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/*-------------------------------------------------------------------------
* snapmgr.c
* PostgreSQL snapshot manager
*
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
* Portions Copyright (c) 1994, Regents of the University of California
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/utils/time/snapmgr.c,v 1.1 2008/03/26 18:48:59 alvherre Exp $
*
*-------------------------------------------------------------------------
*/
#include "postgres.h"
#include "access/xact.h"
#include "access/transam.h"
#include "storage/procarray.h"
#include "utils/snapmgr.h"
#include "utils/tqual.h"
/*
* These SnapshotData structs are static to simplify memory allocation
* (see the hack in GetSnapshotData to avoid repeated malloc/free).
*/
static SnapshotData SerializableSnapshotData = {HeapTupleSatisfiesMVCC};
static SnapshotData LatestSnapshotData = {HeapTupleSatisfiesMVCC};
/* Externally visible pointers to valid snapshots: */
Snapshot SerializableSnapshot = NULL;
Snapshot LatestSnapshot = NULL;
/*
* This pointer is not maintained by this module, but it's convenient
* to declare it here anyway. Callers typically assign a copy of
* GetTransactionSnapshot's result to ActiveSnapshot.
*/
Snapshot ActiveSnapshot = 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;
TransactionId RecentGlobalXmin = FirstNormalTransactionId;
/*
* GetTransactionSnapshot
* Get the appropriate snapshot for a new query in a transaction.
*
* The SerializableSnapshot is the first one taken in a transaction.
* In serializable mode we just use that one throughout the transaction.
* In read-committed mode, we take a new snapshot each time we are called.
*
* Note that the return value points at static storage that will be modified
* by future calls and by CommandCounterIncrement(). Callers should copy
* the result with CopySnapshot() if it is to be used very long.
*/
Snapshot
GetTransactionSnapshot(void)
{
/* First call in transaction? */
if (SerializableSnapshot == NULL)
{
SerializableSnapshot = GetSnapshotData(&SerializableSnapshotData, true);
return SerializableSnapshot;
}
if (IsXactIsoLevelSerializable)
return SerializableSnapshot;
LatestSnapshot = GetSnapshotData(&LatestSnapshotData, false);
return LatestSnapshot;
}
/*
* GetLatestSnapshot
* Get a snapshot that is up-to-date as of the current instant,
* even if we are executing in SERIALIZABLE mode.
*/
Snapshot
GetLatestSnapshot(void)
{
/* Should not be first call in transaction */
if (SerializableSnapshot == NULL)
elog(ERROR, "no snapshot has been set");
LatestSnapshot = GetSnapshotData(&LatestSnapshotData, false);
return LatestSnapshot;
}
/*
* CopySnapshot
* Copy the given snapshot.
*
* The copy is palloc'd in the current memory context.
*/
Snapshot
CopySnapshot(Snapshot snapshot)
{
Snapshot newsnap;
Size subxipoff;
Size size;
/* 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) palloc(size);
memcpy(newsnap, snapshot, sizeof(SnapshotData));
/* 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 */
if (snapshot->subxcnt > 0)
{
newsnap->subxip = (TransactionId *) ((char *) newsnap + subxipoff);
memcpy(newsnap->subxip, snapshot->subxip,
snapshot->subxcnt * sizeof(TransactionId));
}
else
newsnap->subxip = NULL;
return newsnap;
}
/*
* FreeSnapshot
* Free a snapshot previously copied with CopySnapshot.
*
* This is currently identical to pfree, but is provided for cleanliness.
*
* Do *not* apply this to the results of GetTransactionSnapshot or
* GetLatestSnapshot, since those are just static structs.
*/
void
FreeSnapshot(Snapshot snapshot)
{
pfree(snapshot);
}
/*
* FreeXactSnapshot
* Free snapshot(s) at end of transaction.
*/
void
FreeXactSnapshot(void)
{
/*
* We do not free the xip arrays for the static snapshot structs; they
* will be reused soon. So this is now just a state change to prevent
* outside callers from accessing the snapshots.
*/
SerializableSnapshot = NULL;
LatestSnapshot = NULL;
ActiveSnapshot = NULL; /* just for cleanliness */
}
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