procarray.c

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/*-------------------------------------------------------------------------
 *
 * procarray.c
 *    POSTGRES process array code.
 *
 *
 * This module maintains arrays of PGPROC substructures, as well as associated
 * arrays in ProcGlobal, 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 xid (in
 * ProcGlobal->xids[]/MyProc->xid).  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 on the primary (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 primary 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) 1996-2020, 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 "catalog/pg_authid.h"
#include "commands/dbcommands.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/spin.h"
#include "utils/acl.h"
#include "utils/builtins.h"
#include "utils/rel.h"
#include "utils/snapmgr.h"

#define UINT32_ACCESS_ONCE(var)      ((uint32)(*((volatile uint32 *)&(var))))

/* 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 PGPROC
     * 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 allProcs[], has PROCARRAY_MAXPROCS entries */
    int         pgprocnos[FLEXIBLE_ARRAY_MEMBER];
} ProcArrayStruct;

/*
 * State for the GlobalVisTest* family of functions. Those functions can
 * e.g. be used to decide if a deleted row can be removed without violating
 * MVCC semantics: If the deleted row's xmax is not considered to be running
 * by anyone, the row can be removed.
 *
 * To avoid slowing down GetSnapshotData(), we don't calculate a precise
 * cutoff XID while building a snapshot (looking at the frequently changing
 * xmins scales badly). Instead we compute two boundaries while building the
 * snapshot:
 *
 * 1) definitely_needed, indicating that rows deleted by XIDs >=
 *    definitely_needed are definitely still visible.
 *
 * 2) maybe_needed, indicating that rows deleted by XIDs < maybe_needed can
 *    definitely be removed
 *
 * When testing an XID that falls in between the two (i.e. XID >= maybe_needed
 * && XID < definitely_needed), the boundaries can be recomputed (using
 * ComputeXidHorizons()) to get a more accurate answer. This is cheaper than
 * maintaining an accurate value all the time.
 *
 * As it is not cheap to compute accurate boundaries, we limit the number of
 * times that happens in short succession. See GlobalVisTestShouldUpdate().
 *
 *
 * There are three backend lifetime instances of this struct, optimized for
 * different types of relations. As e.g. a normal user defined table in one
 * database is inaccessible to backends connected to another database, a test
 * specific to a relation can be more aggressive than a test for a shared
 * relation.  Currently we track three different states:
 *
 * 1) GlobalVisSharedRels, which only considers an XID's
 *    effects visible-to-everyone if neither snapshots in any database, nor a
 *    replication slot's xmin, nor a replication slot's catalog_xmin might
 *    still consider XID as running.
 *
 * 2) GlobalVisCatalogRels, which only considers an XID's
 *    effects visible-to-everyone if neither snapshots in the current
 *    database, nor a replication slot's xmin, nor a replication slot's
 *    catalog_xmin might still consider XID as running.
 *
 *    I.e. the difference to GlobalVisSharedRels is that
 *    snapshot in other databases are ignored.
 *
 * 3) GlobalVisDataRels, which only considers an XID's
 *    effects visible-to-everyone if neither snapshots in the current
 *    database, nor a replication slot's xmin consider XID as running.
 *
 *    I.e. the difference to GlobalVisCatalogRels is that
 *    replication slot's catalog_xmin is not taken into account.
 *
 * GlobalVisTestFor(relation) returns the appropriate state
 * for the relation.
 *
 * The boundaries are FullTransactionIds instead of TransactionIds to avoid
 * wraparound dangers. There e.g. would otherwise exist no procarray state to
 * prevent maybe_needed to become old enough after the GetSnapshotData()
 * call.
 *
 * The typedef is in the header.
 */
struct GlobalVisState
{
    /* XIDs >= are considered running by some backend */
    FullTransactionId definitely_needed;

    /* XIDs < are not considered to be running by any backend */
    FullTransactionId maybe_needed;
};

/*
 * Result of ComputeXidHorizons().
 */
typedef struct ComputeXidHorizonsResult
{
    /*
     * The value of ShmemVariableCache->latestCompletedXid when
     * ComputeXidHorizons() held ProcArrayLock.
     */
    FullTransactionId latest_completed;

    /*
     * The same for procArray->replication_slot_xmin and.
     * procArray->replication_slot_catalog_xmin.
     */
    TransactionId slot_xmin;
    TransactionId slot_catalog_xmin;

    /*
     * Oldest xid that any backend might still consider running. This needs to
     * include processes running VACUUM, in contrast to the normal visibility
     * cutoffs, as vacuum needs to be able to perform pg_subtrans lookups when
     * determining visibility, but doesn't care about rows above its xmin to
     * be removed.
     *
     * This likely should only be needed to determine whether pg_subtrans can
     * be truncated. It currently includes the effects of replication slots,
     * for historical reasons. But that could likely be changed.
     */
    TransactionId oldest_considered_running;

    /*
     * Oldest xid for which deleted tuples need to be retained in shared
     * tables.
     *
     * This includes the effects of replication slots. If that's not desired,
     * look at shared_oldest_nonremovable_raw;
     */
    TransactionId shared_oldest_nonremovable;

    /*
     * Oldest xid that may be necessary to retain in shared tables. This is
     * the same as shared_oldest_nonremovable, except that is not affected by
     * replication slot's catalog_xmin.
     *
     * This is mainly useful to be able to send the catalog_xmin to upstream
     * streaming replication servers via hot_standby_feedback, so they can
     * apply the limit only when accessing catalog tables.
     */
    TransactionId shared_oldest_nonremovable_raw;

    /*
     * Oldest xid for which deleted tuples need to be retained in non-shared
     * catalog tables.
     */
    TransactionId catalog_oldest_nonremovable;

    /*
     * Oldest xid for which deleted tuples need to be retained in normal user
     * defined tables.
     */
    TransactionId data_oldest_nonremovable;
} ComputeXidHorizonsResult;


static ProcArrayStruct *procArray;

static PGPROC *allProcs;

/*
 * 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;

/*
 * State for visibility checks on different types of relations. See struct
 * GlobalVisState for details. As shared, catalog, and user defined
 * relations can have different horizons, one such state exists for each.
 */
static GlobalVisState GlobalVisSharedRels;
static GlobalVisState GlobalVisCatalogRels;
static GlobalVisState GlobalVisDataRels;

/*
 * This backend's RecentXmin at the last time the accurate xmin horizon was
 * recomputed, or InvalidTransactionId if it has not. Used to limit how many
 * times accurate horizons are recomputed. See GlobalVisTestShouldUpdate().
 */
static TransactionId ComputeXidHorizonsResultLastXmin;

#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, TransactionId latestXid);
static void ProcArrayGroupClearXid(PGPROC *proc, TransactionId latestXid);
static void MaintainLatestCompletedXid(TransactionId latestXid);
static void MaintainLatestCompletedXidRecovery(TransactionId latestXid);

static inline FullTransactionId FullXidRelativeTo(FullTransactionId rel,
                                                  TransactionId xid);
static void GlobalVisUpdateApply(ComputeXidHorizonsResult *horizons);

/*
 * 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->maxKnownAssignedXids = TOTAL_MAX_CACHED_SUBXIDS;
        procArray->numKnownAssignedXids = 0;
        procArray->tailKnownAssignedXids = 0;
        procArray->headKnownAssignedXids = 0;
        SpinLockInit(&procArray->known_assigned_xids_lck);
        procArray->lastOverflowedXid = InvalidTransactionId;
        procArray->replication_slot_xmin = InvalidTransactionId;
        procArray->replication_slot_catalog_xmin = InvalidTransactionId;
        ShmemVariableCache->xactCompletionCount = 1;
    }

    allProcs = ProcGlobal->allProcs;

    /* 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);
    }
}

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

    /* See ProcGlobal comment explaining why both locks are held */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    LWLockAcquire(XidGenLock, 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.)
         */
        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(*arrayP->pgprocnos));
    memmove(&ProcGlobal->xids[index + 1], &ProcGlobal->xids[index],
            (arrayP->numProcs - index) * sizeof(*ProcGlobal->xids));
    memmove(&ProcGlobal->subxidStates[index + 1], &ProcGlobal->subxidStates[index],
            (arrayP->numProcs - index) * sizeof(*ProcGlobal->subxidStates));
    memmove(&ProcGlobal->vacuumFlags[index + 1], &ProcGlobal->vacuumFlags[index],
            (arrayP->numProcs - index) * sizeof(*ProcGlobal->vacuumFlags));

    arrayP->pgprocnos[index] = proc->pgprocno;
    ProcGlobal->xids[index] = proc->xid;
    ProcGlobal->subxidStates[index] = proc->subxidStatus;
    ProcGlobal->vacuumFlags[index] = proc->vacuumFlags;

    arrayP->numProcs++;

    for (; index < arrayP->numProcs; index++)
    {
        allProcs[arrayP->pgprocnos[index]].pgxactoff = index;
    }

    /*
     * Release in reversed acquisition order, to reduce frequency of having to
     * wait for XidGenLock while holding ProcArrayLock.
     */
    LWLockRelease(XidGenLock);
    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

    /* See ProcGlobal comment explaining why both locks are held */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
    LWLockAcquire(XidGenLock, LW_EXCLUSIVE);

    Assert(ProcGlobal->allProcs[arrayP->pgprocnos[proc->pgxactoff]].pgxactoff == proc->pgxactoff);

    if (TransactionIdIsValid(latestXid))
    {
        Assert(TransactionIdIsValid(ProcGlobal->xids[proc->pgxactoff]));

        /* Advance global latestCompletedXid while holding the lock */
        MaintainLatestCompletedXid(latestXid);

        /* Same with xactCompletionCount  */
        ShmemVariableCache->xactCompletionCount++;

        ProcGlobal->xids[proc->pgxactoff] = 0;
        ProcGlobal->subxidStates[proc->pgxactoff].overflowed = false;
        ProcGlobal->subxidStates[proc->pgxactoff].count = 0;
    }
    else
    {
        /* Shouldn't be trying to remove a live transaction here */
        Assert(!TransactionIdIsValid(ProcGlobal->xids[proc->pgxactoff]));
    }

    Assert(TransactionIdIsValid(ProcGlobal->xids[proc->pgxactoff] == 0));
    Assert(TransactionIdIsValid(ProcGlobal->subxidStates[proc->pgxactoff].count == 0));
    Assert(TransactionIdIsValid(ProcGlobal->subxidStates[proc->pgxactoff].overflowed == false));
    ProcGlobal->vacuumFlags[proc->pgxactoff] = 0;

    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(*arrayP->pgprocnos));
            memmove(&ProcGlobal->xids[index], &ProcGlobal->xids[index + 1],
                    (arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->xids));
            memmove(&ProcGlobal->subxidStates[index], &ProcGlobal->subxidStates[index + 1],
                    (arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->subxidStates));
            memmove(&ProcGlobal->vacuumFlags[index], &ProcGlobal->vacuumFlags[index + 1],
                    (arrayP->numProcs - index - 1) * sizeof(*ProcGlobal->vacuumFlags));

            arrayP->pgprocnos[arrayP->numProcs - 1] = -1;   /* for debugging */
            arrayP->numProcs--;

            /* adjust for removed PGPROC */
            for (; index < arrayP->numProcs; index++)
                allProcs[arrayP->pgprocnos[index]].pgxactoff--;

            /*
             * Release in reversed acquisition order, to reduce frequency of
             * having to wait for XidGenLock while holding ProcArrayLock.
             */
            LWLockRelease(XidGenLock);
            LWLockRelease(ProcArrayLock);
            return;
        }
    }

    /* Oops */
    LWLockRelease(XidGenLock);
    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)
{
    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(proc->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, 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(proc->xid));
        Assert(proc->subxidStatus.count == 0);
        Assert(!proc->subxidStatus.overflowed);

        proc->lxid = InvalidLocalTransactionId;
        proc->xmin = InvalidTransactionId;
        proc->delayChkpt = false;   /* be sure this is cleared in abort */
        proc->recoveryConflictPending = false;

        /* must be cleared with xid/xmin: */
        /* avoid unnecessarily dirtying shared cachelines */
        if (proc->vacuumFlags & PROC_VACUUM_STATE_MASK)
        {
            Assert(!LWLockHeldByMe(ProcArrayLock));
            LWLockAcquire(ProcArrayLock, LW_SHARED);
            Assert(proc->vacuumFlags == ProcGlobal->vacuumFlags[proc->pgxactoff]);
            proc->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
            ProcGlobal->vacuumFlags[proc->pgxactoff] = proc->vacuumFlags;
            LWLockRelease(ProcArrayLock);
        }
    }
}

/*
 * 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, TransactionId latestXid)
{
    size_t      pgxactoff = proc->pgxactoff;

    Assert(LWLockHeldByMe(ProcArrayLock));
    Assert(TransactionIdIsValid(ProcGlobal->xids[pgxactoff]));
    Assert(ProcGlobal->xids[pgxactoff] == proc->xid);

    ProcGlobal->xids[pgxactoff] = InvalidTransactionId;
    proc->xid = InvalidTransactionId;
    proc->lxid = InvalidLocalTransactionId;
    proc->xmin = InvalidTransactionId;
    proc->delayChkpt = false;   /* be sure this is cleared in abort */
    proc->recoveryConflictPending = false;

    /* must be cleared with xid/xmin: */
    /* avoid unnecessarily dirtying shared cachelines */
    if (proc->vacuumFlags & PROC_VACUUM_STATE_MASK)
    {
        proc->vacuumFlags &= ~PROC_VACUUM_STATE_MASK;
        ProcGlobal->vacuumFlags[proc->pgxactoff] = proc->vacuumFlags;
    }

    /* Clear the subtransaction-XID cache too while holding the lock */
    Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
           ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
    if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
    {
        ProcGlobal->subxidStates[pgxactoff].count = 0;
        ProcGlobal->subxidStates[pgxactoff].overflowed = false;
        proc->subxidStatus.count = 0;
        proc->subxidStatus.overflowed = false;
    }

    /* Also advance global latestCompletedXid while holding the lock */
    MaintainLatestCompletedXid(latestXid);

    /* Same with xactCompletionCount  */
    ShmemVariableCache->xactCompletionCount++;
}

/*
 * 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)
{
    PROC_HDR   *procglobal = ProcGlobal;
    uint32      nextidx;
    uint32      wakeidx;

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

    /* Add ourselves to the list of processes needing a group XID clear. */
    proc->procArrayGroupMember = true;
    proc->procArrayGroupMemberXid = latestXid;
    nextidx = pg_atomic_read_u32(&procglobal->procArrayGroupFirst);
    while (true)
    {
        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.
     */
    nextidx = pg_atomic_exchange_u32(&procglobal->procArrayGroupFirst,
                                     INVALID_PGPROCNO);

    /* 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];

        ProcArrayEndTransactionInternal(proc, 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 PGPROC.
 */
void
ProcArrayClearTransaction(PGPROC *proc)
{
    size_t      pgxactoff;

    /*
     * Currently we need to lock ProcArrayLock exclusively here, as we
     * increment xactCompletionCount below. We also need it at least in shared
     * mode for pgproc->pgxactoff to stay the same below.
     *
     * We could however, as 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), lower the lock level to
     * shared if we were to make xactCompletionCount an atomic variable. But
     * that doesn't seem worth it currently, as a 2PC commit is heavyweight
     * enough for this not to be the bottleneck.  If it ever becomes a
     * bottleneck it may also be worth considering to combine this with the
     * subsequent ProcArrayRemove()
     */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

    pgxactoff = proc->pgxactoff;

    ProcGlobal->xids[pgxactoff] = InvalidTransactionId;
    proc->xid = InvalidTransactionId;

    proc->lxid = InvalidLocalTransactionId;
    proc->xmin = InvalidTransactionId;
    proc->recoveryConflictPending = false;

    Assert(!(proc->vacuumFlags & PROC_VACUUM_STATE_MASK));
    Assert(!proc->delayChkpt);

    /*
     * Need to increment completion count even though transaction hasn't
     * really committed yet. The reason for that is that GetSnapshotData()
     * omits the xid of the current transaction, thus without the increment we
     * otherwise could end up reusing the snapshot later. Which would be bad,
     * because it might not count the prepared transaction as running.
     */
    ShmemVariableCache->xactCompletionCount++;

    /* Clear the subtransaction-XID cache too */
    Assert(ProcGlobal->subxidStates[pgxactoff].count == proc->subxidStatus.count &&
           ProcGlobal->subxidStates[pgxactoff].overflowed == proc->subxidStatus.overflowed);
    if (proc->subxidStatus.count > 0 || proc->subxidStatus.overflowed)
    {
        ProcGlobal->subxidStates[pgxactoff].count = 0;
        ProcGlobal->subxidStates[pgxactoff].overflowed = false;
        proc->subxidStatus.count = 0;
        proc->subxidStatus.overflowed = false;
    }

    LWLockRelease(ProcArrayLock);
}

/*
 * Update ShmemVariableCache->latestCompletedXid to point to latestXid if
 * currently older.
 */
static void
MaintainLatestCompletedXid(TransactionId latestXid)
{
    FullTransactionId cur_latest = ShmemVariableCache->latestCompletedXid;

    Assert(FullTransactionIdIsValid(cur_latest));
    Assert(!RecoveryInProgress());
    Assert(LWLockHeldByMe(ProcArrayLock));

    if (TransactionIdPrecedes(XidFromFullTransactionId(cur_latest), latestXid))
    {
        ShmemVariableCache->latestCompletedXid =
            FullXidRelativeTo(cur_latest, latestXid);
    }

    Assert(IsBootstrapProcessingMode() ||
           FullTransactionIdIsNormal(ShmemVariableCache->latestCompletedXid));
}

/*
 * Same as MaintainLatestCompletedXid, except for use during WAL replay.
 */
static void
MaintainLatestCompletedXidRecovery(TransactionId latestXid)
{
    FullTransactionId cur_latest = ShmemVariableCache->latestCompletedXid;
    FullTransactionId rel;

    Assert(AmStartupProcess() || !IsUnderPostmaster);
    Assert(LWLockHeldByMe(ProcArrayLock));

    /*
     * Need a FullTransactionId to compare latestXid with. Can't rely on
     * latestCompletedXid to be initialized in recovery. But in recovery it's
     * safe to access nextXid without a lock for the startup process.
     */
    rel = ShmemVariableCache->nextXid;
    Assert(FullTransactionIdIsValid(ShmemVariableCache->nextXid));

    if (!FullTransactionIdIsValid(cur_latest) ||
        TransactionIdPrecedes(XidFromFullTransactionId(cur_latest), latestXid))
    {
        ShmemVariableCache->latestCompletedXid =
            FullXidRelativeTo(rel, latestXid);
    }

    Assert(FullTransactionIdIsNormal(ShmemVariableCache->latestCompletedXid));
}

/*
 * 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 the primary 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;
    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.
     */
    StandbyReleaseOldLocks(running->oldestRunningXid);

    /*
     * 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);

    /*
     * 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 RunningTransactionsData 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.  The running-xacts
         * snapshot may include duplicated xids because of prepared
         * transactions, so ignore them.
         */
        for (i = 0; i < nxids; i++)
        {
            if (i > 0 && TransactionIdEquals(xids[i - 1], xids[i]))
            {
                elog(DEBUG1,
                     "found duplicated transaction %u for KnownAssignedXids insertion",
                     xids[i]);
                continue;
            }
            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 (cf. 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.
     * It also might not yet be set at all.
     */
    MaintainLatestCompletedXidRecovery(running->latestCompletedXid);

    LWLockRelease(ProcArrayLock);

    /* ShmemVariableCache->nextXid must be beyond any observed xid. */
    AdvanceNextFullTransactionIdPastXid(latestObservedXid);

    Assert(FullTransactionIdIsValid(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 ProcGlobal->xids.
 *
 * 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 primary.
 *
 * 4. Search the SubTrans tree to find the Xid's topmost parent, and then see
 * if that is running according to ProcGlobal->xids[] 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
 * ProcGlobal->xids[] again anyway; see GetNewTransactionId).
 */
bool
TransactionIdIsInProgress(TransactionId xid)
{
    static TransactionId *xids = NULL;
    static TransactionId *other_xids;
    XidCacheStatus *other_subxidstates;
    int         nxids = 0;
    ProcArrayStruct *arrayP = procArray;
    TransactionId topxid;
    TransactionId latestCompletedXid;
    int         mypgxactoff;
    size_t      numProcs;
    int         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")));
    }

    other_xids = ProcGlobal->xids;
    other_subxidstates = ProcGlobal->subxidStates;

    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.
     */
    latestCompletedXid =
        XidFromFullTransactionId(ShmemVariableCache->latestCompletedXid);
    if (TransactionIdPrecedes(latestCompletedXid, xid))
    {
        LWLockRelease(ProcArrayLock);
        xc_by_latest_xid_inc();
        return true;
    }

    /* No shortcuts, gotta grovel through the array */
    mypgxactoff = MyProc->pgxactoff;
    numProcs = arrayP->numProcs;
    for (size_t pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
    {
        int         pgprocno;
        PGPROC     *proc;
        TransactionId pxid;
        int         pxids;

        /* Ignore ourselves --- dealt with it above */
        if (pgxactoff == mypgxactoff)
            continue;

        /* Fetch xid just once - see GetNewTransactionId */
        pxid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);

        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
         */
        pxids = other_subxidstates[pgxactoff].count;
        pg_read_barrier();      /* pairs with barrier in GetNewTransactionId() */
        pgprocno = arrayP->pgprocnos[pgxactoff];
        proc = &allProcs[pgprocno];
        for (j = pxids - 1; j >= 0; j--)
        {
            /* Fetch xid just once - see GetNewTransactionId */
            TransactionId cxid = UINT32_ACCESS_ONCE(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 (other_subxidstates[pgxactoff].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 PGPROC 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 (int 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 primary 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;
    TransactionId *other_xids = ProcGlobal->xids;
    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];
        PGPROC     *proc = &allProcs[pgprocno];
        TransactionId pxid;

        /* Fetch xid just once - see GetNewTransactionId */
        pxid = UINT32_ACCESS_ONCE(other_xids[i]);

        if (!TransactionIdIsValid(pxid))
            continue;

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

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

    LWLockRelease(ProcArrayLock);

    return result;
}


/*
 * Determine XID horizons.
 *
 * This is used by wrapper functions like GetOldestNonRemovableTransactionId()
 * (for VACUUM), GetReplicationHorizons() (for hot_standby_feedback), etc as
 * well as "internally" by GlobalVisUpdate() (see comment above struct
 * GlobalVisState).
 *
 * See the definition of ComputedXidHorizonsResult for the various computed
 * horizons.
 *
 * For VACUUM separate horizons (used to to decide which deleted tuples must
 * be preserved), for shared and non-shared tables are computed.  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 also computes a horizon used to truncate pg_subtrans. For that
 * backends in all databases have to be considered, and concurrently running
 * lazy VACUUMs cannot be ignored, as they still may perform pg_subtrans
 * accesses.
 *
 * 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 values to move
 * backwards on repeated calls. The calculated values are conservative, so
 * that anything older is definitely not considered as running by anyone
 * anymore, but the exact values calculated depend on a number of things. For
 * example, if there are no transactions running in the current database, the
 * horizon for normal tables will be 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
 * on the primary but are still being replayed on the standby, thus possibly
 * making the values 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 computed values
 * are also adjusted with vacuum_defer_cleanup_age, so increasing that setting
 * on the fly is another easy way to make horizons move backwards, with no
 * consequences for data integrity.
 *
 * Note: the approximate horizons (see definition of GlobalVisState) are
 * updated by the computations done here. That's currently required for
 * correctness and a small optimization. Without doing so it's possible that
 * heap vacuum's call to heap_page_prune() uses a more conservative horizon
 * than later when deciding which tuples can be removed - which the code
 * doesn't expect (breaking HOT).
 */
static void
ComputeXidHorizons(ComputeXidHorizonsResult *h)
{
    ProcArrayStruct *arrayP = procArray;
    TransactionId kaxmin;
    bool        in_recovery = RecoveryInProgress();
    TransactionId *other_xids = ProcGlobal->xids;

    /* inferred after ProcArrayLock is released */
    h->catalog_oldest_nonremovable = InvalidTransactionId;

    LWLockAcquire(ProcArrayLock, LW_SHARED);

    h->latest_completed = ShmemVariableCache->latestCompletedXid;

    /*
     * 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.
     */
    {
        TransactionId initial;

        initial = XidFromFullTransactionId(h->latest_completed);
        Assert(TransactionIdIsValid(initial));
        TransactionIdAdvance(initial);

        h->oldest_considered_running = initial;
        h->shared_oldest_nonremovable = initial;
        h->data_oldest_nonremovable = initial;
    }

    /*
     * Fetch slot horizons while ProcArrayLock is held - the
     * LWLockAcquire/LWLockRelease are a barrier, ensuring this happens inside
     * the lock.
     */
    h->slot_xmin = procArray->replication_slot_xmin;
    h->slot_catalog_xmin = procArray->replication_slot_catalog_xmin;

    for (int index = 0; index < arrayP->numProcs; index++)
    {
        int         pgprocno = arrayP->pgprocnos[index];
        PGPROC     *proc = &allProcs[pgprocno];
        int8        vacuumFlags = ProcGlobal->vacuumFlags[index];
        TransactionId xid;
        TransactionId xmin;

        /* Fetch xid just once - see GetNewTransactionId */
        xid = UINT32_ACCESS_ONCE(other_xids[index]);
        xmin = UINT32_ACCESS_ONCE(proc->xmin);

        /*
         * Consider both the transaction's Xmin, and its Xid.
         *
         * We must check both 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.
         */
        xmin = TransactionIdOlder(xmin, xid);

        /* if neither is set, this proc doesn't influence the horizon */
        if (!TransactionIdIsValid(xmin))
            continue;

        /*
         * Don't ignore any procs when determining which transactions might be
         * considered running.  While slots should ensure logical decoding
         * backends are protected even without this check, it can't hurt to
         * include them here as well..
         */
        h->oldest_considered_running =
            TransactionIdOlder(h->oldest_considered_running, xmin);

        /*
         * Skip over backends either vacuuming (which is ok with rows being
         * removed, as long as pg_subtrans is not truncated) or doing logical
         * decoding (which manages xmin separately, check below).
         */
        if (vacuumFlags & (PROC_IN_VACUUM | PROC_IN_LOGICAL_DECODING))
            continue;

        /* shared tables need to take backends in all database into account */
        h->shared_oldest_nonremovable =
            TransactionIdOlder(h->shared_oldest_nonremovable, xmin);

        /*
         * Normally queries in other databases are ignored for anything but
         * the shared horizon. But in recovery we cannot compute an accurate
         * per-database horizon as all xids are managed via the
         * KnownAssignedXids machinery.
         */
        if (in_recovery ||
            proc->databaseId == MyDatabaseId ||
            proc->databaseId == 0)  /* always include WalSender */
        {
            h->data_oldest_nonremovable =
                TransactionIdOlder(h->data_oldest_nonremovable, xmin);
        }
    }

    /*
     * If in recovery fetch oldest xid in KnownAssignedXids, will be applied
     * after lock is released.
     */
    if (in_recovery)
        kaxmin = KnownAssignedXidsGetOldestXmin();

    /*
     * No other information from shared state is needed, release the lock
     * immediately. The rest of the computations can be done without a lock.
     */
    LWLockRelease(ProcArrayLock);

    if (in_recovery)
    {
        h->oldest_considered_running =
            TransactionIdOlder(h->oldest_considered_running, kaxmin);
        h->shared_oldest_nonremovable =
            TransactionIdOlder(h->shared_oldest_nonremovable, kaxmin);
        h->data_oldest_nonremovable =
            TransactionIdOlder(h->data_oldest_nonremovable, kaxmin);
    }
    else
    {
        /*
         * Compute the cutoff XID by subtracting vacuum_defer_cleanup_age.
         *
         * 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 primary 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.
         */
        h->oldest_considered_running =
            TransactionIdRetreatedBy(h->oldest_considered_running,
                                     vacuum_defer_cleanup_age);
        h->shared_oldest_nonremovable =
            TransactionIdRetreatedBy(h->shared_oldest_nonremovable,
                                     vacuum_defer_cleanup_age);
        h->data_oldest_nonremovable =
            TransactionIdRetreatedBy(h->data_oldest_nonremovable,
                                     vacuum_defer_cleanup_age);
    }

    /*
     * Check whether there are replication slots requiring an older xmin.
     */
    h->shared_oldest_nonremovable =
        TransactionIdOlder(h->shared_oldest_nonremovable, h->slot_xmin);
    h->data_oldest_nonremovable =
        TransactionIdOlder(h->data_oldest_nonremovable, h->slot_xmin);

    /*
     * The only difference between catalog / data horizons is that the slot's
     * catalog xmin is applied to the catalog one (so catalogs can be accessed
     * for logical decoding). Initialize with data horizon, and then back up
     * further if necessary. Have to back up the shared horizon as well, since
     * that also can contain catalogs.
     */
    h->shared_oldest_nonremovable_raw = h->shared_oldest_nonremovable;
    h->shared_oldest_nonremovable =
        TransactionIdOlder(h->shared_oldest_nonremovable,
                           h->slot_catalog_xmin);
    h->catalog_oldest_nonremovable = h->data_oldest_nonremovable;
    h->catalog_oldest_nonremovable =
        TransactionIdOlder(h->catalog_oldest_nonremovable,
                           h->slot_catalog_xmin);

    /*
     * It's possible that slots / vacuum_defer_cleanup_age backed up the
     * horizons further than oldest_considered_running. Fix.
     */
    h->oldest_considered_running =
        TransactionIdOlder(h->oldest_considered_running,
                           h->shared_oldest_nonremovable);
    h->oldest_considered_running =
        TransactionIdOlder(h->oldest_considered_running,
                           h->catalog_oldest_nonremovable);
    h->oldest_considered_running =
        TransactionIdOlder(h->oldest_considered_running,
                           h->data_oldest_nonremovable);

    /*
     * shared horizons have to be at least as old as the oldest visible in
     * current db
     */
    Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
                                         h->data_oldest_nonremovable));
    Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
                                         h->catalog_oldest_nonremovable));

    /*
     * Horizons need to ensure that pg_subtrans access is still possible for
     * the relevant backends.
     */
    Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
                                         h->shared_oldest_nonremovable));
    Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
                                         h->catalog_oldest_nonremovable));
    Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
                                         h->data_oldest_nonremovable));
    Assert(!TransactionIdIsValid(h->slot_xmin) ||
           TransactionIdPrecedesOrEquals(h->oldest_considered_running,
                                         h->slot_xmin));
    Assert(!TransactionIdIsValid(h->slot_catalog_xmin) ||
           TransactionIdPrecedesOrEquals(h->oldest_considered_running,
                                         h->slot_catalog_xmin));

    /* update approximate horizons with the computed horizons */
    GlobalVisUpdateApply(h);
}

/*
 * Return the oldest XID for which deleted tuples must be preserved in the
 * passed table.
 *
 * If rel is not NULL the horizon may be considerably more recent than
 * otherwise (i.e. fewer tuples will be removable). In the NULL case a horizon
 * that is correct (but not optimal) for all relations will be returned.
 *
 * This is used by VACUUM to decide which deleted tuples must be preserved in
 * the passed in table.
 */
TransactionId
GetOldestNonRemovableTransactionId(Relation rel)
{
    ComputeXidHorizonsResult horizons;

    ComputeXidHorizons(&horizons);

    /* select horizon appropriate for relation */
    if (rel == NULL || rel->rd_rel->relisshared)
        return horizons.shared_oldest_nonremovable;
    else if (RelationIsAccessibleInLogicalDecoding(rel))
        return horizons.catalog_oldest_nonremovable;
    else
        return horizons.data_oldest_nonremovable;
}

/*
 * Return the oldest transaction id any currently running backend might still
 * consider running. This should not be used for visibility / pruning
 * determinations (see GetOldestNonRemovableTransactionId()), but for
 * decisions like up to where pg_subtrans can be truncated.
 */
TransactionId
GetOldestTransactionIdConsideredRunning(void)
{
    ComputeXidHorizonsResult horizons;

    ComputeXidHorizons(&horizons);

    return horizons.oldest_considered_running;
}

/*
 * Return the visibility horizons for a hot standby feedback message.
 */
void
GetReplicationHorizons(TransactionId *xmin, TransactionId *catalog_xmin)
{
    ComputeXidHorizonsResult horizons;

    ComputeXidHorizons(&horizons);

    /*
     * Don't want to use shared_oldest_nonremovable here, as that contains the
     * effect of replication slot's catalog_xmin. We want to send a separate
     * feedback for the catalog horizon, so the primary can remove data table
     * contents more aggressively.
     */
    *xmin = horizons.shared_oldest_nonremovable_raw;
    *catalog_xmin = horizons.slot_catalog_xmin;
}

/*
 * 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;
}

/*
 * Initialize old_snapshot_threshold specific parts of a newly build snapshot.
 */
static void
GetSnapshotDataInitOldSnapshot(Snapshot snapshot)
{
    if (!OldSnapshotThresholdActive())
    {
        /*
         * 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, snapshot->xmin);
    }
}

/*
 * Helper function for GetSnapshotData() that checks if the bulk of the
 * visibility information in the snapshot is still valid. If so, it updates
 * the fields that need to change and returns true. Otherwise it returns
 * false.
 *
 * This very likely can be evolved to not need ProcArrayLock held (at very
 * least in the case we already hold a snapshot), but that's for another day.
 */
static bool
GetSnapshotDataReuse(Snapshot snapshot)
{
    uint64 curXactCompletionCount;

    Assert(LWLockHeldByMe(ProcArrayLock));

    if (unlikely(snapshot->snapXactCompletionCount == 0))
        return false;

    curXactCompletionCount = ShmemVariableCache->xactCompletionCount;
    if (curXactCompletionCount != snapshot->snapXactCompletionCount)
        return false;

    /*
     * If the current xactCompletionCount is still the same as it was at the
     * time the snapshot was built, we can be sure that rebuilding the
     * contents of the snapshot the hard way would result in the same snapshot
     * contents:
     *
     * As explained in transam/README, the set of xids considered running by
     * GetSnapshotData() cannot change while ProcArrayLock is held. Snapshot
     * contents only depend on transactions with xids and xactCompletionCount
     * is incremented whenever a transaction with an xid finishes (while
     * holding ProcArrayLock) exclusively). Thus the xactCompletionCount check
     * ensures we would detect if the snapshot would have changed.
     *
     * As the snapshot contents are the same as it was before, it is is safe
     * to re-enter the snapshot's xmin into the PGPROC array. None of the rows
     * visible under the snapshot could already have been removed (that'd
     * require the set of running transactions to change) and it fulfills the
     * requirement that concurrent GetSnapshotData() calls yield the same
     * xmin.
     */
    if (!TransactionIdIsValid(MyProc->xmin))
        MyProc->xmin = TransactionXmin = snapshot->xmin;

    RecentXmin = snapshot->xmin;
    Assert(TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin));

    snapshot->curcid = GetCurrentCommandId(false);
    snapshot->active_count = 0;
    snapshot->regd_count = 0;
    snapshot->copied = false;

    GetSnapshotDataInitOldSnapshot(snapshot);

    return true;
}

/*
 * 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 heapam_visibility.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 MyProc->xmin).
 *      RecentXmin: the xmin computed for the most recent snapshot.  XIDs
 *          older than this are known not running any more.
 *
 * And try to advance the bounds of GlobalVisSharedRels, GlobalVisCatalogRels,
 * GlobalVisDataRels for the benefit of theGlobalVisTest* family of functions.
 *
 * 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 *other_xids = ProcGlobal->xids;
    TransactionId xmin;
    TransactionId xmax;
    size_t      count = 0;
    int         subcount = 0;
    bool        suboverflowed = false;
    FullTransactionId latest_completed;
    TransactionId oldestxid;
    int         mypgxactoff;
    TransactionId myxid;
    uint64      curXactCompletionCount;

    TransactionId replication_slot_xmin = InvalidTransactionId;
    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 MyProc->xmin.
     */
    LWLockAcquire(ProcArrayLock, LW_SHARED);

    if (GetSnapshotDataReuse(snapshot))
    {
        LWLockRelease(ProcArrayLock);
        return snapshot;
    }

    latest_completed = ShmemVariableCache->latestCompletedXid;
    mypgxactoff = MyProc->pgxactoff;
    myxid = other_xids[mypgxactoff];
    Assert(myxid == MyProc->xid);

    oldestxid = ShmemVariableCache->oldestXid;
    curXactCompletionCount = ShmemVariableCache->xactCompletionCount;

    /* xmax is always latestCompletedXid + 1 */
    xmax = XidFromFullTransactionId(latest_completed);
    TransactionIdAdvance(xmax);
    Assert(TransactionIdIsNormal(xmax));

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

    /* take own xid into account, saves a check inside the loop */
    if (TransactionIdIsNormal(myxid) && NormalTransactionIdPrecedes(myxid, xmin))
        xmin = myxid;

    snapshot->takenDuringRecovery = RecoveryInProgress();

    if (!snapshot->takenDuringRecovery)
    {
        size_t      numProcs = arrayP->numProcs;
        TransactionId *xip = snapshot->xip;
        int        *pgprocnos = arrayP->pgprocnos;
        XidCacheStatus *subxidStates = ProcGlobal->subxidStates;
        uint8      *allVacuumFlags = ProcGlobal->vacuumFlags;

        /*
         * First collect set of pgxactoff/xids that need to be included in the
         * snapshot.
         */
        for (size_t pgxactoff = 0; pgxactoff < numProcs; pgxactoff++)
        {
            /* Fetch xid just once - see GetNewTransactionId */
            TransactionId xid = UINT32_ACCESS_ONCE(other_xids[pgxactoff]);
            uint8       vacuumFlags;

            Assert(allProcs[arrayP->pgprocnos[pgxactoff]].pgxactoff == pgxactoff);

            /*
             * If the transaction has no XID assigned, we can skip it; it
             * won't have sub-XIDs either.
             */
            if (likely(xid == InvalidTransactionId))
                continue;

            /*
             * We don't include our own XIDs (if any) in the snapshot. It
             * needs to be includeded in the xmin computation, but we did so
             * outside the loop.
             */
            if (pgxactoff == mypgxactoff)
                continue;

            /*
             * The only way we are able to get here with a non-normal xid
             * is during bootstrap - with this backend using
             * BootstrapTransactionId. But the above test should filter
             * that out.
             */
            Assert(TransactionIdIsNormal(xid));

            /*
             * If the XID is >= xmax, we can skip it; such transactions will
             * be treated as running anyway (and any sub-XIDs will also be >=
             * xmax).
             */
            if (!NormalTransactionIdPrecedes(xid, xmax))
                continue;

            /*
             * Skip over backends doing logical decoding which manages xmin
             * separately (check below) and ones running LAZY VACUUM.
             */
            vacuumFlags = allVacuumFlags[pgxactoff];
            if (vacuumFlags & (PROC_IN_LOGICAL_DECODING | PROC_IN_VACUUM))
                continue;

            if (NormalTransactionIdPrecedes(xid, xmin))
                xmin = xid;

            /* Add XID to 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 (subxidStates[pgxactoff].overflowed)
                    suboverflowed = true;
                else
                {
                    int         nsubxids = subxidStates[pgxactoff].count;

                    if (nsubxids > 0)
                    {
                        int         pgprocno = pgprocnos[pgxactoff];
                        PGPROC     *proc = &allProcs[pgprocno];

                        pg_read_barrier();  /* pairs with GetNewTransactionId */

                        memcpy(snapshot->subxip + subcount,
                               (void *) proc->subxids.xids,
                               nsubxids * sizeof(TransactionId));
                        subcount += nsubxids;
                    }
                }
            }
        }
    }
    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 local variable while ProcArrayLock is held - the
     * LWLockRelease below is a barrier, ensuring this happens inside the
     * lock.
     */
    replication_slot_xmin = procArray->replication_slot_xmin;
    replication_slot_catalog_xmin = procArray->replication_slot_catalog_xmin;

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

    LWLockRelease(ProcArrayLock);

    /* maintain state for GlobalVis* */
    {
        TransactionId def_vis_xid;
        TransactionId def_vis_xid_data;
        FullTransactionId def_vis_fxid;
        FullTransactionId def_vis_fxid_data;
        FullTransactionId oldestfxid;

        /*
         * Converting oldestXid is only safe when xid horizon cannot advance,
         * i.e. holding locks. While we don't hold the lock anymore, all the
         * necessary data has been gathered with lock held.
         */
        oldestfxid = FullXidRelativeTo(latest_completed, oldestxid);

        /* apply vacuum_defer_cleanup_age */
        def_vis_xid_data =
            TransactionIdRetreatedBy(xmin, vacuum_defer_cleanup_age);

        /* Check whether there's a replication slot requiring an older xmin. */
        def_vis_xid_data =
            TransactionIdOlder(def_vis_xid_data, replication_slot_xmin);

        /*
         * Rows in non-shared, non-catalog tables possibly could be vacuumed
         * if older than this xid.
         */
        def_vis_xid = def_vis_xid_data;

        /*
         * Check whether there's a replication slot requiring an older catalog
         * xmin.
         */
        def_vis_xid =
            TransactionIdOlder(replication_slot_catalog_xmin, def_vis_xid);

        def_vis_fxid = FullXidRelativeTo(latest_completed, def_vis_xid);
        def_vis_fxid_data = FullXidRelativeTo(latest_completed, def_vis_xid_data);

        /*
         * Check if we can increase upper bound. As a previous
         * GlobalVisUpdate() might have computed more aggressive values, don't
         * overwrite them if so.
         */
        GlobalVisSharedRels.definitely_needed =
            FullTransactionIdNewer(def_vis_fxid,
                                   GlobalVisSharedRels.definitely_needed);
        GlobalVisCatalogRels.definitely_needed =
            FullTransactionIdNewer(def_vis_fxid,
                                   GlobalVisCatalogRels.definitely_needed);
        GlobalVisDataRels.definitely_needed =
            FullTransactionIdNewer(def_vis_fxid_data,
                                   GlobalVisDataRels.definitely_needed);

        /*
         * Check if we know that we can initialize or increase the lower
         * bound. Currently the only cheap way to do so is to use
         * ShmemVariableCache->oldestXid as input.
         *
         * We should definitely be able to do better. We could e.g. put a
         * global lower bound value into ShmemVariableCache.
         */
        GlobalVisSharedRels.maybe_needed =
            FullTransactionIdNewer(GlobalVisSharedRels.maybe_needed,
                                   oldestfxid);
        GlobalVisCatalogRels.maybe_needed =
            FullTransactionIdNewer(GlobalVisCatalogRels.maybe_needed,
                                   oldestfxid);
        GlobalVisDataRels.maybe_needed =
            FullTransactionIdNewer(GlobalVisDataRels.maybe_needed,
                                   oldestfxid);
    }

    RecentXmin = xmin;
    Assert(TransactionIdPrecedesOrEquals(TransactionXmin, RecentXmin));

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

    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;

    GetSnapshotDataInitOldSnapshot(snapshot);

    return snapshot;
}

/*
 * ProcArrayInstallImportedXmin -- install imported xmin into MyProc->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];
        PGPROC     *proc = &allProcs[pgprocno];
        int         vacuumFlags = ProcGlobal->vacuumFlags[index];
        TransactionId xid;

        /* Ignore procs running LAZY VACUUM */
        if (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 = UINT32_ACCESS_ONCE(proc->xmin);
        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.)
         */
        MyProc->xmin = TransactionXmin = xmin;

        result = true;
        break;
    }

    LWLockRelease(ProcArrayLock);

    return result;
}

/*
 * ProcArrayInstallRestoredXmin -- install restored xmin into MyProc->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;

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

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

    /*
     * 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 = UINT32_ACCESS_ONCE(proc->xmin);
    if (proc->databaseId == MyDatabaseId &&
        TransactionIdIsNormal(xid) &&
        TransactionIdPrecedesOrEquals(xid, xmin))
    {
        MyProc->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 PGPROCs with an assigned TransactionId, even VACUUM processes and
 * prepared transactions.
 *
 * 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.
 *
 * Dummy PGPROCs from prepared transaction are included, meaning that this
 * may return entries with duplicated TransactionId values coming from
 * transaction finishing to prepare.  Nothing is done about duplicated
 * entries here to not hold on ProcArrayLock more than necessary.
 *
 * 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.
 */
RunningTransactions
GetRunningTransactionData(void)
{
    /* result workspace */
    static RunningTransactionsData CurrentRunningXactsData;

    ProcArrayStruct *arrayP = procArray;
    TransactionId *other_xids = ProcGlobal->xids;
    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 =
        XidFromFullTransactionId(ShmemVariableCache->latestCompletedXid);
    oldestRunningXid =
        XidFromFullTransactionId(ShmemVariableCache->nextXid);

    /*
     * Spin over procArray collecting all xids
     */
    for (index = 0; index < arrayP->numProcs; index++)
    {
        TransactionId xid;

        /* Fetch xid just once - see GetNewTransactionId */
        xid = UINT32_ACCESS_ONCE(other_xids[index]);

        /*
         * 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;

        /*
         * Be careful not to exclude any xids before calculating the values of
         * oldestRunningXid and suboverflowed, since these are used to clean
         * up transaction information held on standbys.
         */
        if (TransactionIdPrecedes(xid, oldestRunningXid))
            oldestRunningXid = xid;

        if (ProcGlobal->subxidStates[index].overflowed)
            suboverflowed = true;

        /*
         * If we wished to exclude xids this would be the right place for it.
         * Procs with the PROC_IN_VACUUM flag set don't usually assign xids,
         * but they do during truncation at the end when they get the lock and
         * truncate, so it is not much of a problem to include them if they
         * are seen and it is cleaner to include them.
         */

        xids[count++] = xid;
    }

    /*
     * Spin over procArray collecting all subxids, but only if there hasn't
     * been a suboverflow.
     */
    if (!suboverflowed)
    {
        XidCacheStatus *other_subxidstates = ProcGlobal->subxidStates;

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

            /*
             * Save subtransaction XIDs. Other backends can't add or remove
             * entries while we're holding XidGenLock.
             */
            nsubxids = other_subxidstates[index].count;
            if (nsubxids > 0)
            {
                /* barrier not really required, as XidGenLock is held, but ... */
                pg_read_barrier();  /* pairs with GetNewTransactionId */

                memcpy(&xids[count], (void *) proc->subxids.xids,
                       nsubxids * sizeof(TransactionId));
                count += nsubxids;
                subcount += nsubxids;

                /*
                 * 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 = XidFromFullTransactionId(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 PGPROCs 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 *other_xids = ProcGlobal->xids;
    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 = XidFromFullTransactionId(ShmemVariableCache->nextXid);
    LWLockRelease(XidGenLock);

    /*
     * Spin over procArray collecting all xids and subxids.
     */
    LWLockAcquire(ProcArrayLock, LW_SHARED);
    for (index = 0; index < arrayP->numProcs; index++)
    {
        TransactionId xid;

        /* Fetch xid just once - see GetNewTransactionId */
        xid = UINT32_ACCESS_ONCE(other_xids[index]);

        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 = XidFromFullTransactionId(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 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
     * ProcGlobal->xids[i] 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)
    {
        TransactionId *other_xids = ProcGlobal->xids;

        /*
         * Spin over procArray collecting min(ProcGlobal->xids[i])
         */
        for (index = 0; index < arrayP->numProcs; index++)
        {
            TransactionId xid;

            /* Fetch xid just once - see GetNewTransactionId */
            xid = UINT32_ACCESS_ONCE(other_xids[index]);

            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 PGPROC.
 *
 * 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];
        PGPROC     *proc = &allProcs[pgprocno];

        if (proc->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];
        PGPROC     *proc = &allProcs[pgprocno];
        VirtualTransactionId vxid;

        GET_VXID_FROM_PGPROC(vxid, *proc);

        if (proc->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;
    TransactionId *other_xids = ProcGlobal->xids;
    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];
        PGPROC     *proc = &allProcs[pgprocno];

        if (other_xids[index] == 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];
        PGPROC     *proc = &allProcs[pgprocno];
        uint8       vacuumFlags = ProcGlobal->vacuumFlags[index];

        if (proc == MyProc)
            continue;

        if (excludeVacuum & vacuumFlags)
            continue;

        if (allDbs || proc->databaseId == MyDatabaseId)
        {
            /* Fetch xmin just once - might change on us */
            TransactionId pxmin = UINT32_ACCESS_ONCE(proc->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];
        PGPROC     *proc = &allProcs[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 = UINT32_ACCESS_ONCE(proc->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];
        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];
        PGPROC     *proc = &allProcs[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 (proc->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];
        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];
        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;

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

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

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

            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];
        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];
            PGPROC     *proc = &allProcs[pgprocno];
            uint8       vacuumFlags = ProcGlobal->vacuumFlags[index];

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

            found = true;

            if (proc->pid == 0)
                (*nprepared)++;
            else
            {
                (*nbackends)++;
                if ((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 */
}

/*
 * Terminate existing connections to the specified database. This routine
 * is used by the DROP DATABASE command when user has asked to forcefully
 * drop the database.
 *
 * 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.
 *
 * It doesn't allow to terminate the connections even if there is a one
 * backend with the prepared transaction in the target database.
 */
void
TerminateOtherDBBackends(Oid databaseId)
{
    ProcArrayStruct *arrayP = procArray;
    List       *pids = NIL;
    int         nprepared = 0;
    int         i;

    LWLockAcquire(ProcArrayLock, LW_SHARED);

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

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

        if (proc->pid != 0)
            pids = lappend_int(pids, proc->pid);
        else
            nprepared++;
    }

    LWLockRelease(ProcArrayLock);

    if (nprepared > 0)
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_IN_USE),
                 errmsg("database \"%s\" is being used by prepared transactions",
                        get_database_name(databaseId)),
                 errdetail_plural("There is %d prepared transaction using the database.",
                                  "There are %d prepared transactions using the database.",
                                  nprepared,
                                  nprepared)));

    if (pids)
    {
        ListCell   *lc;

        /*
         * Check whether we have the necessary rights to terminate other
         * sessions.  We don't terminate any session until we ensure that we
         * have rights on all the sessions to be terminated.  These checks are
         * the same as we do in pg_terminate_backend.
         *
         * In this case we don't raise some warnings - like "PID %d is not a
         * PostgreSQL server process", because for us already finished session
         * is not a problem.
         */
        foreach(lc, pids)
        {
            int         pid = lfirst_int(lc);
            PGPROC     *proc = BackendPidGetProc(pid);

            if (proc != NULL)
            {
                /* Only allow superusers to signal superuser-owned backends. */
                if (superuser_arg(proc->roleId) && !superuser())
                    ereport(ERROR,
                            (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                             errmsg("must be a superuser to terminate superuser process")));

                /* Users can signal backends they have role membership in. */
                if (!has_privs_of_role(GetUserId(), proc->roleId) &&
                    !has_privs_of_role(GetUserId(), DEFAULT_ROLE_SIGNAL_BACKENDID))
                    ereport(ERROR,
                            (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
                             errmsg("must be a member of the role whose process is being terminated or member of pg_signal_backend")));
            }
        }

        /*
         * There's a race condition here: once we release the ProcArrayLock,
         * it's possible for the session to exit before we issue kill.  That
         * race condition possibility seems too unlikely to worry about.  See
         * pg_signal_backend.
         */
        foreach(lc, pids)
        {
            int         pid = lfirst_int(lc);
            PGPROC     *proc = BackendPidGetProc(pid);

            if (proc != NULL)
            {
                /*
                 * If we have setsid(), signal the backend's whole process
                 * group
                 */
#ifdef HAVE_SETSID
                (void) kill(-pid, SIGTERM);
#else
                (void) kill(pid, SIGTERM);
#endif
            }
        }
    }
}

/*
 * 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
 * replication 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);
}

/*
 * 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;
    XidCacheStatus *mysubxidstat;

    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.
     *
     * Note that we do not have to be careful about memory ordering of our own
     * reads wrt. GetNewTransactionId() here - only this process can modify
     * relevant fields of MyProc/ProcGlobal->xids[].  But we do have to be
     * careful about our own writes being well ordered.
     */
    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

    mysubxidstat = &ProcGlobal->subxidStates[MyProc->pgxactoff];

    /*
     * 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 = MyProc->subxidStatus.count - 1; j >= 0; j--)
        {
            if (TransactionIdEquals(MyProc->subxids.xids[j], anxid))
            {
                MyProc->subxids.xids[j] = MyProc->subxids.xids[MyProc->subxidStatus.count - 1];
                pg_write_barrier();
                mysubxidstat->count--;
                MyProc->subxidStatus.count--;
                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 && !MyProc->subxidStatus.overflowed)
            elog(WARNING, "did not find subXID %u in MyProc", anxid);
    }

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

    /* Also advance global latestCompletedXid while holding the lock */
    MaintainLatestCompletedXid(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 */

/*
 * If rel != NULL, return test state appropriate for relation, otherwise
 * return state usable for all relations.  The latter may consider XIDs as
 * not-yet-visible-to-everyone that a state for a specific relation would
 * already consider visible-to-everyone.
 *
 * This needs to be called while a snapshot is active or registered, otherwise
 * there are wraparound and other dangers.
 *
 * See comment for GlobalVisState for details.
 */
GlobalVisState *
GlobalVisTestFor(Relation rel)
{
    bool        need_shared;
    bool        need_catalog;
    GlobalVisState *state;

    /* XXX: we should assert that a snapshot is pushed or registered */
    Assert(RecentXmin);

    if (!rel)
        need_shared = need_catalog = true;
    else
    {
        /*
         * Other kinds currently don't contain xids, nor always the necessary
         * logical decoding markers.
         */
        Assert(rel->rd_rel->relkind == RELKIND_RELATION ||
               rel->rd_rel->relkind == RELKIND_MATVIEW ||
               rel->rd_rel->relkind == RELKIND_TOASTVALUE);

        need_shared = rel->rd_rel->relisshared || RecoveryInProgress();
        need_catalog = IsCatalogRelation(rel) || RelationIsAccessibleInLogicalDecoding(rel);
    }

    if (need_shared)
        state = &GlobalVisSharedRels;
    else if (need_catalog)
        state = &GlobalVisCatalogRels;
    else
        state = &GlobalVisDataRels;

    Assert(FullTransactionIdIsValid(state->definitely_needed) &&
           FullTransactionIdIsValid(state->maybe_needed));

    return state;
}

/*
 * Return true if it's worth updating the accurate maybe_needed boundary.
 *
 * As it is somewhat expensive to determine xmin horizons, we don't want to
 * repeatedly do so when there is a low likelihood of it being beneficial.
 *
 * The current heuristic is that we update only if RecentXmin has changed
 * since the last update. If the oldest currently running transaction has not
 * finished, it is unlikely that recomputing the horizon would be useful.
 */
static bool
GlobalVisTestShouldUpdate(GlobalVisState *state)
{
    /* hasn't been updated yet */
    if (!TransactionIdIsValid(ComputeXidHorizonsResultLastXmin))
        return true;

    /*
     * If the maybe_needed/definitely_needed boundaries are the same, it's
     * unlikely to be beneficial to refresh boundaries.
     */
    if (FullTransactionIdFollowsOrEquals(state->maybe_needed,
                                         state->definitely_needed))
        return false;

    /* does the last snapshot built have a different xmin? */
    return RecentXmin != ComputeXidHorizonsResultLastXmin;
}

static void
GlobalVisUpdateApply(ComputeXidHorizonsResult *horizons)
{
    GlobalVisSharedRels.maybe_needed =
        FullXidRelativeTo(horizons->latest_completed,
                          horizons->shared_oldest_nonremovable);
    GlobalVisCatalogRels.maybe_needed =
        FullXidRelativeTo(horizons->latest_completed,
                          horizons->catalog_oldest_nonremovable);
    GlobalVisDataRels.maybe_needed =
        FullXidRelativeTo(horizons->latest_completed,
                          horizons->data_oldest_nonremovable);

    /*
     * In longer running transactions it's possible that transactions we
     * previously needed to treat as running aren't around anymore. So update
     * definitely_needed to not be earlier than maybe_needed.
     */
    GlobalVisSharedRels.definitely_needed =
        FullTransactionIdNewer(GlobalVisSharedRels.maybe_needed,
                               GlobalVisSharedRels.definitely_needed);
    GlobalVisCatalogRels.definitely_needed =
        FullTransactionIdNewer(GlobalVisCatalogRels.maybe_needed,
                               GlobalVisCatalogRels.definitely_needed);
    GlobalVisDataRels.definitely_needed =
        FullTransactionIdNewer(GlobalVisDataRels.maybe_needed,
                               GlobalVisDataRels.definitely_needed);

    ComputeXidHorizonsResultLastXmin = RecentXmin;
}

/*
 * Update boundaries in GlobalVis{Shared,Catalog, Data}Rels
 * using ComputeXidHorizons().
 */
static void
GlobalVisUpdate(void)
{
    ComputeXidHorizonsResult horizons;

    /* updates the horizons as a side-effect */
    ComputeXidHorizons(&horizons);
}

/*
 * Return true if no snapshot still considers fxid to be running.
 *
 * The state passed needs to have been initialized for the relation fxid is
 * from (NULL is also OK), otherwise the result may not be correct.
 *
 * See comment for GlobalVisState for details.
 */
bool
GlobalVisTestIsRemovableFullXid(GlobalVisState *state,
                                FullTransactionId fxid)
{
    /*
     * If fxid is older than maybe_needed bound, it definitely is visible to
     * everyone.
     */
    if (FullTransactionIdPrecedes(fxid, state->maybe_needed))
        return true;

    /*
     * If fxid is >= definitely_needed bound, it is very likely to still be
     * considered running.
     */
    if (FullTransactionIdFollowsOrEquals(fxid, state->definitely_needed))
        return false;

    /*
     * fxid is between maybe_needed and definitely_needed, i.e. there might or
     * might not exist a snapshot considering fxid running. If it makes sense,
     * update boundaries and recheck.
     */
    if (GlobalVisTestShouldUpdate(state))
    {
        GlobalVisUpdate();

        Assert(FullTransactionIdPrecedes(fxid, state->definitely_needed));

        return FullTransactionIdPrecedes(fxid, state->maybe_needed);
    }
    else
        return false;
}

/*
 * Wrapper around GlobalVisTestIsRemovableFullXid() for 32bit xids.
 *
 * It is crucial that this only gets called for xids from a source that
 * protects against xid wraparounds (e.g. from a table and thus protected by
 * relfrozenxid).
 */
bool
GlobalVisTestIsRemovableXid(GlobalVisState *state, TransactionId xid)
{
    FullTransactionId fxid;

    /*
     * Convert 32 bit argument to FullTransactionId. We can do so safely
     * because we know the xid has to, at the very least, be between
     * [oldestXid, nextFullXid), i.e. within 2 billion of xid. To avoid taking
     * a lock to determine either, we can just compare with
     * state->definitely_needed, which was based on those value at the time
     * the current snapshot was built.
     */
    fxid = FullXidRelativeTo(state->definitely_needed, xid);

    return GlobalVisTestIsRemovableFullXid(state, fxid);
}

/*
 * Return FullTransactionId below which all transactions are not considered
 * running anymore.
 *
 * Note: This is less efficient than testing with
 * GlobalVisTestIsRemovableFullXid as it likely requires building an accurate
 * cutoff, even in the case all the XIDs compared with the cutoff are outside
 * [maybe_needed, definitely_needed).
 */
FullTransactionId
GlobalVisTestNonRemovableFullHorizon(GlobalVisState *state)
{
    /* acquire accurate horizon if not already done */
    if (GlobalVisTestShouldUpdate(state))
        GlobalVisUpdate();

    return state->maybe_needed;
}

/* Convenience wrapper around GlobalVisTestNonRemovableFullHorizon */
TransactionId
GlobalVisTestNonRemovableHorizon(GlobalVisState *state)
{
    FullTransactionId cutoff;

    cutoff = GlobalVisTestNonRemovableFullHorizon(state);

    return XidFromFullTransactionId(cutoff);
}

/*
 * Convenience wrapper around GlobalVisTestFor() and
 * GlobalVisTestIsRemovableFullXid(), see their comments.
 */
bool
GlobalVisIsRemovableFullXid(Relation rel, FullTransactionId fxid)
{
    GlobalVisState *state;

    state = GlobalVisTestFor(rel);

    return GlobalVisTestIsRemovableFullXid(state, fxid);
}

/*
 * Convenience wrapper around GlobalVisTestFor() and
 * GlobalVisTestIsRemovableXid(), see their comments.
 */
bool
GlobalVisCheckRemovableXid(Relation rel, TransactionId xid)
{
    GlobalVisState *state;

    state = GlobalVisTestFor(rel);

    return GlobalVisTestIsRemovableXid(state, xid);
}

/*
 * Convert a 32 bit transaction id into 64 bit transaction id, by assuming it
 * is within MaxTransactionId / 2 of XidFromFullTransactionId(rel).
 *
 * Be very careful about when to use this function. It can only safely be used
 * when there is a guarantee that xid is within MaxTransactionId / 2 xids of
 * rel. That e.g. can be guaranteed if the caller assures a snapshot is
 * held by the backend and xid is from a table (where vacuum/freezing ensures
 * the xid has to be within that range), or if xid is from the procarray and
 * prevents xid wraparound that way.
 */
static inline FullTransactionId
FullXidRelativeTo(FullTransactionId rel, TransactionId xid)
{
    TransactionId rel_xid = XidFromFullTransactionId(rel);

    Assert(TransactionIdIsValid(xid));
    Assert(TransactionIdIsValid(rel_xid));

    /* not guaranteed to find issues, but likely to catch mistakes */
    AssertTransactionIdInAllowableRange(xid);

    return FullTransactionIdFromU64(U64FromFullTransactionId(rel)
                                    + (int32) (xid - rel_xid));
}


/* ----------------------------------------------
 *      KnownAssignedTransactionIds sub-module
 * ----------------------------------------------
 */

/*
 * In Hot Standby mode, we maintain a list of transactions that are (or were)
 * running on the primary 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 PGPROC 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 primary 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 */
        AdvanceNextFullTransactionIdPastXid(latestObservedXid);
        next_expected_xid = latestObservedXid;
        TransactionIdAdvance(next_expected_xid);
    }
}

/*
 * 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 */
    MaintainLatestCompletedXidRecovery(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 primary 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)
{
    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)
{
    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)
{
    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)
{
    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)
{
    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)
{
    ProcArrayStruct *pArray = procArray;

    LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);

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

    LWLockRelease(ProcArrayLock);
}