path: root/src/backend/utils/time/snapmgr.c

/*-------------------------------------------------------------------------
 *
 * 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 MyProc->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 MyProc->xmin except when the active-snapshot
 * stack is empty.
 *
 *
 * Portions Copyright (c) 1996-2021, 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 "datatype/timestamp.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/old_snapshot.h"
#include "utils/rel.h"
#include "utils/resowner_private.h"
#include "utils/snapmgr.h"
#include "utils/syscache.h"
#include "utils/timestamp.h"


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

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.
 */
TransactionId TransactionXmin = FirstNormalTransactionId;
TransactionId RecentXmin = FirstNormalTransactionId;

/* (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 MyProc->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 PGPROC->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
	 * the state for GlobalVis*.
	 */
	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 */

	CurrentSnapshot->snapXactCompletionCount = 0;

	/*
	 * Now we have to fix what GetSnapshotData did with MyProc->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;
	newsnap->snapXactCompletionCount = 0;

	/* 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)
{
	PushActiveSnapshotWithLevel(snap, GetCurrentTransactionNestLevel());
}

/*
 * PushActiveSnapshotWithLevel
 *		Set the given snapshot as the current active snapshot
 *
 * Same as PushActiveSnapshot except that caller can specify the
 * transaction nesting level that "owns" the snapshot.  This level
 * must not be deeper than the current top of the snapshot stack.
 */
void
PushActiveSnapshotWithLevel(Snapshot snap, int snap_level)
{
	ActiveSnapshotElt *newactive;

	Assert(snap != InvalidSnapshot);
	Assert(ActiveSnapshot == NULL || snap_level >= ActiveSnapshot->as_level);

	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 = snap_level;

	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;
}

/*
 * SnapshotResetXmin
 *
 * If there are no more snapshots, we can reset our PGPROC->xmin to
 * InvalidTransactionId. 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
 * PGPROC->xmin to some degree.  This typically happens when a portal is
 * dropped.  For efficiency, we only consider recomputing PGPROC->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))
	{
		MyProc->xmin = InvalidTransactionId;
		return;
	}

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

	if (TransactionIdPrecedes(MyProc->xmin, minSnapshot->xmin))
		MyProc->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 MyProc->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 || MyProc->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;
}

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

/*
 * XXX: Magic to keep old_snapshot_threshold tests appear "working". They
 * currently are broken, and discussion of what to do about them is
 * ongoing. See
 * https://www.postgresql.org/message-id/20200403001235.e6jfdll3gh2ygbuc%40alap3.anarazel.de
 */
void
SnapshotTooOldMagicForTest(void)
{
	TimestampTz ts = GetSnapshotCurrentTimestamp();

	Assert(old_snapshot_threshold == 0);

	ts -= 5 * USECS_PER_SEC;

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

/*
 * If there is a valid mapping for the timestamp, set *xlimitp to
 * that. Returns whether there is such a mapping.
 */
static bool
GetOldSnapshotFromTimeMapping(TimestampTz ts, TransactionId *xlimitp)
{
	bool		in_mapping = false;

	Assert(ts == AlignTimestampToMinuteBoundary(ts));

	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;

		*xlimitp = oldSnapshotControl->xid_by_minute[offset];

		in_mapping = true;
	}

	LWLockRelease(OldSnapshotTimeMapLock);

	return in_mapping;
}

/*
 * TransactionIdLimitedForOldSnapshots
 *
 * Apply old snapshot limit.  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.
 *
 * If the limited horizon allows a cleanup action that otherwise would not be
 * possible, SetOldSnapshotThresholdTimestamp(*limit_ts, *limit_xid) needs to
 * be called before that cleanup action.
 */
bool
TransactionIdLimitedForOldSnapshots(TransactionId recentXmin,
									Relation relation,
									TransactionId *limit_xid,
									TimestampTz *limit_ts)
{
	TimestampTz ts;
	TransactionId xlimit = recentXmin;
	TransactionId latest_xmin;
	TimestampTz next_map_update_ts;
	TransactionId threshold_timestamp;
	TransactionId threshold_xid;

	Assert(TransactionIdIsNormal(recentXmin));
	Assert(OldSnapshotThresholdActive());
	Assert(limit_ts != NULL && limit_xid != NULL);

	/*
	 * TestForOldSnapshot() assumes early pruning advances the page LSN, so we
	 * can't prune early when skipping WAL.
	 */
	if (!RelationAllowsEarlyPruning(relation) || !RelationNeedsWAL(relation))
		return false;

	ts = GetSnapshotCurrentTimestamp();

	SpinLockAcquire(&oldSnapshotControl->mutex_latest_xmin);
	latest_xmin = oldSnapshotControl->latest_xmin;
	next_map_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, MyProc->xmin)
			&& TransactionIdFollows(latest_xmin, xlimit))
			xlimit = latest_xmin;

		ts -= 5 * USECS_PER_SEC;
	}
	else
	{
		ts = AlignTimestampToMinuteBoundary(ts)
			- (old_snapshot_threshold * USECS_PER_MINUTE);

		/* Check for fast exit without LW locking. */
		SpinLockAcquire(&oldSnapshotControl->mutex_threshold);
		threshold_timestamp = oldSnapshotControl->threshold_timestamp;
		threshold_xid = oldSnapshotControl->threshold_xid;
		SpinLockRelease(&oldSnapshotControl->mutex_threshold);

		if (ts == threshold_timestamp)
		{
			/*
			 * Current timestamp is in same bucket as the last limit that was
			 * applied. Reuse.
			 */
			xlimit = threshold_xid;
		}
		else if (ts == next_map_update_ts)
		{
			/*
			 * FIXME: This branch is super iffy - but that should probably
			 * fixed separately.
			 */
			xlimit = latest_xmin;
		}
		else if (GetOldSnapshotFromTimeMapping(ts, &xlimit))
		{
		}

		/*
		 * 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 (TransactionIdIsValid(xlimit) &&
		TransactionIdFollowsOrEquals(xlimit, recentXmin))
	{
		*limit_ts = ts;
		*limit_xid = xlimit;

		return true;
	}

	return false;
}

/*
 * 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			distance_to_new_tail;
		int			distance_to_current_tail;
		int			advance;

		/*
		 * Our goal is for the new "tail" of the mapping, that is, the entry
		 * which is newest and thus furthest from the "head" entry, to
		 * correspond to "ts". Since there's one entry per minute, the
		 * distance between the current head and the new tail is just the
		 * number of minutes of difference between ts and the current
		 * head_timestamp.
		 *
		 * The distance from the current head to the current tail is one less
		 * than the number of entries in the mapping, because the entry at the
		 * head_offset is for 0 minutes after head_timestamp.
		 *
		 * The difference between these two values is the number of minutes by
		 * which we need to advance the mapping, either adding new entries or
		 * rotating old ones out.
		 */
		distance_to_new_tail =
			(ts - oldSnapshotControl->head_timestamp) / USECS_PER_MINUTE;
		distance_to_current_tail =
			oldSnapshotControl->count_used - 1;
		advance = distance_to_new_tail - distance_to_current_tail;
		Assert(advance > 0);

		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;
			oldSnapshotControl->head_timestamp = ts;
		}
		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;
					oldSnapshotControl->head_timestamp += USECS_PER_MINUTE;
				}
				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;
	snapshot->snapXactCompletionCount = 0;

	/* 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 *source_pgproc)
{
	SetTransactionSnapshot(snapshot, NULL, InvalidPid, source_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;
}