruning from removing affected rows still needed by clients with
 * replicaton slots.
 */
void
ProcArraySetReplicationSlotXmin(TransactionId xmin, TransactionId catalog_xmin,
                                                                bool already_locked)
{
        Assert(!already_locked || LWLockHeldByMe(ProcArrayLock));

        if (!already_locked)
                LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        procArray->replication_slot_xmin = xmin;
        procArray->replication_slot_catalog_xmin = catalog_xmin;

        if (!already_locked)
                LWLockRelease(ProcArrayLock);
}

/*
 * ProcArrayGetReplicationSlotXmin
 *
 * Return the current slot xmin limits. That's useful to be able to remove
 * data that's older than those limits.
 */
void
ProcArrayGetReplicationSlotXmin(TransactionId *xmin,
                                                                TransactionId *catalog_xmin)
{
        LWLockAcquire(ProcArrayLock, LW_SHARED);

        if (xmin != NULL)
                *xmin = procArray->replication_slot_xmin;

        if (catalog_xmin != NULL)
                *catalog_xmin = procArray->replication_slot_catalog_xmin;

        LWLockRelease(ProcArrayLock);
}


#define XidCacheRemove(i) \
        do { \
                MyProc->subxids.xids[i] = MyProc->subxids.xids[MyPgXact->nxids - 1]; \
                MyPgXact->nxids--; \
        } while (0)

/*
 * XidCacheRemoveRunningXids
 *
 * Remove a bunch of TransactionIds from the list of known-running
 * subtransactions for my backend.  Both the specified xid and those in
 * the xids[] array (of length nxids) are removed from the subxids cache.
 * latestXid must be the latest XID among the group.
 */
void
XidCacheRemoveRunningXids(TransactionId xid,
                                                  int nxids, const TransactionId *xids,
                                                  TransactionId latestXid)
{
        int                     i,
                                j;

        Assert(TransactionIdIsValid(xid));

        /*
         * We must hold ProcArrayLock exclusively in order to remove transactions
         * from the PGPROC array.  (See src/backend/access/transam/README.)  It's
         * possible this could be relaxed since we know this routine is only used
         * to abort subtransactions, but pending closer analysis we'd best be
         * conservative.
         */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        /*
         * Under normal circumstances xid and xids[] will be in increasing order,
         * as will be the entries in subxids.  Scan backwards to avoid O(N^2)
         * behavior when removing a lot of xids.
         */
        for (i = nxids - 1; i >= 0; i--)
        {
                TransactionId anxid = xids[i];

                for (j = MyPgXact->nxids - 1; j >= 0; j--)
                {
                        if (TransactionIdEquals(MyProc->subxids.xids[j], anxid))
                        {
                                XidCacheRemove(j);
                                break;
                        }
                }

                /*
                 * Ordinarily we should have found it, unless the cache has
                 * overflowed. However it's also possible for this routine to be
                 * invoked multiple times for the same subtransaction, in case of an
                 * error during AbortSubTransaction.  So instead of Assert, emit a
                 * debug warning.
                 */
                if (j < 0 && !MyPgXact->overflowed)
                        elog(WARNING, "did not find subXID %u in MyProc", anxid);
        }

        for (j = MyPgXact->nxids - 1; j >= 0; j--)
        {
                if (TransactionIdEquals(MyProc->subxids.xids[j], xid))
                {
                        XidCacheRemove(j);
                        break;
                }
        }
        /* Ordinarily we should have found it, unless the cache has overflowed */
        if (j < 0 && !MyPgXact->overflowed)
                elog(WARNING, "did not find subXID %u in MyProc", xid);

        /* Also advance global latestCompletedXid while holding the lock */
        if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
                                                          latestXid))
                ShmemVariableCache->latestCompletedXid = latestXid;

        LWLockRelease(ProcArrayLock);
}

#ifdef XIDCACHE_DEBUG

/*
 * Print stats about effectiveness of XID cache
 */
static void
DisplayXidCache(void)
{
        fprintf(stderr,
                        "XidCache: xmin: %ld, known: %ld, myxact: %ld, latest: %ld, mainxid: %ld, childxid: %ld, knownassigned: %ld, nooflo: %ld, slow: %ld\n",
                        xc_by_recent_xmin,
                        xc_by_known_xact,
                        xc_by_my_xact,
                        xc_by_latest_xid,
                        xc_by_main_xid,
                        xc_by_child_xid,
                        xc_by_known_assigned,
                        xc_no_overflow,
                        xc_slow_answer);
}
#endif                                                  /* XIDCACHE_DEBUG */


/* ----------------------------------------------
 *              KnownAssignedTransactions sub-module
 * ----------------------------------------------
 */

/*
 * In Hot Standby mode, we maintain a list of transactions that are (or were)
 * running in the master at the current point in WAL.  These XIDs must be
 * treated as running by standby transactions, even though they are not in
 * the standby server's PGXACT array.
 *
 * We record all XIDs that we know have been assigned.  That includes all the
 * XIDs seen in WAL records, plus all unobserved XIDs that we can deduce have
 * been assigned.  We can deduce the existence of unobserved XIDs because we
 * know XIDs are assigned in sequence, with no gaps.  The KnownAssignedXids
 * list expands as new XIDs are observed or inferred, and contracts when
 * transaction completion records arrive.
 *
 * During hot standby we do not fret too much about the distinction between
 * top-level XIDs and subtransaction XIDs. We store both together in the
 * KnownAssignedXids list.  In backends, this is copied into snapshots in
 * GetSnapshotData(), taking advantage of the fact that XidInMVCCSnapshot()
 * doesn't care about the distinction either.  Subtransaction XIDs are
 * effectively treated as top-level XIDs and in the typical case pg_subtrans
 * links are *not* maintained (which does not affect visibility).
 *
 * We have room in KnownAssignedXids and in snapshots to hold maxProcs *
 * (1 + PGPROC_MAX_CACHED_SUBXIDS) XIDs, so every master transaction must
 * report its subtransaction XIDs in a WAL XLOG_XACT_ASSIGNMENT record at
 * least every PGPROC_MAX_CACHED_SUBXIDS.  When we receive one of these
 * records, we mark the subXIDs as children of the top XID in pg_subtrans,
 * and then remove them from KnownAssignedXids.  This prevents overflow of
 * KnownAssignedXids and snapshots, at the cost that status checks for these
 * subXIDs will take a slower path through TransactionIdIsInProgress().
 * This means that KnownAssignedXids is not necessarily complete for subXIDs,
 * though it should be complete for top-level XIDs; this is the same situation
 * that holds with respect to the PGPROC entries in normal running.
 *
 * When we throw away subXIDs from KnownAssignedXids, we need to keep track of
 * that, similarly to tracking overflow of a PGPROC's subxids array.  We do
 * that by remembering the lastOverflowedXID, ie the last thrown-away subXID.
 * As long as that is within the range of interesting XIDs, we have to assume
 * that subXIDs are missing from snapshots.  (Note that subXID overflow occurs
 * on primary when 65th subXID arrives, whereas on standby it occurs when 64th
 * subXID arrives - that is not an error.)
 *
 * Should a backend on primary somehow disappear before it can write an abort
 * record, then we just leave those XIDs in KnownAssignedXids. They actually
 * aborted but we think they were running; the distinction is irrelevant
 * because either way any changes done by the transaction are not visible to
 * backends in the standby.  We prune KnownAssignedXids when
 * XLOG_RUNNING_XACTS arrives, to forestall possible overflow of the
 * array due to such dead XIDs.
 */

/*
 * RecordKnownAssignedTransactionIds
 *              Record the given XID in KnownAssignedXids, as well as any preceding
 *              unobserved XIDs.
 *
 * RecordKnownAssignedTransactionIds() should be run for *every* WAL record
 * associated with a transaction. Must be called for each record after we
 * have executed StartupCLOG() et al, since we must ExtendCLOG() etc..
 *
 * Called during recovery in analogy with and in place of GetNewTransactionId()
 */
void
RecordKnownAssignedTransactionIds(TransactionId xid)
{
        Assert(standbyState >= STANDBY_INITIALIZED);
        Assert(TransactionIdIsValid(xid));
        Assert(TransactionIdIsValid(latestObservedXid));

        elog(trace_recovery(DEBUG4), "record known xact %u latestObservedXid %u",
                 xid, latestObservedXid);

        /*
         * When a newly observed xid arrives, it is frequently the case that it is
         * *not* the next xid in sequence. When this occurs, we must treat the
         * intervening xids as running also.
         */
        if (TransactionIdFollows(xid, latestObservedXid))
        {
                TransactionId next_expected_xid;

                /*
                 * Extend subtrans like we do in GetNewTransactionId() during normal
                 * operation using individual extend steps. Note that we do not need
                 * to extend clog since its extensions are WAL logged.
                 *
                 * This part has to be done regardless of standbyState since we
                 * immediately start assigning subtransactions to their toplevel
                 * transactions.
                 */
                next_expected_xid = latestObservedXid;
                while (TransactionIdPrecedes(next_expected_xid, xid))
                {
                        TransactionIdAdvance(next_expected_xid);
                        ExtendSUBTRANS(next_expected_xid);
                }
                Assert(next_expected_xid == xid);

                /*
                 * If the KnownAssignedXids machinery isn't up yet, there's nothing
                 * more to do since we don't track assigned xids yet.
                 */
                if (standbyState <= STANDBY_INITIALIZED)
                {
                        latestObservedXid = xid;
                        return;
                }

                /*
                 * Add (latestObservedXid, xid] onto the KnownAssignedXids array.
                 */
                next_expected_xid = latestObservedXid;
                TransactionIdAdvance(next_expected_xid);
                KnownAssignedXidsAdd(next_expected_xid, xid, false);

                /*
                 * Now we can advance latestObservedXid
                 */
                latestObservedXid = xid;

                /* ShmemVariableCache->nextXid must be beyond any observed xid */
                next_expected_xid = latestObservedXid;
                TransactionIdAdvance(next_expected_xid);
                LWLockAcquire(XidGenLock, LW_EXCLUSIVE);
                ShmemVariableCache->nextXid = next_expected_xid;
                LWLockRelease(XidGenLock);
        }
}

/*
 * ExpireTreeKnownAssignedTransactionIds
 *              Remove the given XIDs from KnownAssignedXids.
 *
 * Called during recovery in analogy with and in place of ProcArrayEndTransaction()
 */
void
ExpireTreeKnownAssignedTransactionIds(TransactionId xid, int nsubxids,
                                                                          TransactionId *subxids, TransactionId max_xid)
{
        Assert(standbyState >= STANDBY_INITIALIZED);

        /*
         * Uses same locking as transaction commit
         */
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        KnownAssignedXidsRemoveTree(xid, nsubxids, subxids);

        /* As in ProcArrayEndTransaction, advance latestCompletedXid */
        if (TransactionIdPrecedes(ShmemVariableCache->latestCompletedXid,
                                                          max_xid))
                ShmemVariableCache->latestCompletedXid = max_xid;

        LWLockRelease(ProcArrayLock);
}

/*
 * ExpireAllKnownAssignedTransactionIds
 *              Remove all entries in KnownAssignedXids
 */
void
ExpireAllKnownAssignedTransactionIds(void)
{
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        KnownAssignedXidsRemovePreceding(InvalidTransactionId);
        LWLockRelease(ProcArrayLock);
}

/*
 * ExpireOldKnownAssignedTransactionIds
 *              Remove KnownAssignedXids entries preceding the given XID
 */
void
ExpireOldKnownAssignedTransactionIds(TransactionId xid)
{
        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
        KnownAssignedXidsRemovePreceding(xid);
        LWLockRelease(ProcArrayLock);
}


/*
 * Private module functions to manipulate KnownAssignedXids
 *
 * There are 5 main uses of the KnownAssignedXids data structure:
 *
 *      * backends taking snapshots - all valid XIDs need to be copied out
 *      * backends seeking to determine presence of a specific XID
 *      * startup process adding new known-assigned XIDs
 *      * startup process removing specific XIDs as transactions end
 *      * startup process pruning array when special WAL records arrive
 *
 * This data structure is known to be a hot spot during Hot Standby, so we
 * go to some lengths to make these operations as efficient and as concurrent
 * as possible.
 *
 * The XIDs are stored in an array in sorted order --- TransactionIdPrecedes
 * order, to be exact --- to allow binary search for specific XIDs.  Note:
 * in general TransactionIdPrecedes would not provide a total order, but
 * we know that the entries present at any instant should not extend across
 * a large enough fraction of XID space to wrap around (the master would
 * shut down for fear of XID wrap long before that happens).  So it's OK to
 * use TransactionIdPrecedes as a binary-search comparator.
 *
 * It's cheap to maintain the sortedness during insertions, since new known
 * XIDs are always reported in XID order; we just append them at the right.
 *
 * To keep individual deletions cheap, we need to allow gaps in the array.
 * This is implemented by marking array elements as valid or invalid using
 * the parallel boolean array KnownAssignedXidsValid[].  A deletion is done
 * by setting KnownAssignedXidsValid[i] to false, *without* clearing the
 * XID entry itself.  This preserves the property that the XID entries are
 * sorted, so we can do binary searches easily.  Periodically we compress
 * out the unused entries; that's much cheaper than having to compress the
 * array immediately on every deletion.
 *
 * The actually valid items in KnownAssignedXids[] and KnownAssignedXidsValid[]
 * are those with indexes tail <= i < head; items outside this subscript range
 * have unspecified contents.  When head reaches the end of the array, we
 * force compression of unused entries rather than wrapping around, since
 * allowing wraparound would greatly complicate the search logic.  We maintain
 * an explicit tail pointer so that pruning of old XIDs can be done without
 * immediately moving the array contents.  In most cases only a small fraction
 * of the array contains valid entries at any instant.
 *
 * Although only the startup process can ever change the KnownAssignedXids
 * data structure, we still need interlocking so that standby backends will
 * not observe invalid intermediate states.  The convention is that backends
 * must hold shared ProcArrayLock to examine the array.  To remove XIDs from
 * the array, the startup process must hold ProcArrayLock exclusively, for
 * the usual transactional reasons (compare commit/abort of a transaction
 * during normal running).  Compressing unused entries out of the array
 * likewise requires exclusive lock.  To add XIDs to the array, we just insert
 * them into slots to the right of the head pointer and then advance the head
 * pointer.  This wouldn't require any lock at all, except that on machines
 * with weak memory ordering we need to be careful that other processors
 * see the array element changes before they see the head pointer change.
 * We handle this by using a spinlock to protect reads and writes of the
 * head/tail pointers.  (We could dispense with the spinlock if we were to
 * create suitable memory access barrier primitives and use those instead.)
 * The spinlock must be taken to read or write the head/tail pointers unless
 * the caller holds ProcArrayLock exclusively.
 *
 * Algorithmic analysis:
 *
 * If we have a maximum of M slots, with N XIDs currently spread across
 * S elements then we have N <= S <= M always.
 *
 *      * Adding a new XID is O(1) and needs little locking (unless compression
 *              must happen)
 *      * Compressing the array is O(S) and requires exclusive lock
 *      * Removing an XID is O(logS) and requires exclusive lock
 *      * Taking a snapshot is O(S) and requires shared lock
 *      * Checking for an XID is O(logS) and requires shared lock
 *
 * In comparison, using a hash table for KnownAssignedXids would mean that
 * taking snapshots would be O(M). If we can maintain S << M then the
 * sorted array technique will deliver significantly faster snapshots.
 * If we try to keep S too small then we will spend too much time compressing,
 * so there is an optimal point for any workload mix. We use a heuristic to
 * decide when to compress the array, though trimming also helps reduce
 * frequency of compressing. The heuristic requires us to track the number of
 * currently valid XIDs in the array.
 */


/*
 * Compress KnownAssignedXids by shifting valid data down to the start of the
 * array, removing any gaps.
 *
 * A compression step is forced if "force" is true, otherwise we do it
 * only if a heuristic indicates it's a good time to do it.
 *
 * Caller must hold ProcArrayLock in exclusive mode.
 */
static void
KnownAssignedXidsCompress(bool force)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;
        int                     head,
                                tail;
        int                     compress_index;
        int                     i;

        /* no spinlock required since we hold ProcArrayLock exclusively */
        head = pArray->headKnownAssignedXids;
        tail = pArray->tailKnownAssignedXids;

        if (!force)
        {
                /*
                 * If we can choose how much to compress, use a heuristic to avoid
                 * compressing too often or not often enough.
                 *
                 * Heuristic is if we have a large enough current spread and less than
                 * 50% of the elements are currently in use, then compress. This
                 * should ensure we compress fairly infrequently. We could compress
                 * less often though the virtual array would spread out more and
                 * snapshots would become more expensive.
                 */
                int                     nelements = head - tail;

                if (nelements < 4 * PROCARRAY_MAXPROCS ||
                        nelements < 2 * pArray->numKnownAssignedXids)
                        return;
        }

        /*
         * We compress the array by reading the valid values from tail to head,
         * re-aligning data to 0th element.
         */
        compress_index = 0;
        for (i = tail; i < head; i++)
        {
                if (KnownAssignedXidsValid[i])
                {
                        KnownAssignedXids[compress_index] = KnownAssignedXids[i];
                        KnownAssignedXidsValid[compress_index] = true;
                        compress_index++;
                }
        }

        pArray->tailKnownAssignedXids = 0;
        pArray->headKnownAssignedXids = compress_index;
}

/*
 * Add xids into KnownAssignedXids at the head of the array.
 *
 * xids from from_xid to to_xid, inclusive, are added to the array.
 *
 * If exclusive_lock is true then caller already holds ProcArrayLock in
 * exclusive mode, so we need no extra locking here.  Else caller holds no
 * lock, so we need to be sure we maintain sufficient interlocks against
 * concurrent readers.  (Only the startup process ever calls this, so no need
 * to worry about concurrent writers.)
 */
static void
KnownAssignedXidsAdd(TransactionId from_xid, TransactionId to_xid,
                                         bool exclusive_lock)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;
        TransactionId next_xid;
        int                     head,
                                tail;
        int                     nxids;
        int                     i;

        Assert(TransactionIdPrecedesOrEquals(from_xid, to_xid));

        /*
         * Calculate how many array slots we'll need.  Normally this is cheap; in
         * the unusual case where the XIDs cross the wrap point, we do it the hard
         * way.
         */
        if (to_xid >= from_xid)
                nxids = to_xid - from_xid + 1;
        else
        {
                nxids = 1;
                next_xid = from_xid;
                while (TransactionIdPrecedes(next_xid, to_xid))
                {
                        nxids++;
                        TransactionIdAdvance(next_xid);
                }
        }

        /*
         * Since only the startup process modifies the head/tail pointers, we
         * don't need a lock to read them here.
         */
        head = pArray->headKnownAssignedXids;
        tail = pArray->tailKnownAssignedXids;

        Assert(head >= 0 && head <= pArray->maxKnownAssignedXids);
        Assert(tail >= 0 && tail < pArray->maxKnownAssignedXids);

        /*
         * Verify that insertions occur in TransactionId sequence.  Note that even
         * if the last existing element is marked invalid, it must still have a
         * correctly sequenced XID value.
         */
        if (head > tail &&
                TransactionIdFollowsOrEquals(KnownAssignedXids[head - 1], from_xid))
        {
                KnownAssignedXidsDisplay(LOG);
                elog(ERROR, "out-of-order XID insertion in KnownAssignedXids");
        }

        /*
         * If our xids won't fit in the remaining space, compress out free space
         */
        if (head + nxids > pArray->maxKnownAssignedXids)
        {
                /* must hold lock to compress */
                if (!exclusive_lock)
                        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

                KnownAssignedXidsCompress(true);

                head = pArray->headKnownAssignedXids;
                /* note: we no longer care about the tail pointer */

                if (!exclusive_lock)
                        LWLockRelease(ProcArrayLock);

                /*
                 * If it still won't fit then we're out of memory
                 */
                if (head + nxids > pArray->maxKnownAssignedXids)
                        elog(ERROR, "too many KnownAssignedXids");
        }

        /* Now we can insert the xids into the space starting at head */
        next_xid = from_xid;
        for (i = 0; i < nxids; i++)
        {
                KnownAssignedXids[head] = next_xid;
                KnownAssignedXidsValid[head] = true;
                TransactionIdAdvance(next_xid);
                head++;
        }

        /* Adjust count of number of valid entries */
        pArray->numKnownAssignedXids += nxids;

        /*
         * Now update the head pointer.  We use a spinlock to protect this
         * pointer, not because the update is likely to be non-atomic, but to
         * ensure that other processors see the above array updates before they
         * see the head pointer change.
         *
         * If we're holding ProcArrayLock exclusively, there's no need to take the
         * spinlock.
         */
        if (exclusive_lock)
                pArray->headKnownAssignedXids = head;
        else
        {
                SpinLockAcquire(&pArray->known_assigned_xids_lck);
                pArray->headKnownAssignedXids = head;
                SpinLockRelease(&pArray->known_assigned_xids_lck);
        }
}

/*
 * KnownAssignedXidsSearch
 *
 * Searches KnownAssignedXids for a specific xid and optionally removes it.
 * Returns true if it was found, false if not.
 *
 * Caller must hold ProcArrayLock in shared or exclusive mode.
 * Exclusive lock must be held for remove = true.
 */
static bool
KnownAssignedXidsSearch(TransactionId xid, bool remove)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;
        int                     first,
                                last;
        int                     head;
        int                     tail;
        int                     result_index = -1;

        if (remove)
        {
                /* we hold ProcArrayLock exclusively, so no need for spinlock */
                tail = pArray->tailKnownAssignedXids;
                head = pArray->headKnownAssignedXids;
        }
        else
        {
                /* take spinlock to ensure we see up-to-date array contents */
                SpinLockAcquire(&pArray->known_assigned_xids_lck);
                tail = pArray->tailKnownAssignedXids;
                head = pArray->headKnownAssignedXids;
                SpinLockRelease(&pArray->known_assigned_xids_lck);
        }

        /*
         * Standard binary search.  Note we can ignore the KnownAssignedXidsValid
         * array here, since even invalid entries will contain sorted XIDs.
         */
        first = tail;
        last = head - 1;
        while (first <= last)
        {
                int                     mid_index;
                TransactionId mid_xid;

                mid_index = (first + last) / 2;
                mid_xid = KnownAssignedXids[mid_index];

                if (xid == mid_xid)
                {
                        result_index = mid_index;
                        break;
                }
                else if (TransactionIdPrecedes(xid, mid_xid))
                        last = mid_index - 1;
                else
                        first = mid_index + 1;
        }

        if (result_index < 0)
                return false;                   /* not in array */

        if (!KnownAssignedXidsValid[result_index])
                return false;                   /* in array, but invalid */

        if (remove)
        {
                KnownAssignedXidsValid[result_index] = false;

                pArray->numKnownAssignedXids--;
                Assert(pArray->numKnownAssignedXids >= 0);

                /*
                 * If we're removing the tail element then advance tail pointer over
                 * any invalid elements.  This will speed future searches.
                 */
                if (result_index == tail)
                {
                        tail++;
                        while (tail < head && !KnownAssignedXidsValid[tail])
                                tail++;
                        if (tail >= head)
                        {
                                /* Array is empty, so we can reset both pointers */
                                pArray->headKnownAssignedXids = 0;
                                pArray->tailKnownAssignedXids = 0;
                        }
                        else
                        {
                                pArray->tailKnownAssignedXids = tail;
                        }
                }
        }

        return true;
}

/*
 * Is the specified XID present in KnownAssignedXids[]?
 *
 * Caller must hold ProcArrayLock in shared or exclusive mode.
 */
static bool
KnownAssignedXidExists(TransactionId xid)
{
        Assert(TransactionIdIsValid(xid));

        return KnownAssignedXidsSearch(xid, false);
}

/*
 * Remove the specified XID from KnownAssignedXids[].
 *
 * Caller must hold ProcArrayLock in exclusive mode.
 */
static void
KnownAssignedXidsRemove(TransactionId xid)
{
        Assert(TransactionIdIsValid(xid));

        elog(trace_recovery(DEBUG4), "remove KnownAssignedXid %u", xid);

        /*
         * Note: we cannot consider it an error to remove an XID that's not
         * present.  We intentionally remove subxact IDs while processing
         * XLOG_XACT_ASSIGNMENT, to avoid array overflow.  Then those XIDs will be
         * removed again when the top-level xact commits or aborts.
         *
         * It might be possible to track such XIDs to distinguish this case from
         * actual errors, but it would be complicated and probably not worth it.
         * So, just ignore the search result.
         */
        (void) KnownAssignedXidsSearch(xid, true);
}

/*
 * KnownAssignedXidsRemoveTree
 *              Remove xid (if it's not InvalidTransactionId) and all the subxids.
 *
 * Caller must hold ProcArrayLock in exclusive mode.
 */
static void
KnownAssignedXidsRemoveTree(TransactionId xid, int nsubxids,
                                                        TransactionId *subxids)
{
        int                     i;

        if (TransactionIdIsValid(xid))
                KnownAssignedXidsRemove(xid);

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

        /* Opportunistically compress the array */
        KnownAssignedXidsCompress(false);
}

/*
 * Prune KnownAssignedXids up to, but *not* including xid. If xid is invalid
 * then clear the whole table.
 *
 * Caller must hold ProcArrayLock in exclusive mode.
 */
static void
KnownAssignedXidsRemovePreceding(TransactionId removeXid)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;
        int                     count = 0;
        int                     head,
                                tail,
                                i;

        if (!TransactionIdIsValid(removeXid))
        {
                elog(trace_recovery(DEBUG4), "removing all KnownAssignedXids");
                pArray->numKnownAssignedXids = 0;
                pArray->headKnownAssignedXids = pArray->tailKnownAssignedXids = 0;
                return;
        }

        elog(trace_recovery(DEBUG4), "prune KnownAssignedXids to %u", removeXid);

        /*
         * Mark entries invalid starting at the tail.  Since array is sorted, we
         * can stop as soon as we reach an entry >= removeXid.
         */
        tail = pArray->tailKnownAssignedXids;
        head = pArray->headKnownAssignedXids;

        for (i = tail; i < head; i++)
        {
                if (KnownAssignedXidsValid[i])
                {
                        TransactionId knownXid = KnownAssignedXids[i];

                        if (TransactionIdFollowsOrEquals(knownXid, removeXid))
                                break;

                        if (!StandbyTransactionIdIsPrepared(knownXid))
                        {
                                KnownAssignedXidsValid[i] = false;
                                count++;
                        }
                }
        }

        pArray->numKnownAssignedXids -= count;
        Assert(pArray->numKnownAssignedXids >= 0);

        /*
         * Advance the tail pointer if we've marked the tail item invalid.
         */
        for (i = tail; i < head; i++)
        {
                if (KnownAssignedXidsValid[i])
                        break;
        }
        if (i >= head)
        {
                /* Array is empty, so we can reset both pointers */
                pArray->headKnownAssignedXids = 0;
                pArray->tailKnownAssignedXids = 0;
        }
        else
        {
                pArray->tailKnownAssignedXids = i;
        }

        /* Opportunistically compress the array */
        KnownAssignedXidsCompress(false);
}

/*
 * KnownAssignedXidsGet - Get an array of xids by scanning KnownAssignedXids.
 * We filter out anything >= xmax.
 *
 * Returns the number of XIDs stored into xarray[].  Caller is responsible
 * that array is large enough.
 *
 * Caller must hold ProcArrayLock in (at least) shared mode.
 */
static int
KnownAssignedXidsGet(TransactionId *xarray, TransactionId xmax)
{
        TransactionId xtmp = InvalidTransactionId;

        return KnownAssignedXidsGetAndSetXmin(xarray, &xtmp, xmax);
}

/*
 * KnownAssignedXidsGetAndSetXmin - as KnownAssignedXidsGet, plus
 * we reduce *xmin to the lowest xid value seen if not already lower.
 *
 * Caller must hold ProcArrayLock in (at least) shared mode.
 */
static int
KnownAssignedXidsGetAndSetXmin(TransactionId *xarray, TransactionId *xmin,
                                                           TransactionId xmax)
{
        int                     count = 0;
        int                     head,
                                tail;
        int                     i;

        /*
         * Fetch head just once, since it may change while we loop. We can stop
         * once we reach the initially seen head, since we are certain that an xid
         * cannot enter and then leave the array while we hold ProcArrayLock.  We
         * might miss newly-added xids, but they should be >= xmax so irrelevant
         * anyway.
         *
         * Must take spinlock to ensure we see up-to-date array contents.
         */
        SpinLockAcquire(&procArray->known_assigned_xids_lck);
        tail = procArray->tailKnownAssignedXids;
        head = procArray->headKnownAssignedXids;
        SpinLockRelease(&procArray->known_assigned_xids_lck);

        for (i = tail; i < head; i++)
        {
                /* Skip any gaps in the array */
                if (KnownAssignedXidsValid[i])
                {
                        TransactionId knownXid = KnownAssignedXids[i];

                        /*
                         * Update xmin if required.  Only the first XID need be checked,
                         * since the array is sorted.
                         */
                        if (count == 0 &&
                                TransactionIdPrecedes(knownXid, *xmin))
                                *xmin = knownXid;

                        /*
                         * Filter out anything >= xmax, again relying on sorted property
                         * of array.
                         */
                        if (TransactionIdIsValid(xmax) &&
                                TransactionIdFollowsOrEquals(knownXid, xmax))
                                break;

                        /* Add knownXid into output array */
                        xarray[count++] = knownXid;
                }
        }

        return count;
}

/*
 * Get oldest XID in the KnownAssignedXids array, or InvalidTransactionId
 * if nothing there.
 */
static TransactionId
KnownAssignedXidsGetOldestXmin(void)
{
        int                     head,
                                tail;
        int                     i;

        /*
         * Fetch head just once, since it may change while we loop.
         */
        SpinLockAcquire(&procArray->known_assigned_xids_lck);
        tail = procArray->tailKnownAssignedXids;
        head = procArray->headKnownAssignedXids;
        SpinLockRelease(&procArray->known_assigned_xids_lck);

        for (i = tail; i < head; i++)
        {
                /* Skip any gaps in the array */
                if (KnownAssignedXidsValid[i])
                        return KnownAssignedXids[i];
        }

        return InvalidTransactionId;
}

/*
 * Display KnownAssignedXids to provide debug trail
 *
 * Currently this is only called within startup process, so we need no
 * special locking.
 *
 * Note this is pretty expensive, and much of the expense will be incurred
 * even if the elog message will get discarded.  It's not currently called
 * in any performance-critical places, however, so no need to be tenser.
 */
static void
KnownAssignedXidsDisplay(int trace_level)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;
        StringInfoData buf;
        int                     head,
                                tail,
                                i;
        int                     nxids = 0;

        tail = pArray->tailKnownAssignedXids;
        head = pArray->headKnownAssignedXids;

        initStringInfo(&buf);

        for (i = tail; i < head; i++)
        {
                if (KnownAssignedXidsValid[i])
                {
                        nxids++;
                        appendStringInfo(&buf, "[%d]=%u ", i, KnownAssignedXids[i]);
                }
        }

        elog(trace_level, "%d KnownAssignedXids (num=%d tail=%d head=%d) %s",
                 nxids,
                 pArray->numKnownAssignedXids,
                 pArray->tailKnownAssignedXids,
                 pArray->headKnownAssignedXids,
                 buf.data);

        pfree(buf.data);
}

/*
 * KnownAssignedXidsReset
 *              Resets KnownAssignedXids to be empty
 */
static void
KnownAssignedXidsReset(void)
{
        /* use volatile pointer to prevent code rearrangement */
        volatile ProcArrayStruct *pArray = procArray;

        LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

        pArray->numKnownAssignedXids = 0;
        pArray->tailKnownAssignedXids = 0;
        pArray->headKnownAssignedXids = 0;

        LWLockRelease(ProcArrayLock);
}