snapmgr.c

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/*-------------------------------------------------------------------------
 *
 * snapmgr.c
 *      PostgreSQL snapshot manager
 *
 * We keep track of snapshots in two ways: those "registered" by resowner.c,
 * and the "active snapshot" stack.  All snapshots in either of them live in
 * persistent memory.  When a snapshot is no longer in any of these lists
 * (tracked by separate refcounts on each snapshot), its memory can be freed.
 *
 * The FirstXactSnapshot, if any, is treated a bit specially: we increment its
 * regd_count and list it in RegisteredSnapshots, but this reference is not
 * tracked by a resource owner. We used to use the TopTransactionResourceOwner
 * to track this snapshot reference, but that introduces logical circularity
 * and thus makes it impossible to clean up in a sane fashion.  It's better to
 * handle this reference as an internally-tracked registration, so that this
 * module is entirely lower-level than ResourceOwners.
 *
 * Likewise, any snapshots that have been exported by pg_export_snapshot
 * have regd_count = 1 and are listed in RegisteredSnapshots, but are not
 * tracked by any resource owner.
 *
 * Likewise, the CatalogSnapshot is listed in RegisteredSnapshots when it
 * is valid, but is not tracked by any resource owner.
 *
 * The same is true for historic snapshots used during logical decoding,
 * their lifetime is managed separately (as they live longer than one xact.c
 * transaction).
 *
 * These arrangements let us reset MyPgXact->xmin when there are no snapshots
 * referenced by this transaction, and advance it when the one with oldest
 * Xmin is no longer referenced.  For simplicity however, only registered
 * snapshots not active snapshots participate in tracking which one is oldest;
 * we don't try to change MyPgXact->xmin except when the active-snapshot
 * stack is empty.
 *
 *
 * Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *    src/backend/utils/time/snapmgr.c
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include <sys/stat.h>
#include <unistd.h>

#include "access/subtrans.h"
#include "access/transam.h"
#include "access/xact.h"
#include "access/xlog.h"
#include "catalog/catalog.h"
#include "lib/pairingheap.h"
#include "miscadmin.h"
#include "storage/predicate.h"
#include "storage/proc.h"
#include "storage/procarray.h"
#include "storage/sinval.h"
#include "storage/sinvaladt.h"
#include "storage/spin.h"
#include "utils/builtins.h"
#include "utils/memutils.h"
#include "utils/rel.h"
#include "utils/resowner_private.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"


/*
 * GUC parameters
 */
int         old_snapshot_threshold; /* number of minutes, -1 disables */

/*
 * Structure for dealing with old_snapshot_threshold implementation.
 */
typedef struct OldSnapshotControlData
{
    /*
     * Variables for old snapshot handling are shared among processes and are
     * only allowed to move forward.
     */
    slock_t     mutex_current;  /* protect current_timestamp */
    TimestampTz current_timestamp;  /* latest snapshot timestamp */
    slock_t     mutex_latest_xmin;  /* protect latest_xmin and next_map_update */
    TransactionId latest_xmin;  /* latest snapshot xmin */
    TimestampTz next_map_update;    /* latest snapshot valid up to */
    slock_t     mutex_threshold;    /* protect threshold fields */
    TimestampTz threshold_timestamp;    /* earlier snapshot is old */
    TransactionId threshold_xid;    /* earlier xid may be gone */

    /*
     * Keep one xid per minute for old snapshot error handling.
     *
     * Use a circular buffer with a head offset, a count of entries currently
     * used, and a timestamp corresponding to the xid at the head offset.  A
     * count_used value of zero means that there are no times stored; a
     * count_used value of OLD_SNAPSHOT_TIME_MAP_ENTRIES means that the buffer
     * is full and the head must be advanced to add new entries.  Use
     * timestamps aligned to minute boundaries, since that seems less
     * surprising than aligning based on the first usage timestamp.  The
     * latest bucket is effectively stored within latest_xmin.  The circular
     * buffer is updated when we get a new xmin value that doesn't fall into
     * the same interval.
     *
     * It is OK if the xid for a given time slot is from earlier than
     * calculated by adding the number of minutes corresponding to the
     * (possibly wrapped) distance from the head offset to the time of the
     * head entry, since that just results in the vacuuming of old tuples
     * being slightly less aggressive.  It would not be OK for it to be off in
     * the other direction, since it might result in vacuuming tuples that are
     * still expected to be there.
     *
     * Use of an SLRU was considered but not chosen because it is more
     * heavyweight than is needed for this, and would probably not be any less
     * code to implement.
     *
     * Persistence is not needed.
     */
    int         head_offset;    /* subscript of oldest tracked time */
    TimestampTz head_timestamp; /* time corresponding to head xid */
    int         count_used;     /* how many slots are in use */
    TransactionId xid_by_minute[FLEXIBLE_ARRAY_MEMBER];
} OldSnapshotControlData;

static volatile OldSnapshotControlData *oldSnapshotControl;


/*
 * CurrentSnapshot points to the only snapshot taken in transaction-snapshot
 * mode, and to the latest one taken in a read-committed transaction.
 * SecondarySnapshot is a snapshot that's always up-to-date as of the current
 * instant, even in transaction-snapshot mode.  It should only be used for
 * special-purpose code (say, RI checking.)  CatalogSnapshot points to an
 * MVCC snapshot intended to be used for catalog scans; we must invalidate it
 * whenever a system catalog change occurs.
 *
 * These SnapshotData structs are static to simplify memory allocation
 * (see the hack in GetSnapshotData to avoid repeated malloc/free).
 */
static SnapshotData CurrentSnapshotData = {SNAPSHOT_MVCC};
static SnapshotData SecondarySnapshotData = {SNAPSHOT_MVCC};
SnapshotData CatalogSnapshotData = {SNAPSHOT_MVCC};
SnapshotData SnapshotSelfData = {SNAPSHOT_SELF};
SnapshotData SnapshotAnyData = {SNAPSHOT_ANY};

/* Pointers to valid snapshots */
static Snapshot CurrentSnapshot = NULL;
static Snapshot SecondarySnapshot = NULL;
static Snapshot CatalogSnapshot = NULL;
static Snapshot HistoricSnapshot = NULL;

/*
 * These are updated by GetSnapshotData.  We initialize them this way
 * for the convenience of TransactionIdIsInProgress: even in bootstrap
 * mode, we don't want it to say that BootstrapTransactionId is in progress.
 *
 * RecentGlobalXmin and RecentGlobalDataXmin are initialized to
 * InvalidTransactionId, to ensure that no one tries to use a stale
 * value. Readers should ensure that it has been set to something else
 * before using it.
 */
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;
TransactionId RecentGlobalXmin = InvalidTransactionId;
TransactionId RecentGlobalDataXmin = InvalidTransactionId;

/* (table, ctid) => (cmin, cmax) mapping during timetravel */
static HTAB *tuplecid_data = NULL;

/*
 * Elements of the active snapshot stack.
 *
 * Each element here accounts for exactly one active_count on SnapshotData.
 *
 * NB: the code assumes that elements in this list are in non-increasing
 * order of as_level; also, the list must be NULL-terminated.
 */
typedef struct ActiveSnapshotElt
{
    Snapshot    as_snap;
    int         as_level;
    struct ActiveSnapshotElt *as_next;
} ActiveSnapshotElt;

/* Top of the stack of active snapshots */
static ActiveSnapshotElt *ActiveSnapshot = NULL;

/* Bottom of the stack of active snapshots */
static ActiveSnapshotElt *OldestActiveSnapshot = NULL;

/*
 * Currently registered Snapshots.  Ordered in a heap by xmin, so that we can
 * quickly find the one with lowest xmin, to advance our MyPgXact->xmin.
 */
static int  xmin_cmp(const pairingheap_node *a, const pairingheap_node *b,
                     void *arg);

static pairingheap RegisteredSnapshots = {&xmin_cmp, NULL, NULL};

/* first GetTransactionSnapshot call in a transaction? */
bool        FirstSnapshotSet = false;

/*
 * Remember the serializable transaction snapshot, if any.  We cannot trust
 * FirstSnapshotSet in combination with IsolationUsesXactSnapshot(), because
 * GUC may be reset before us, changing the value of IsolationUsesXactSnapshot.
 */
static Snapshot FirstXactSnapshot = NULL;

/* Define pathname of exported-snapshot files */
#define SNAPSHOT_EXPORT_DIR "pg_snapshots"

/* Structure holding info about exported snapshot. */
typedef struct ExportedSnapshot
{
    char       *snapfile;
    Snapshot    snapshot;
} ExportedSnapshot;

/* Current xact's exported snapshots (a list of ExportedSnapshot structs) */
static List *exportedSnapshots = NIL;

/* Prototypes for local functions */
static TimestampTz AlignTimestampToMinuteBoundary(TimestampTz ts);
static Snapshot CopySnapshot(Snapshot snapshot);
static void FreeSnapshot(Snapshot snapshot);
static void SnapshotResetXmin(void);

/*
 * Snapshot fields to be serialized.
 *
 * Only these fields need to be sent to the cooperating backend; the
 * remaining ones can (and must) be set by the receiver upon restore.
 */
typedef struct SerializedSnapshotData
{
    TransactionId xmin;
    TransactionId xmax;
    uint32      xcnt;
    int32       subxcnt;
    bool        suboverflowed;
    bool        takenDuringRecovery;
    CommandId   curcid;
    TimestampTz whenTaken;
    XLogRecPtr  lsn;
} SerializedSnapshotData;

Size
SnapMgrShmemSize(void)
{
    Size        size;

    size = offsetof(OldSnapshotControlData, xid_by_minute);
    if (old_snapshot_threshold > 0)
        size = add_size(size, mul_size(sizeof(TransactionId),
                                       OLD_SNAPSHOT_TIME_MAP_ENTRIES));

    return size;
}

/*
 * Initialize for managing old snapshot detection.
 */
void
SnapMgrInit(void)
{
    bool        found;

    /*
     * Create or attach to the OldSnapshotControlData structure.
     */
    oldSnapshotControl = (volatile OldSnapshotControlData *)
        ShmemInitStruct("OldSnapshotControlData",
                        SnapMgrShmemSize(), &found);

    if (!found)
    {
        SpinLockInit(&oldSnapshotControl->mutex_current);
        oldSnapshotControl->current_timestamp = 0;
        SpinLockInit(&oldSnapshotControl->mutex_latest_xmin);
        oldSnapshotControl->latest_xmin = InvalidTransactionId;
        oldSnapshotControl->next_map_update = 0;
        SpinLockInit(&oldSnapshotControl->mutex_threshold);
        oldSnapshotControl->threshold_timestamp = 0;
        oldSnapshotControl->threshold_xid = InvalidTransactionId;
        oldSnapshotControl->head_offset = 0;
        oldSnapshotControl->head_timestamp = 0;
        oldSnapshotControl->count_used = 0;
    }
}

/*
 * GetTransactionSnapshot
 *      Get the appropriate snapshot for a new query in a transaction.
 *
 * Note that the return value may point at static storage that will be modified
 * by future calls and by CommandCounterIncrement().  Callers should call
 * RegisterSnapshot or PushActiveSnapshot on the returned snap if it is to be
 * used very long.
 */
Snapshot
GetTransactionSnapshot(void)
{
    /*
     * Return historic snapshot if doing logical decoding. We'll never need a
     * non-historic transaction snapshot in this (sub-)transaction, so there's
     * no need to be careful to set one up for later calls to
     * GetTransactionSnapshot().
     */
    if (HistoricSnapshotActive())
    {
        Assert(!FirstSnapshotSet);
        return HistoricSnapshot;
    }

    /* First call in transaction? */
    if (!FirstSnapshotSet)
    {
        /*
         * Don't allow catalog snapshot to be older than xact snapshot.  Must
         * do this first to allow the empty-heap Assert to succeed.
         */
        InvalidateCatalogSnapshot();

        Assert(pairingheap_is_empty(&RegisteredSnapshots));
        Assert(FirstXactSnapshot == NULL);

        if (IsInParallelMode())
            elog(ERROR,
                 "cannot take query snapshot during a parallel operation");

        /*
         * In transaction-snapshot mode, the first snapshot must live until
         * end of xact regardless of what the caller does with it, so we must
         * make a copy of it rather than returning CurrentSnapshotData
         * directly.  Furthermore, if we're running in serializable mode,
         * predicate.c needs to wrap the snapshot fetch in its own processing.
         */
        if (IsolationUsesXactSnapshot())
        {
            /* First, create the snapshot in CurrentSnapshotData */
            if (IsolationIsSerializable())
                CurrentSnapshot = GetSerializableTransactionSnapshot(&CurrentSnapshotData);
            else
                CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);
            /* Make a saved copy */
            CurrentSnapshot = CopySnapshot(CurrentSnapshot);
            FirstXactSnapshot = CurrentSnapshot;
            /* Mark it as "registered" in FirstXactSnapshot */
            FirstXactSnapshot->regd_count++;
            pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
        }
        else
            CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

        FirstSnapshotSet = true;
        return CurrentSnapshot;
    }

    if (IsolationUsesXactSnapshot())
        return CurrentSnapshot;

    /* Don't allow catalog snapshot to be older than xact snapshot. */
    InvalidateCatalogSnapshot();

    CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

    return CurrentSnapshot;
}

/*
 * GetLatestSnapshot
 *      Get a snapshot that is up-to-date as of the current instant,
 *      even if we are executing in transaction-snapshot mode.
 */
Snapshot
GetLatestSnapshot(void)
{
    /*
     * We might be able to relax this, but nothing that could otherwise work
     * needs it.
     */
    if (IsInParallelMode())
        elog(ERROR,
             "cannot update SecondarySnapshot during a parallel operation");

    /*
     * So far there are no cases requiring support for GetLatestSnapshot()
     * during logical decoding, but it wouldn't be hard to add if required.
     */
    Assert(!HistoricSnapshotActive());

    /* If first call in transaction, go ahead and set the xact snapshot */
    if (!FirstSnapshotSet)
        return GetTransactionSnapshot();

    SecondarySnapshot = GetSnapshotData(&SecondarySnapshotData);

    return SecondarySnapshot;
}

/*
 * GetOldestSnapshot
 *
 *      Get the transaction's oldest known snapshot, as judged by the LSN.
 *      Will return NULL if there are no active or registered snapshots.
 */
Snapshot
GetOldestSnapshot(void)
{
    Snapshot    OldestRegisteredSnapshot = NULL;
    XLogRecPtr  RegisteredLSN = InvalidXLogRecPtr;

    if (!pairingheap_is_empty(&RegisteredSnapshots))
    {
        OldestRegisteredSnapshot = pairingheap_container(SnapshotData, ph_node,
                                                         pairingheap_first(&RegisteredSnapshots));
        RegisteredLSN = OldestRegisteredSnapshot->lsn;
    }

    if (OldestActiveSnapshot != NULL)
    {
        XLogRecPtr  ActiveLSN = OldestActiveSnapshot->as_snap->lsn;

        if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
            return OldestActiveSnapshot->as_snap;
    }

    return OldestRegisteredSnapshot;
}

/*
 * GetCatalogSnapshot
 *      Get a snapshot that is sufficiently up-to-date for scan of the
 *      system catalog with the specified OID.
 */
Snapshot
GetCatalogSnapshot(Oid relid)
{
    /*
     * Return historic snapshot while we're doing logical decoding, so we can
     * see the appropriate state of the catalog.
     *
     * This is the primary reason for needing to reset the system caches after
     * finishing decoding.
     */
    if (HistoricSnapshotActive())
        return HistoricSnapshot;

    return GetNonHistoricCatalogSnapshot(relid);
}

/*
 * GetNonHistoricCatalogSnapshot
 *      Get a snapshot that is sufficiently up-to-date for scan of the system
 *      catalog with the specified OID, even while historic snapshots are set
 *      up.
 */
Snapshot
GetNonHistoricCatalogSnapshot(Oid relid)
{
    /*
     * If the caller is trying to scan a relation that has no syscache, no
     * catcache invalidations will be sent when it is updated.  For a few key
     * relations, snapshot invalidations are sent instead.  If we're trying to
     * scan a relation for which neither catcache nor snapshot invalidations
     * are sent, we must refresh the snapshot every time.
     */
    if (CatalogSnapshot &&
        !RelationInvalidatesSnapshotsOnly(relid) &&
        !RelationHasSysCache(relid))
        InvalidateCatalogSnapshot();

    if (CatalogSnapshot == NULL)
    {
        /* Get new snapshot. */
        CatalogSnapshot = GetSnapshotData(&CatalogSnapshotData);

        /*
         * Make sure the catalog snapshot will be accounted for in decisions
         * about advancing PGXACT->xmin.  We could apply RegisterSnapshot, but
         * that would result in making a physical copy, which is overkill; and
         * it would also create a dependency on some resource owner, which we
         * do not want for reasons explained at the head of this file. Instead
         * just shove the CatalogSnapshot into the pairing heap manually. This
         * has to be reversed in InvalidateCatalogSnapshot, of course.
         *
         * NB: it had better be impossible for this to throw error, since the
         * CatalogSnapshot pointer is already valid.
         */
        pairingheap_add(&RegisteredSnapshots, &CatalogSnapshot->ph_node);
    }

    return CatalogSnapshot;
}

/*
 * InvalidateCatalogSnapshot
 *      Mark the current catalog snapshot, if any, as invalid
 *
 * We could change this API to allow the caller to provide more fine-grained
 * invalidation details, so that a change to relation A wouldn't prevent us
 * from using our cached snapshot to scan relation B, but so far there's no
 * evidence that the CPU cycles we spent tracking such fine details would be
 * well-spent.
 */
void
InvalidateCatalogSnapshot(void)
{
    if (CatalogSnapshot)
    {
        pairingheap_remove(&RegisteredSnapshots, &CatalogSnapshot->ph_node);
        CatalogSnapshot = NULL;
        SnapshotResetXmin();
    }
}

/*
 * InvalidateCatalogSnapshotConditionally
 *      Drop catalog snapshot if it's the only one we have
 *
 * This is called when we are about to wait for client input, so we don't
 * want to continue holding the catalog snapshot if it might mean that the
 * global xmin horizon can't advance.  However, if there are other snapshots
 * still active or registered, the catalog snapshot isn't likely to be the
 * oldest one, so we might as well keep it.
 */
void
InvalidateCatalogSnapshotConditionally(void)
{
    if (CatalogSnapshot &&
        ActiveSnapshot == NULL &&
        pairingheap_is_singular(&RegisteredSnapshots))
        InvalidateCatalogSnapshot();
}

/*
 * SnapshotSetCommandId
 *      Propagate CommandCounterIncrement into the static snapshots, if set
 */
void
SnapshotSetCommandId(CommandId curcid)
{
    if (!FirstSnapshotSet)
        return;

    if (CurrentSnapshot)
        CurrentSnapshot->curcid = curcid;
    if (SecondarySnapshot)
        SecondarySnapshot->curcid = curcid;
    /* Should we do the same with CatalogSnapshot? */
}

/*
 * SetTransactionSnapshot
 *      Set the transaction's snapshot from an imported MVCC snapshot.
 *
 * Note that this is very closely tied to GetTransactionSnapshot --- it
 * must take care of all the same considerations as the first-snapshot case
 * in GetTransactionSnapshot.
 */
static void
SetTransactionSnapshot(Snapshot sourcesnap, VirtualTransactionId *sourcevxid,
                       int sourcepid, PGPROC *sourceproc)
{
    /* Caller should have checked this already */
    Assert(!FirstSnapshotSet);

    /* Better do this to ensure following Assert succeeds. */
    InvalidateCatalogSnapshot();

    Assert(pairingheap_is_empty(&RegisteredSnapshots));
    Assert(FirstXactSnapshot == NULL);
    Assert(!HistoricSnapshotActive());

    /*
     * Even though we are not going to use the snapshot it computes, we must
     * call GetSnapshotData, for two reasons: (1) to be sure that
     * CurrentSnapshotData's XID arrays have been allocated, and (2) to update
     * RecentXmin and RecentGlobalXmin.  (We could alternatively include those
     * two variables in exported snapshot files, but it seems better to have
     * snapshot importers compute reasonably up-to-date values for them.)
     */
    CurrentSnapshot = GetSnapshotData(&CurrentSnapshotData);

    /*
     * Now copy appropriate fields from the source snapshot.
     */
    CurrentSnapshot->xmin = sourcesnap->xmin;
    CurrentSnapshot->xmax = sourcesnap->xmax;
    CurrentSnapshot->xcnt = sourcesnap->xcnt;
    Assert(sourcesnap->xcnt <= GetMaxSnapshotXidCount());
    memcpy(CurrentSnapshot->xip, sourcesnap->xip,
           sourcesnap->xcnt * sizeof(TransactionId));
    CurrentSnapshot->subxcnt = sourcesnap->subxcnt;
    Assert(sourcesnap->subxcnt <= GetMaxSnapshotSubxidCount());
    memcpy(CurrentSnapshot->subxip, sourcesnap->subxip,
           sourcesnap->subxcnt * sizeof(TransactionId));
    CurrentSnapshot->suboverflowed = sourcesnap->suboverflowed;
    CurrentSnapshot->takenDuringRecovery = sourcesnap->takenDuringRecovery;
    /* NB: curcid should NOT be copied, it's a local matter */

    /*
     * Now we have to fix what GetSnapshotData did with MyPgXact->xmin and
     * TransactionXmin.  There is a race condition: to make sure we are not
     * causing the global xmin to go backwards, we have to test that the
     * source transaction is still running, and that has to be done
     * atomically. So let procarray.c do it.
     *
     * Note: in serializable mode, predicate.c will do this a second time. It
     * doesn't seem worth contorting the logic here to avoid two calls,
     * especially since it's not clear that predicate.c *must* do this.
     */
    if (sourceproc != NULL)
    {
        if (!ProcArrayInstallRestoredXmin(CurrentSnapshot->xmin, sourceproc))
            ereport(ERROR,
                    (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                     errmsg("could not import the requested snapshot"),
                     errdetail("The source transaction is not running anymore.")));
    }
    else if (!ProcArrayInstallImportedXmin(CurrentSnapshot->xmin, sourcevxid))
        ereport(ERROR,
                (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
                 errmsg("could not import the requested snapshot"),
                 errdetail("The source process with PID %d is not running anymore.",
                           sourcepid)));

    /*
     * In transaction-snapshot mode, the first snapshot must live until end of
     * xact, so we must make a copy of it.  Furthermore, if we're running in
     * serializable mode, predicate.c needs to do its own processing.
     */
    if (IsolationUsesXactSnapshot())
    {
        if (IsolationIsSerializable())
            SetSerializableTransactionSnapshot(CurrentSnapshot, sourcevxid,
                                               sourcepid);
        /* Make a saved copy */
        CurrentSnapshot = CopySnapshot(CurrentSnapshot);
        FirstXactSnapshot = CurrentSnapshot;
        /* Mark it as "registered" in FirstXactSnapshot */
        FirstXactSnapshot->regd_count++;
        pairingheap_add(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
    }

    FirstSnapshotSet = true;
}

/*
 * CopySnapshot
 *      Copy the given snapshot.
 *
 * The copy is palloc'd in TopTransactionContext and has initial refcounts set
 * to 0.  The returned snapshot has the copied flag set.
 */
static Snapshot
CopySnapshot(Snapshot snapshot)
{
    Snapshot    newsnap;
    Size        subxipoff;
    Size        size;

    Assert(snapshot != InvalidSnapshot);

    /* We allocate any XID arrays needed in the same palloc block. */
    size = subxipoff = sizeof(SnapshotData) +
        snapshot->xcnt * sizeof(TransactionId);
    if (snapshot->subxcnt > 0)
        size += snapshot->subxcnt * sizeof(TransactionId);

    newsnap = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
    memcpy(newsnap, snapshot, sizeof(SnapshotData));

    newsnap->regd_count = 0;
    newsnap->active_count = 0;
    newsnap->copied = true;

    /* setup XID array */
    if (snapshot->xcnt > 0)
    {
        newsnap->xip = (TransactionId *) (newsnap + 1);
        memcpy(newsnap->xip, snapshot->xip,
               snapshot->xcnt * sizeof(TransactionId));
    }
    else
        newsnap->xip = NULL;

    /*
     * Setup subXID array. Don't bother to copy it if it had overflowed,
     * though, because it's not used anywhere in that case. Except if it's a
     * snapshot taken during recovery; all the top-level XIDs are in subxip as
     * well in that case, so we mustn't lose them.
     */
    if (snapshot->subxcnt > 0 &&
        (!snapshot->suboverflowed || snapshot->takenDuringRecovery))
    {
        newsnap->subxip = (TransactionId *) ((char *) newsnap + subxipoff);
        memcpy(newsnap->subxip, snapshot->subxip,
               snapshot->subxcnt * sizeof(TransactionId));
    }
    else
        newsnap->subxip = NULL;

    return newsnap;
}

/*
 * FreeSnapshot
 *      Free the memory associated with a snapshot.
 */
static void
FreeSnapshot(Snapshot snapshot)
{
    Assert(snapshot->regd_count == 0);
    Assert(snapshot->active_count == 0);
    Assert(snapshot->copied);

    pfree(snapshot);
}

/*
 * PushActiveSnapshot
 *      Set the given snapshot as the current active snapshot
 *
 * If the passed snapshot is a statically-allocated one, or it is possibly
 * subject to a future command counter update, create a new long-lived copy
 * with active refcount=1.  Otherwise, only increment the refcount.
 */
void
PushActiveSnapshot(Snapshot snap)
{
    ActiveSnapshotElt *newactive;

    Assert(snap != InvalidSnapshot);

    newactive = MemoryContextAlloc(TopTransactionContext, sizeof(ActiveSnapshotElt));

    /*
     * Checking SecondarySnapshot is probably useless here, but it seems
     * better to be sure.
     */
    if (snap == CurrentSnapshot || snap == SecondarySnapshot || !snap->copied)
        newactive->as_snap = CopySnapshot(snap);
    else
        newactive->as_snap = snap;

    newactive->as_next = ActiveSnapshot;
    newactive->as_level = GetCurrentTransactionNestLevel();

    newactive->as_snap->active_count++;

    ActiveSnapshot = newactive;
    if (OldestActiveSnapshot == NULL)
        OldestActiveSnapshot = ActiveSnapshot;
}

/*
 * PushCopiedSnapshot
 *      As above, except forcibly copy the presented snapshot.
 *
 * This should be used when the ActiveSnapshot has to be modifiable, for
 * example if the caller intends to call UpdateActiveSnapshotCommandId.
 * The new snapshot will be released when popped from the stack.
 */
void
PushCopiedSnapshot(Snapshot snapshot)
{
    PushActiveSnapshot(CopySnapshot(snapshot));
}

/*
 * UpdateActiveSnapshotCommandId
 *
 * Update the current CID of the active snapshot.  This can only be applied
 * to a snapshot that is not referenced elsewhere.
 */
void
UpdateActiveSnapshotCommandId(void)
{
    CommandId   save_curcid,
                curcid;

    Assert(ActiveSnapshot != NULL);
    Assert(ActiveSnapshot->as_snap->active_count == 1);
    Assert(ActiveSnapshot->as_snap->regd_count == 0);

    /*
     * Don't allow modification of the active snapshot during parallel
     * operation.  We share the snapshot to worker backends at the beginning
     * of parallel operation, so any change to the snapshot can lead to
     * inconsistencies.  We have other defenses against
     * CommandCounterIncrement, but there are a few places that call this
     * directly, so we put an additional guard here.
     */
    save_curcid = ActiveSnapshot->as_snap->curcid;
    curcid = GetCurrentCommandId(false);
    if (IsInParallelMode() && save_curcid != curcid)
        elog(ERROR, "cannot modify commandid in active snapshot during a parallel operation");
    ActiveSnapshot->as_snap->curcid = curcid;
}

/*
 * PopActiveSnapshot
 *
 * Remove the topmost snapshot from the active snapshot stack, decrementing the
 * reference count, and free it if this was the last reference.
 */
void
PopActiveSnapshot(void)
{
    ActiveSnapshotElt *newstack;

    newstack = ActiveSnapshot->as_next;

    Assert(ActiveSnapshot->as_snap->active_count > 0);

    ActiveSnapshot->as_snap->active_count--;

    if (ActiveSnapshot->as_snap->active_count == 0 &&
        ActiveSnapshot->as_snap->regd_count == 0)
        FreeSnapshot(ActiveSnapshot->as_snap);

    pfree(ActiveSnapshot);
    ActiveSnapshot = newstack;
    if (ActiveSnapshot == NULL)
        OldestActiveSnapshot = NULL;

    SnapshotResetXmin();
}

/*
 * GetActiveSnapshot
 *      Return the topmost snapshot in the Active stack.
 */
Snapshot
GetActiveSnapshot(void)
{
    Assert(ActiveSnapshot != NULL);

    return ActiveSnapshot->as_snap;
}

/*
 * ActiveSnapshotSet
 *      Return whether there is at least one snapshot in the Active stack
 */
bool
ActiveSnapshotSet(void)
{
    return ActiveSnapshot != NULL;
}

/*
 * RegisterSnapshot
 *      Register a snapshot as being in use by the current resource owner
 *
 * If InvalidSnapshot is passed, it is not registered.
 */
Snapshot
RegisterSnapshot(Snapshot snapshot)
{
    if (snapshot == InvalidSnapshot)
        return InvalidSnapshot;

    return RegisterSnapshotOnOwner(snapshot, CurrentResourceOwner);
}

/*
 * RegisterSnapshotOnOwner
 *      As above, but use the specified resource owner
 */
Snapshot
RegisterSnapshotOnOwner(Snapshot snapshot, ResourceOwner owner)
{
    Snapshot    snap;

    if (snapshot == InvalidSnapshot)
        return InvalidSnapshot;

    /* Static snapshot?  Create a persistent copy */
    snap = snapshot->copied ? snapshot : CopySnapshot(snapshot);

    /* and tell resowner.c about it */
    ResourceOwnerEnlargeSnapshots(owner);
    snap->regd_count++;
    ResourceOwnerRememberSnapshot(owner, snap);

    if (snap->regd_count == 1)
        pairingheap_add(&RegisteredSnapshots, &snap->ph_node);

    return snap;
}

/*
 * UnregisterSnapshot
 *
 * Decrement the reference count of a snapshot, remove the corresponding
 * reference from CurrentResourceOwner, and free the snapshot if no more
 * references remain.
 */
void
UnregisterSnapshot(Snapshot snapshot)
{
    if (snapshot == NULL)
        return;

    UnregisterSnapshotFromOwner(snapshot, CurrentResourceOwner);
}

/*
 * UnregisterSnapshotFromOwner
 *      As above, but use the specified resource owner
 */
void
UnregisterSnapshotFromOwner(Snapshot snapshot, ResourceOwner owner)
{
    if (snapshot == NULL)
        return;

    Assert(snapshot->regd_count > 0);
    Assert(!pairingheap_is_empty(&RegisteredSnapshots));

    ResourceOwnerForgetSnapshot(owner, snapshot);

    snapshot->regd_count--;
    if (snapshot->regd_count == 0)
        pairingheap_remove(&RegisteredSnapshots, &snapshot->ph_node);

    if (snapshot->regd_count == 0 && snapshot->active_count == 0)
    {
        FreeSnapshot(snapshot);
        SnapshotResetXmin();
    }
}

/*
 * Comparison function for RegisteredSnapshots heap.  Snapshots are ordered
 * by xmin, so that the snapshot with smallest xmin is at the top.
 */
static int
xmin_cmp(const pairingheap_node *a, const pairingheap_node *b, void *arg)
{
    const SnapshotData *asnap = pairingheap_const_container(SnapshotData, ph_node, a);
    const SnapshotData *bsnap = pairingheap_const_container(SnapshotData, ph_node, b);

    if (TransactionIdPrecedes(asnap->xmin, bsnap->xmin))
        return 1;
    else if (TransactionIdFollows(asnap->xmin, bsnap->xmin))
        return -1;
    else
        return 0;
}

/*
 * Get current RecentGlobalXmin value, as a FullTransactionId.
 */
FullTransactionId
GetFullRecentGlobalXmin(void)
{
    FullTransactionId nextxid_full;
    uint32      nextxid_epoch;
    TransactionId nextxid_xid;
    uint32      epoch;

    Assert(TransactionIdIsNormal(RecentGlobalXmin));

    /*
     * Compute the epoch from the next XID's epoch. This relies on the fact
     * that RecentGlobalXmin must be within the 2 billion XID horizon from the
     * next XID.
     */
    nextxid_full = ReadNextFullTransactionId();
    nextxid_epoch = EpochFromFullTransactionId(nextxid_full);
    nextxid_xid = XidFromFullTransactionId(nextxid_full);

    if (RecentGlobalXmin > nextxid_xid)
        epoch = nextxid_epoch - 1;
    else
        epoch = nextxid_epoch;

    return FullTransactionIdFromEpochAndXid(epoch, RecentGlobalXmin);
}

/*
 * SnapshotResetXmin
 *
 * If there are no more snapshots, we can reset our PGXACT->xmin to InvalidXid.
 * Note we can do this without locking because we assume that storing an Xid
 * is atomic.
 *
 * Even if there are some remaining snapshots, we may be able to advance our
 * PGXACT->xmin to some degree.  This typically happens when a portal is
 * dropped.  For efficiency, we only consider recomputing PGXACT->xmin when
 * the active snapshot stack is empty; this allows us not to need to track
 * which active snapshot is oldest.
 *
 * Note: it's tempting to use GetOldestSnapshot() here so that we can include
 * active snapshots in the calculation.  However, that compares by LSN not
 * xmin so it's not entirely clear that it's the same thing.  Also, we'd be
 * critically dependent on the assumption that the bottommost active snapshot
 * stack entry has the oldest xmin.  (Current uses of GetOldestSnapshot() are
 * not actually critical, but this would be.)
 */
static void
SnapshotResetXmin(void)
{
    Snapshot    minSnapshot;

    if (ActiveSnapshot != NULL)
        return;

    if (pairingheap_is_empty(&RegisteredSnapshots))
    {
        MyPgXact->xmin = InvalidTransactionId;
        return;
    }

    minSnapshot = pairingheap_container(SnapshotData, ph_node,
                                        pairingheap_first(&RegisteredSnapshots));

    if (TransactionIdPrecedes(MyPgXact->xmin, minSnapshot->xmin))
        MyPgXact->xmin = minSnapshot->xmin;
}

/*
 * AtSubCommit_Snapshot
 */
void
AtSubCommit_Snapshot(int level)
{
    ActiveSnapshotElt *active;

    /*
     * Relabel the active snapshots set in this subtransaction as though they
     * are owned by the parent subxact.
     */
    for (active = ActiveSnapshot; active != NULL; active = active->as_next)
    {
        if (active->as_level < level)
            break;
        active->as_level = level - 1;
    }
}

/*
 * AtSubAbort_Snapshot
 *      Clean up snapshots after a subtransaction abort
 */
void
AtSubAbort_Snapshot(int level)
{
    /* Forget the active snapshots set by this subtransaction */
    while (ActiveSnapshot && ActiveSnapshot->as_level >= level)
    {
        ActiveSnapshotElt *next;

        next = ActiveSnapshot->as_next;

        /*
         * Decrement the snapshot's active count.  If it's still registered or
         * marked as active by an outer subtransaction, we can't free it yet.
         */
        Assert(ActiveSnapshot->as_snap->active_count >= 1);
        ActiveSnapshot->as_snap->active_count -= 1;

        if (ActiveSnapshot->as_snap->active_count == 0 &&
            ActiveSnapshot->as_snap->regd_count == 0)
            FreeSnapshot(ActiveSnapshot->as_snap);

        /* and free the stack element */
        pfree(ActiveSnapshot);

        ActiveSnapshot = next;
        if (ActiveSnapshot == NULL)
            OldestActiveSnapshot = NULL;
    }

    SnapshotResetXmin();
}

/*
 * AtEOXact_Snapshot
 *      Snapshot manager's cleanup function for end of transaction
 */
void
AtEOXact_Snapshot(bool isCommit, bool resetXmin)
{
    /*
     * In transaction-snapshot mode we must release our privately-managed
     * reference to the transaction snapshot.  We must remove it from
     * RegisteredSnapshots to keep the check below happy.  But we don't bother
     * to do FreeSnapshot, for two reasons: the memory will go away with
     * TopTransactionContext anyway, and if someone has left the snapshot
     * stacked as active, we don't want the code below to be chasing through a
     * dangling pointer.
     */
    if (FirstXactSnapshot != NULL)
    {
        Assert(FirstXactSnapshot->regd_count > 0);
        Assert(!pairingheap_is_empty(&RegisteredSnapshots));
        pairingheap_remove(&RegisteredSnapshots, &FirstXactSnapshot->ph_node);
    }
    FirstXactSnapshot = NULL;

    /*
     * If we exported any snapshots, clean them up.
     */
    if (exportedSnapshots != NIL)
    {
        ListCell   *lc;

        /*
         * Get rid of the files.  Unlink failure is only a WARNING because (1)
         * it's too late to abort the transaction, and (2) leaving a leaked
         * file around has little real consequence anyway.
         *
         * We also need to remove the snapshots from RegisteredSnapshots to
         * prevent a warning below.
         *
         * As with the FirstXactSnapshot, we don't need to free resources of
         * the snapshot itself as it will go away with the memory context.
         */
        foreach(lc, exportedSnapshots)
        {
            ExportedSnapshot *esnap = (ExportedSnapshot *) lfirst(lc);

            if (unlink(esnap->snapfile))
                elog(WARNING, "could not unlink file \"%s\": %m",
                     esnap->snapfile);

            pairingheap_remove(&RegisteredSnapshots,
                               &esnap->snapshot->ph_node);
        }

        exportedSnapshots = NIL;
    }

    /* Drop catalog snapshot if any */
    InvalidateCatalogSnapshot();

    /* On commit, complain about leftover snapshots */
    if (isCommit)
    {
        ActiveSnapshotElt *active;

        if (!pairingheap_is_empty(&RegisteredSnapshots))
            elog(WARNING, "registered snapshots seem to remain after cleanup");

        /* complain about unpopped active snapshots */
        for (active = ActiveSnapshot; active != NULL; active = active->as_next)
            elog(WARNING, "snapshot %p still active", active);
    }

    /*
     * And reset our state.  We don't need to free the memory explicitly --
     * it'll go away with TopTransactionContext.
     */
    ActiveSnapshot = NULL;
    OldestActiveSnapshot = NULL;
    pairingheap_reset(&RegisteredSnapshots);

    CurrentSnapshot = NULL;
    SecondarySnapshot = NULL;

    FirstSnapshotSet = false;

    /*
     * During normal commit processing, we call ProcArrayEndTransaction() to
     * reset the MyPgXact->xmin. That call happens prior to the call to
     * AtEOXact_Snapshot(), so we need not touch xmin here at all.
     */
    if (resetXmin)
        SnapshotResetXmin();

    Assert(resetXmin || MyPgXact->xmin == 0);
}


/*
 * ExportSnapshot
 *      Export the snapshot to a file so that other backends can import it.
 *      Returns the token (the file name) that can be used to import this
 *      snapshot.
 */
char *
ExportSnapshot(Snapshot snapshot)
{
    TransactionId topXid;
    TransactionId *children;
    ExportedSnapshot *esnap;
    int         nchildren;
    int         addTopXid;
    StringInfoData buf;
    FILE       *f;
    int         i;
    MemoryContext oldcxt;
    char        path[MAXPGPATH];
    char        pathtmp[MAXPGPATH];

    /*
     * It's tempting to call RequireTransactionBlock here, since it's not very
     * useful to export a snapshot that will disappear immediately afterwards.
     * However, we haven't got enough information to do that, since we don't
     * know if we're at top level or not.  For example, we could be inside a
     * plpgsql function that is going to fire off other transactions via
     * dblink.  Rather than disallow perfectly legitimate usages, don't make a
     * check.
     *
     * Also note that we don't make any restriction on the transaction's
     * isolation level; however, importers must check the level if they are
     * serializable.
     */

    /*
     * Get our transaction ID if there is one, to include in the snapshot.
     */
    topXid = GetTopTransactionIdIfAny();

    /*
     * We cannot export a snapshot from a subtransaction because there's no
     * easy way for importers to verify that the same subtransaction is still
     * running.
     */
    if (IsSubTransaction())
        ereport(ERROR,
                (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
                 errmsg("cannot export a snapshot from a subtransaction")));

    /*
     * We do however allow previous committed subtransactions to exist.
     * Importers of the snapshot must see them as still running, so get their
     * XIDs to add them to the snapshot.
     */
    nchildren = xactGetCommittedChildren(&children);

    /*
     * Generate file path for the snapshot.  We start numbering of snapshots
     * inside the transaction from 1.
     */
    snprintf(path, sizeof(path), SNAPSHOT_EXPORT_DIR "/%08X-%08X-%d",
             MyProc->backendId, MyProc->lxid, list_length(exportedSnapshots) + 1);

    /*
     * Copy the snapshot into TopTransactionContext, add it to the
     * exportedSnapshots list, and mark it pseudo-registered.  We do this to
     * ensure that the snapshot's xmin is honored for the rest of the
     * transaction.
     */
    snapshot = CopySnapshot(snapshot);

    oldcxt = MemoryContextSwitchTo(TopTransactionContext);
    esnap = (ExportedSnapshot *) palloc(sizeof(ExportedSnapshot));
    esnap->snapfile = pstrdup(path);
    esnap->snapshot = snapshot;
    exportedSnapshots = lappend(exportedSnapshots, esnap);
    MemoryContextSwitchTo(oldcxt);

    snapshot->regd_count++;
    pairingheap_add(&RegisteredSnapshots, &snapshot->ph_node);

    /*
     * Fill buf with a text serialization of the snapshot, plus identification
     * data about this transaction.  The format expected by ImportSnapshot is
     * pretty rigid: each line must be fieldname:value.
     */
    initStringInfo(&buf);

    appendStringInfo(&buf, "vxid:%d/%u\n", MyProc->backendId, MyProc->lxid);
    appendStringInfo(&buf, "pid:%d\n", MyProcPid);
    appendStringInfo(&buf, "dbid:%u\n", MyDatabaseId);
    appendStringInfo(&buf, "iso:%d\n", XactIsoLevel);
    appendStringInfo(&buf, "ro:%d\n", XactReadOnly);

    appendStringInfo(&buf, "xmin:%u\n", snapshot->xmin);
    appendStringInfo(&buf, "xmax:%u\n", snapshot->xmax);

    /*
     * We must include our own top transaction ID in the top-xid data, since
     * by definition we will still be running when the importing transaction
     * adopts the snapshot, but GetSnapshotData never includes our own XID in
     * the snapshot.  (There must, therefore, be enough room to add it.)
     *
     * However, it could be that our topXid is after the xmax, in which case
     * we shouldn't include it because xip[] members are expected to be before
     * xmax.  (We need not make the same check for subxip[] members, see
     * snapshot.h.)
     */
    addTopXid = (TransactionIdIsValid(topXid) &&
                 TransactionIdPrecedes(topXid, snapshot->xmax)) ? 1 : 0;
    appendStringInfo(&buf, "xcnt:%d\n", snapshot->xcnt + addTopXid);
    for (i = 0; i < snapshot->xcnt; i++)
        appendStringInfo(&buf, "xip:%u\n", snapshot->xip[i]);
    if (addTopXid)
        appendStringInfo(&buf, "xip:%u\n", topXid);

    /*
     * Similarly, we add our subcommitted child XIDs to the subxid data. Here,
     * we have to cope with possible overflow.
     */
    if (snapshot->suboverflowed ||
        snapshot->subxcnt + nchildren > GetMaxSnapshotSubxidCount())
        appendStringInfoString(&buf, "sof:1\n");
    else
    {
        appendStringInfoString(&buf, "sof:0\n");
        appendStringInfo(&buf, "sxcnt:%d\n", snapshot->subxcnt + nchildren);
        for (i = 0; i < snapshot->subxcnt; i++)
            appendStringInfo(&buf, "sxp:%u\n", snapshot->subxip[i]);
        for (i = 0; i < nchildren; i++)
            appendStringInfo(&buf, "sxp:%u\n", children[i]);
    }
    appendStringInfo(&buf, "rec:%u\n", snapshot->takenDuringRecovery);

    /*
     * Now write the text representation into a file.  We first write to a
     * ".tmp" filename, and rename to final filename if no error.  This
     * ensures that no other backend can read an incomplete file
     * (ImportSnapshot won't allow it because of its valid-characters check).
     */
    snprintf(pathtmp, sizeof(pathtmp), "%s.tmp", path);
    if (!(f = AllocateFile(pathtmp, PG_BINARY_W)))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not create file \"%s\": %m", pathtmp)));

    if (fwrite(buf.data, buf.len, 1, f) != 1)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to file \"%s\": %m", pathtmp)));

    /* no fsync() since file need not survive a system crash */

    if (FreeFile(f))
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not write to file \"%s\": %m", pathtmp)));

    /*
     * Now that we have written everything into a .tmp file, rename the file
     * to remove the .tmp suffix.
     */
    if (rename(pathtmp, path) < 0)
        ereport(ERROR,
                (errcode_for_file_access(),
                 errmsg("could not rename file \"%s\" to \"%s\": %m",
                        pathtmp, path)));

    /*
     * The basename of the file is what we return from pg_export_snapshot().
     * It's already in path in a textual format and we know that the path
     * starts with SNAPSHOT_EXPORT_DIR.  Skip over the prefix and the slash
     * and pstrdup it so as not to return the address of a local variable.
     */
    return pstrdup(path + strlen(SNAPSHOT_EXPORT_DIR) + 1);
}

/*
 * pg_export_snapshot
 *      SQL-callable wrapper for ExportSnapshot.
 */
Datum
pg_export_snapshot(PG_FUNCTION_ARGS)
{
    char       *snapshotName;

    snapshotName = ExportSnapshot(GetActiveSnapshot());
    PG_RETURN_TEXT_P(cstring_to_text(snapshotName));
}


/*
 * Parsing subroutines for ImportSnapshot: parse a line with the given
 * prefix followed by a value, and advance *s to the next line.  The
 * filename is provided for use in error messages.
 */
static int
parseIntFromText(const char *prefix, char **s, const char *filename)
{
    char       *ptr = *s;
    int         prefixlen = strlen(prefix);
    int         val;

    if (strncmp(ptr, prefix, prefixlen) != 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr += prefixlen;
    if (sscanf(ptr, "%d", &val) != 1)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr = strchr(ptr, '\n');
    if (!ptr)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    *s = ptr + 1;
    return val;
}

static TransactionId
parseXidFromText(const char *prefix, char **s, const char *filename)
{
    char       *ptr = *s;
    int         prefixlen = strlen(prefix);
    TransactionId val;

    if (strncmp(ptr, prefix, prefixlen) != 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr += prefixlen;
    if (sscanf(ptr, "%u", &val) != 1)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr = strchr(ptr, '\n');
    if (!ptr)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    *s = ptr + 1;
    return val;
}

static void
parseVxidFromText(const char *prefix, char **s, const char *filename,
                  VirtualTransactionId *vxid)
{
    char       *ptr = *s;
    int         prefixlen = strlen(prefix);

    if (strncmp(ptr, prefix, prefixlen) != 0)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr += prefixlen;
    if (sscanf(ptr, "%d/%u", &vxid->backendId, &vxid->localTransactionId) != 2)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    ptr = strchr(ptr, '\n');
    if (!ptr)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", filename)));
    *s = ptr + 1;
}

/*
 * ImportSnapshot
 *      Import a previously exported snapshot.  The argument should be a
 *      filename in SNAPSHOT_EXPORT_DIR.  Load the snapshot from that file.
 *      This is called by "SET TRANSACTION SNAPSHOT 'foo'".
 */
void
ImportSnapshot(const char *idstr)
{
    char        path[MAXPGPATH];
    FILE       *f;
    struct stat stat_buf;
    char       *filebuf;
    int         xcnt;
    int         i;
    VirtualTransactionId src_vxid;
    int         src_pid;
    Oid         src_dbid;
    int         src_isolevel;
    bool        src_readonly;
    SnapshotData snapshot;

    /*
     * Must be at top level of a fresh transaction.  Note in particular that
     * we check we haven't acquired an XID --- if we have, it's conceivable
     * that the snapshot would show it as not running, making for very screwy
     * behavior.
     */
    if (FirstSnapshotSet ||
        GetTopTransactionIdIfAny() != InvalidTransactionId ||
        IsSubTransaction())
        ereport(ERROR,
                (errcode(ERRCODE_ACTIVE_SQL_TRANSACTION),
                 errmsg("SET TRANSACTION SNAPSHOT must be called before any query")));

    /*
     * If we are in read committed mode then the next query would execute with
     * a new snapshot thus making this function call quite useless.
     */
    if (!IsolationUsesXactSnapshot())
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("a snapshot-importing transaction must have isolation level SERIALIZABLE or REPEATABLE READ")));

    /*
     * Verify the identifier: only 0-9, A-F and hyphens are allowed.  We do
     * this mainly to prevent reading arbitrary files.
     */
    if (strspn(idstr, "0123456789ABCDEF-") != strlen(idstr))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid snapshot identifier: \"%s\"", idstr)));

    /* OK, read the file */
    snprintf(path, MAXPGPATH, SNAPSHOT_EXPORT_DIR "/%s", idstr);

    f = AllocateFile(path, PG_BINARY_R);
    if (!f)
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
                 errmsg("invalid snapshot identifier: \"%s\"", idstr)));

    /* get the size of the file so that we know how much memory we need */
    if (fstat(fileno(f), &stat_buf))
        elog(ERROR, "could not stat file \"%s\": %m", path);

    /* and read the file into a palloc'd string */
    filebuf = (char *) palloc(stat_buf.st_size + 1);
    if (fread(filebuf, stat_buf.st_size, 1, f) != 1)
        elog(ERROR, "could not read file \"%s\": %m", path);

    filebuf[stat_buf.st_size] = '\0';

    FreeFile(f);

    /*
     * Construct a snapshot struct by parsing the file content.
     */
    memset(&snapshot, 0, sizeof(snapshot));

    parseVxidFromText("vxid:", &filebuf, path, &src_vxid);
    src_pid = parseIntFromText("pid:", &filebuf, path);
    /* we abuse parseXidFromText a bit here ... */
    src_dbid = parseXidFromText("dbid:", &filebuf, path);
    src_isolevel = parseIntFromText("iso:", &filebuf, path);
    src_readonly = parseIntFromText("ro:", &filebuf, path);

    snapshot.snapshot_type = SNAPSHOT_MVCC;

    snapshot.xmin = parseXidFromText("xmin:", &filebuf, path);
    snapshot.xmax = parseXidFromText("xmax:", &filebuf, path);

    snapshot.xcnt = xcnt = parseIntFromText("xcnt:", &filebuf, path);

    /* sanity-check the xid count before palloc */
    if (xcnt < 0 || xcnt > GetMaxSnapshotXidCount())
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", path)));

    snapshot.xip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
    for (i = 0; i < xcnt; i++)
        snapshot.xip[i] = parseXidFromText("xip:", &filebuf, path);

    snapshot.suboverflowed = parseIntFromText("sof:", &filebuf, path);

    if (!snapshot.suboverflowed)
    {
        snapshot.subxcnt = xcnt = parseIntFromText("sxcnt:", &filebuf, path);

        /* sanity-check the xid count before palloc */
        if (xcnt < 0 || xcnt > GetMaxSnapshotSubxidCount())
            ereport(ERROR,
                    (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                     errmsg("invalid snapshot data in file \"%s\"", path)));

        snapshot.subxip = (TransactionId *) palloc(xcnt * sizeof(TransactionId));
        for (i = 0; i < xcnt; i++)
            snapshot.subxip[i] = parseXidFromText("sxp:", &filebuf, path);
    }
    else
    {
        snapshot.subxcnt = 0;
        snapshot.subxip = NULL;
    }

    snapshot.takenDuringRecovery = parseIntFromText("rec:", &filebuf, path);

    /*
     * Do some additional sanity checking, just to protect ourselves.  We
     * don't trouble to check the array elements, just the most critical
     * fields.
     */
    if (!VirtualTransactionIdIsValid(src_vxid) ||
        !OidIsValid(src_dbid) ||
        !TransactionIdIsNormal(snapshot.xmin) ||
        !TransactionIdIsNormal(snapshot.xmax))
        ereport(ERROR,
                (errcode(ERRCODE_INVALID_TEXT_REPRESENTATION),
                 errmsg("invalid snapshot data in file \"%s\"", path)));

    /*
     * If we're serializable, the source transaction must be too, otherwise
     * predicate.c has problems (SxactGlobalXmin could go backwards).  Also, a
     * non-read-only transaction can't adopt a snapshot from a read-only
     * transaction, as predicate.c handles the cases very differently.
     */
    if (IsolationIsSerializable())
    {
        if (src_isolevel != XACT_SERIALIZABLE)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("a serializable transaction cannot import a snapshot from a non-serializable transaction")));
        if (src_readonly && !XactReadOnly)
            ereport(ERROR,
                    (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                     errmsg("a non-read-only serializable transaction cannot import a snapshot from a read-only transaction")));
    }

    /*
     * We cannot import a snapshot that was taken in a different database,
     * because vacuum calculates OldestXmin on a per-database basis; so the
     * source transaction's xmin doesn't protect us from data loss.  This
     * restriction could be removed if the source transaction were to mark its
     * xmin as being globally applicable.  But that would require some
     * additional syntax, since that has to be known when the snapshot is
     * initially taken.  (See pgsql-hackers discussion of 2011-10-21.)
     */
    if (src_dbid != MyDatabaseId)
        ereport(ERROR,
                (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
                 errmsg("cannot import a snapshot from a different database")));

    /* OK, install the snapshot */
    SetTransactionSnapshot(&snapshot, &src_vxid, src_pid, NULL);
}

/*
 * XactHasExportedSnapshots
 *      Test whether current transaction has exported any snapshots.
 */
bool
XactHasExportedSnapshots(void)
{
    return (exportedSnapshots != NIL);
}

/*
 * DeleteAllExportedSnapshotFiles
 *      Clean up any files that have been left behind by a crashed backend
 *      that had exported snapshots before it died.
 *
 * This should be called during database startup or crash recovery.
 */
void
DeleteAllExportedSnapshotFiles(void)
{
    char        buf[MAXPGPATH + sizeof(SNAPSHOT_EXPORT_DIR)];
    DIR        *s_dir;
    struct dirent *s_de;

    /*
     * Problems in reading the directory, or unlinking files, are reported at
     * LOG level.  Since we're running in the startup process, ERROR level
     * would prevent database start, and it's not important enough for that.
     */
    s_dir = AllocateDir(SNAPSHOT_EXPORT_DIR);

    while ((s_de = ReadDirExtended(s_dir, SNAPSHOT_EXPORT_DIR, LOG)) != NULL)
    {
        if (strcmp(s_de->d_name, ".") == 0 ||
            strcmp(s_de->d_name, "..") == 0)
            continue;

        snprintf(buf, sizeof(buf), SNAPSHOT_EXPORT_DIR "/%s", s_de->d_name);

        if (unlink(buf) != 0)
            ereport(LOG,
                    (errcode_for_file_access(),
                     errmsg("could not remove file \"%s\": %m", buf)));
    }

    FreeDir(s_dir);
}

/*
 * ThereAreNoPriorRegisteredSnapshots
 *      Is the registered snapshot count less than or equal to one?
 *
 * Don't use this to settle important decisions.  While zero registrations and
 * no ActiveSnapshot would confirm a certain idleness, the system makes no
 * guarantees about the significance of one registered snapshot.
 */
bool
ThereAreNoPriorRegisteredSnapshots(void)
{
    if (pairingheap_is_empty(&RegisteredSnapshots) ||
        pairingheap_is_singular(&RegisteredSnapshots))
        return true;

    return false;
}


/*
 * Return a timestamp that is exactly on a minute boundary.
 *
 * If the argument is already aligned, return that value, otherwise move to
 * the next minute boundary following the given time.
 */
static TimestampTz
AlignTimestampToMinuteBoundary(TimestampTz ts)
{
    TimestampTz retval = ts + (USECS_PER_MINUTE - 1);

    return retval - (retval % USECS_PER_MINUTE);
}

/*
 * Get current timestamp for snapshots
 *
 * This is basically GetCurrentTimestamp(), but with a guarantee that
 * the result never moves backward.
 */
TimestampTz
GetSnapshotCurrentTimestamp(void)
{
    TimestampTz now = GetCurrentTimestamp();

    /*
     * Don't let time move backward; if it hasn't advanced, use the old value.
     */
    SpinLockAcquire(&oldSnapshotControl->mutex_current);
    if (now <= oldSnapshotControl->current_timestamp)
        now = oldSnapshotControl->current_timestamp;
    else
        oldSnapshotControl->current_timestamp = now;
    SpinLockRelease(&oldSnapshotControl->mutex_current);

    return now;
}

/*
 * Get timestamp through which vacuum may have processed based on last stored
 * value for threshold_timestamp.
 *
 * XXX: So far, we never trust that a 64-bit value can be read atomically; if
 * that ever changes, we could get rid of the spinlock here.
 */
TimestampTz
GetOldSnapshotThresholdTimestamp(void)
{
    TimestampTz threshold_timestamp;

    SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
    threshold_timestamp = oldSnapshotControl->threshold_timestamp;
    SpinLockRelease(&oldSnapshotControl->mutex_threshold);

    return threshold_timestamp;
}

static void
SetOldSnapshotThresholdTimestamp(TimestampTz ts, TransactionId xlimit)
{
    SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
    oldSnapshotControl->threshold_timestamp = ts;
    oldSnapshotControl->threshold_xid = xlimit;
    SpinLockRelease(&oldSnapshotControl->mutex_threshold);
}

/*
 * TransactionIdLimitedForOldSnapshots
 *
 * Apply old snapshot limit, if any.  This is intended to be called for page
 * pruning and table vacuuming, to allow old_snapshot_threshold to override
 * the normal global xmin value.  Actual testing for snapshot too old will be
 * based on whether a snapshot timestamp is prior to the threshold timestamp
 * set in this function.
 */
TransactionId
TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
                                    Relation relation)
{
    if (TransactionIdIsNormal(recentXmin)
        && old_snapshot_threshold >= 0
        && RelationAllowsEarlyPruning(relation))
    {
        TimestampTz ts = GetSnapshotCurrentTimestamp();
        TransactionId xlimit = recentXmin;
        TransactionId latest_xmin;
        TimestampTz update_ts;
        bool        same_ts_as_threshold = false;

        SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
        latest_xmin = oldSnapshotControl->latest_xmin;
        update_ts = oldSnapshotControl->next_map_update;
        SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);

        /*
         * Zero threshold always overrides to latest xmin, if valid.  Without
         * some heuristic it will find its own snapshot too old on, for
         * example, a simple UPDATE -- which would make it useless for most
         * testing, but there is no principled way to ensure that it doesn't
         * fail in this way.  Use a five-second delay to try to get useful
         * testing behavior, but this may need adjustment.
         */
        if (old_snapshot_threshold == 0)
        {
            if (TransactionIdPrecedes(latest_xmin, MyPgXact->xmin)
                && TransactionIdFollows(latest_xmin, xlimit))
                xlimit = latest_xmin;

            ts -= 5 * USECS_PER_SEC;
            SetOldSnapshotThresholdTimestamp(ts, xlimit);

            return xlimit;
        }

        ts = AlignTimestampToMinuteBoundary(ts)
            - (old_snapshot_threshold * USECS_PER_MINUTE);

        /* Check for fast exit without LW locking. */
        SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
        if (ts == oldSnapshotControl->threshold_timestamp)
        {
            xlimit = oldSnapshotControl->threshold_xid;
            same_ts_as_threshold = true;
        }
        SpinLockRelease(&oldSnapshotControl->mutex_threshold);

        if (!same_ts_as_threshold)
        {
            if (ts == update_ts)
            {
                xlimit = latest_xmin;
                if (NormalTransactionIdFollows(xlimit, recentXmin))
                    SetOldSnapshotThresholdTimestamp(ts, xlimit);
            }
            else
            {
                LWLockAcquire(OldSnapshotTimeMapLock, LW_SHARED);

                if (oldSnapshotControl->count_used > 0
                    && ts >= oldSnapshotControl->head_timestamp)
                {
                    int         offset;

                    offset = ((ts - oldSnapshotControl->head_timestamp)
                              / USECS_PER_MINUTE);
                    if (offset > oldSnapshotControl->count_used - 1)
                        offset = oldSnapshotControl->count_used - 1;
                    offset = (oldSnapshotControl->head_offset + offset)
                        % OLD_SNAPSHOT_TIME_MAP_ENTRIES;
                    xlimit = oldSnapshotControl->xid_by_minute[offset];

                    if (NormalTransactionIdFollows(xlimit, recentXmin))
                        SetOldSnapshotThresholdTimestamp(ts, xlimit);
                }

                LWLockRelease(OldSnapshotTimeMapLock);
            }
        }

        /*
         * Failsafe protection against vacuuming work of active transaction.
         *
         * This is not an assertion because we avoid the spinlock for
         * performance, leaving open the possibility that xlimit could advance
         * and be more current; but it seems prudent to apply this limit.  It
         * might make pruning a tiny bit less aggressive than it could be, but
         * protects against data loss bugs.
         */
        if (TransactionIdIsNormal(latest_xmin)
            && TransactionIdPrecedes(latest_xmin, xlimit))
            xlimit = latest_xmin;

        if (NormalTransactionIdFollows(xlimit, recentXmin))
            return xlimit;
    }

    return recentXmin;
}

/*
 * Take care of the circular buffer that maps time to xid.
 */
void
MaintainOldSnapshotTimeMapping(TimestampTz whenTaken, TransactionId xmin)
{
    TimestampTz ts;
    TransactionId latest_xmin;
    TimestampTz update_ts;
    bool        map_update_required = false;

    /* Never call this function when old snapshot checking is disabled. */
    Assert(old_snapshot_threshold >= 0);

    ts = AlignTimestampToMinuteBoundary(whenTaken);

    /*
     * Keep track of the latest xmin seen by any process. Update mapping with
     * a new value when we have crossed a bucket boundary.
     */
    SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
    latest_xmin = oldSnapshotControl->latest_xmin;
    update_ts = oldSnapshotControl->next_map_update;
    if (ts > update_ts)
    {
        oldSnapshotControl->next_map_update = ts;
        map_update_required = true;
    }
    if (TransactionIdFollows(xmin, latest_xmin))
        oldSnapshotControl->latest_xmin = xmin;
    SpinLockRelease(&oldSnapshotControl->mutex_latest_xmin);

    /* We only needed to update the most recent xmin value. */
    if (!map_update_required)
        return;

    /* No further tracking needed for 0 (used for testing). */
    if (old_snapshot_threshold == 0)
        return;

    /*
     * We don't want to do something stupid with unusual values, but we don't
     * want to litter the log with warnings or break otherwise normal
     * processing for this feature; so if something seems unreasonable, just
     * log at DEBUG level and return without doing anything.
     */
    if (whenTaken < 0)
    {
        elog(DEBUG1,
             "MaintainOldSnapshotTimeMapping called with negative whenTaken = %ld",
             (long) whenTaken);
        return;
    }
    if (!TransactionIdIsNormal(xmin))
    {
        elog(DEBUG1,
             "MaintainOldSnapshotTimeMapping called with xmin = %lu",
             (unsigned long) xmin);
        return;
    }

    LWLockAcquire(OldSnapshotTimeMapLock, LW_EXCLUSIVE);

    Assert(oldSnapshotControl->head_offset >= 0);
    Assert(oldSnapshotControl->head_offset < OLD_SNAPSHOT_TIME_MAP_ENTRIES);
    Assert((oldSnapshotControl->head_timestamp % USECS_PER_MINUTE) == 0);
    Assert(oldSnapshotControl->count_used >= 0);
    Assert(oldSnapshotControl->count_used <= OLD_SNAPSHOT_TIME_MAP_ENTRIES);

    if (oldSnapshotControl->count_used == 0)
    {
        /* set up first entry for empty mapping */
        oldSnapshotControl->head_offset = 0;
        oldSnapshotControl->head_timestamp = ts;
        oldSnapshotControl->count_used = 1;
        oldSnapshotControl->xid_by_minute[0] = xmin;
    }
    else if (ts < oldSnapshotControl->head_timestamp)
    {
        /* old ts; log it at DEBUG */
        LWLockRelease(OldSnapshotTimeMapLock);
        elog(DEBUG1,
             "MaintainOldSnapshotTimeMapping called with old whenTaken = %ld",
             (long) whenTaken);
        return;
    }
    else if (ts <= (oldSnapshotControl->head_timestamp +
                    ((oldSnapshotControl->count_used - 1)
                     * USECS_PER_MINUTE)))
    {
        /* existing mapping; advance xid if possible */
        int         bucket = (oldSnapshotControl->head_offset
                              + ((ts - oldSnapshotControl->head_timestamp)
                                 / USECS_PER_MINUTE))
        % OLD_SNAPSHOT_TIME_MAP_ENTRIES;

        if (TransactionIdPrecedes(oldSnapshotControl->xid_by_minute[bucket], xmin))
            oldSnapshotControl->xid_by_minute[bucket] = xmin;
    }
    else
    {
        /* We need a new bucket, but it might not be the very next one. */
        int         advance = ((ts - oldSnapshotControl->head_timestamp)
                               / USECS_PER_MINUTE);

        oldSnapshotControl->head_timestamp = ts;

        if (advance >= OLD_SNAPSHOT_TIME_MAP_ENTRIES)
        {
            /* Advance is so far that all old data is junk; start over. */
            oldSnapshotControl->head_offset = 0;
            oldSnapshotControl->count_used = 1;
            oldSnapshotControl->xid_by_minute[0] = xmin;
        }
        else
        {
            /* Store the new value in one or more buckets. */
            int         i;

            for (i = 0; i < advance; i++)
            {
                if (oldSnapshotControl->count_used == OLD_SNAPSHOT_TIME_MAP_ENTRIES)
                {
                    /* Map full and new value replaces old head. */
                    int         old_head = oldSnapshotControl->head_offset;

                    if (old_head == (OLD_SNAPSHOT_TIME_MAP_ENTRIES - 1))
                        oldSnapshotControl->head_offset = 0;
                    else
                        oldSnapshotControl->head_offset = old_head + 1;
                    oldSnapshotControl->xid_by_minute[old_head] = xmin;
                }
                else
                {
                    /* Extend map to unused entry. */
                    int         new_tail = (oldSnapshotControl->head_offset
                                            + oldSnapshotControl->count_used)
                    % OLD_SNAPSHOT_TIME_MAP_ENTRIES;

                    oldSnapshotControl->count_used++;
                    oldSnapshotControl->xid_by_minute[new_tail] = xmin;
                }
            }
        }
    }

    LWLockRelease(OldSnapshotTimeMapLock);
}


/*
 * Setup a snapshot that replaces normal catalog snapshots that allows catalog
 * access to behave just like it did at a certain point in the past.
 *
 * Needed for logical decoding.
 */
void
SetupHistoricSnapshot(Snapshot historic_snapshot, HTAB *tuplecids)
{
    Assert(historic_snapshot != NULL);

    /* setup the timetravel snapshot */
    HistoricSnapshot = historic_snapshot;

    /* setup (cmin, cmax) lookup hash */
    tuplecid_data = tuplecids;
}


/*
 * Make catalog snapshots behave normally again.
 */
void
TeardownHistoricSnapshot(bool is_error)
{
    HistoricSnapshot = NULL;
    tuplecid_data = NULL;
}

bool
HistoricSnapshotActive(void)
{
    return HistoricSnapshot != NULL;
}

HTAB *
HistoricSnapshotGetTupleCids(void)
{
    Assert(HistoricSnapshotActive());
    return tuplecid_data;
}

/*
 * EstimateSnapshotSpace
 *      Returns the size needed to store the given snapshot.
 *
 * We are exporting only required fields from the Snapshot, stored in
 * SerializedSnapshotData.
 */
Size
EstimateSnapshotSpace(Snapshot snap)
{
    Size        size;

    Assert(snap != InvalidSnapshot);
    Assert(snap->snapshot_type == SNAPSHOT_MVCC);

    /* We allocate any XID arrays needed in the same palloc block. */
    size = add_size(sizeof(SerializedSnapshotData),
                    mul_size(snap->xcnt, sizeof(TransactionId)));
    if (snap->subxcnt > 0 &&
        (!snap->suboverflowed || snap->takenDuringRecovery))
        size = add_size(size,
                        mul_size(snap->subxcnt, sizeof(TransactionId)));

    return size;
}

/*
 * SerializeSnapshot
 *      Dumps the serialized snapshot (extracted from given snapshot) onto the
 *      memory location at start_address.
 */
void
SerializeSnapshot(Snapshot snapshot, char *start_address)
{
    SerializedSnapshotData serialized_snapshot;

    Assert(snapshot->subxcnt >= 0);

    /* Copy all required fields */
    serialized_snapshot.xmin = snapshot->xmin;
    serialized_snapshot.xmax = snapshot->xmax;
    serialized_snapshot.xcnt = snapshot->xcnt;
    serialized_snapshot.subxcnt = snapshot->subxcnt;
    serialized_snapshot.suboverflowed = snapshot->suboverflowed;
    serialized_snapshot.takenDuringRecovery = snapshot->takenDuringRecovery;
    serialized_snapshot.curcid = snapshot->curcid;
    serialized_snapshot.whenTaken = snapshot->whenTaken;
    serialized_snapshot.lsn = snapshot->lsn;

    /*
     * Ignore the SubXID array if it has overflowed, unless the snapshot was
     * taken during recovery - in that case, top-level XIDs are in subxip as
     * well, and we mustn't lose them.
     */
    if (serialized_snapshot.suboverflowed && !snapshot->takenDuringRecovery)
        serialized_snapshot.subxcnt = 0;

    /* Copy struct to possibly-unaligned buffer */
    memcpy(start_address,
           &serialized_snapshot, sizeof(SerializedSnapshotData));

    /* Copy XID array */
    if (snapshot->xcnt > 0)
        memcpy((TransactionId *) (start_address +
                                  sizeof(SerializedSnapshotData)),
               snapshot->xip, snapshot->xcnt * sizeof(TransactionId));

    /*
     * Copy SubXID array. Don't bother to copy it if it had overflowed,
     * though, because it's not used anywhere in that case. Except if it's a
     * snapshot taken during recovery; all the top-level XIDs are in subxip as
     * well in that case, so we mustn't lose them.
     */
    if (serialized_snapshot.subxcnt > 0)
    {
        Size        subxipoff = sizeof(SerializedSnapshotData) +
        snapshot->xcnt * sizeof(TransactionId);

        memcpy((TransactionId *) (start_address + subxipoff),
               snapshot->subxip, snapshot->subxcnt * sizeof(TransactionId));
    }
}

/*
 * RestoreSnapshot
 *      Restore a serialized snapshot from the specified address.
 *
 * The copy is palloc'd in TopTransactionContext and has initial refcounts set
 * to 0.  The returned snapshot has the copied flag set.
 */
Snapshot
RestoreSnapshot(char *start_address)
{
    SerializedSnapshotData serialized_snapshot;
    Size        size;
    Snapshot    snapshot;
    TransactionId *serialized_xids;

    memcpy(&serialized_snapshot, start_address,
           sizeof(SerializedSnapshotData));
    serialized_xids = (TransactionId *)
        (start_address + sizeof(SerializedSnapshotData));

    /* We allocate any XID arrays needed in the same palloc block. */
    size = sizeof(SnapshotData)
        + serialized_snapshot.xcnt * sizeof(TransactionId)
        + serialized_snapshot.subxcnt * sizeof(TransactionId);

    /* Copy all required fields */
    snapshot = (Snapshot) MemoryContextAlloc(TopTransactionContext, size);
    snapshot->snapshot_type = SNAPSHOT_MVCC;
    snapshot->xmin = serialized_snapshot.xmin;
    snapshot->xmax = serialized_snapshot.xmax;
    snapshot->xip = NULL;
    snapshot->xcnt = serialized_snapshot.xcnt;
    snapshot->subxip = NULL;
    snapshot->subxcnt = serialized_snapshot.subxcnt;
    snapshot->suboverflowed = serialized_snapshot.suboverflowed;
    snapshot->takenDuringRecovery = serialized_snapshot.takenDuringRecovery;
    snapshot->curcid = serialized_snapshot.curcid;
    snapshot->whenTaken = serialized_snapshot.whenTaken;
    snapshot->lsn = serialized_snapshot.lsn;

    /* Copy XIDs, if present. */
    if (serialized_snapshot.xcnt > 0)
    {
        snapshot->xip = (TransactionId *) (snapshot + 1);
        memcpy(snapshot->xip, serialized_xids,
               serialized_snapshot.xcnt * sizeof(TransactionId));
    }

    /* Copy SubXIDs, if present. */
    if (serialized_snapshot.subxcnt > 0)
    {
        snapshot->subxip = ((TransactionId *) (snapshot + 1)) +
            serialized_snapshot.xcnt;
        memcpy(snapshot->subxip, serialized_xids + serialized_snapshot.xcnt,
               serialized_snapshot.subxcnt * sizeof(TransactionId));
    }

    /* Set the copied flag so that the caller will set refcounts correctly. */
    snapshot->regd_count = 0;
    snapshot->active_count = 0;
    snapshot->copied = true;

    return snapshot;
}

/*
 * Install a restored snapshot as the transaction snapshot.
 *
 * The second argument is of type void * so that snapmgr.h need not include
 * the declaration for PGPROC.
 */
void
RestoreTransactionSnapshot(Snapshot snapshot, void *master_pgproc)
{
    SetTransactionSnapshot(snapshot, NULL, InvalidPid, master_pgproc);
}

/*
 * XidInMVCCSnapshot
 *      Is the given XID still-in-progress according to the snapshot?
 *
 * Note: GetSnapshotData never stores either top xid or subxids of our own
 * backend into a snapshot, so these xids will not be reported as "running"
 * by this function.  This is OK for current uses, because we always check
 * TransactionIdIsCurrentTransactionId first, except when it's known the
 * XID could not be ours anyway.
 */
bool
XidInMVCCSnapshot(TransactionId xid, Snapshot snapshot)
{
    uint32      i;

    /*
     * Make a quick range check to eliminate most XIDs without looking at the
     * xip arrays.  Note that this is OK even if we convert a subxact XID to
     * its parent below, because a subxact with XID < xmin has surely also got
     * a parent with XID < xmin, while one with XID >= xmax must belong to a
     * parent that was not yet committed at the time of this snapshot.
     */

    /* Any xid < xmin is not in-progress */
    if (TransactionIdPrecedes(xid, snapshot->xmin))
        return false;
    /* Any xid >= xmax is in-progress */
    if (TransactionIdFollowsOrEquals(xid, snapshot->xmax))
        return true;

    /*
     * Snapshot information is stored slightly differently in snapshots taken
     * during recovery.
     */
    if (!snapshot->takenDuringRecovery)
    {
        /*
         * If the snapshot contains full subxact data, the fastest way to
         * check things is just to compare the given XID against both subxact
         * XIDs and top-level XIDs.  If the snapshot overflowed, we have to
         * use pg_subtrans to convert a subxact XID to its parent XID, but
         * then we need only look at top-level XIDs not subxacts.
         */
        if (!snapshot->suboverflowed)
        {
            /* we have full data, so search subxip */
            int32       j;

            for (j = 0; j < snapshot->subxcnt; j++)
            {
                if (TransactionIdEquals(xid, snapshot->subxip[j]))
                    return true;
            }

            /* not there, fall through to search xip[] */
        }
        else
        {
            /*
             * Snapshot overflowed, so convert xid to top-level.  This is safe
             * because we eliminated too-old XIDs above.
             */
            xid = SubTransGetTopmostTransaction(xid);

            /*
             * If xid was indeed a subxact, we might now have an xid < xmin,
             * so recheck to avoid an array scan.  No point in rechecking
             * xmax.
             */
            if (TransactionIdPrecedes(xid, snapshot->xmin))
                return false;
        }

        for (i = 0; i < snapshot->xcnt; i++)
        {
            if (TransactionIdEquals(xid, snapshot->xip[i]))
                return true;
        }
    }
    else
    {
        int32       j;

        /*
         * In recovery we store all xids in the subxact array because it is by
         * far the bigger array, and we mostly don't know which xids are
         * top-level and which are subxacts. The xip array is empty.
         *
         * We start by searching subtrans, if we overflowed.
         */
        if (snapshot->suboverflowed)
        {
            /*
             * Snapshot overflowed, so convert xid to top-level.  This is safe
             * because we eliminated too-old XIDs above.
             */
            xid = SubTransGetTopmostTransaction(xid);

            /*
             * If xid was indeed a subxact, we might now have an xid < xmin,
             * so recheck to avoid an array scan.  No point in rechecking
             * xmax.
             */
            if (TransactionIdPrecedes(xid, snapshot->xmin))
                return false;
        }

        /*
         * We now have either a top-level xid higher than xmin or an
         * indeterminate xid. We don't know whether it's top level or subxact
         * but it doesn't matter. If it's present, the xid is visible.
         */
        for (j = 0; j < snapshot->subxcnt; j++)
        {
            if (TransactionIdEquals(xid, snapshot->subxip[j]))
                return true;
        }
    }

    return false;
}