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Support synchronization of snapshots through an export/import procedure.

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A transaction can export a snapshot with pg_export_snapshot(), and then
others can import it with SET TRANSACTION SNAPSHOT.  The data does not
leave the server so there are not security issues.  A snapshot can only
be imported while the exporting transaction is still running, and there
are some other restrictions.

I'm not totally convinced that we've covered all the bases for SSI (true
serializable) mode, but it works fine for lesser isolation modes.

Joachim Wieland, reviewed by Marko Tiikkaja, and rather heavily modified
by Tom Lane
This commit is contained in:
Tom Lane
2011-10-22 18:22:45 -04:00
parent b436c72f61
commit bb446b689b
17 changed files with 1030 additions and 34 deletions
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97
src/backend/storage/ipc/procarray.c
View File

@ -1122,6 +1122,28 @@ GetOldestXmin(bool allDbs, bool ignoreVacuum)
return result;
}
/*
* GetMaxSnapshotXidCount -- get max size for snapshot XID array
*
* We have to export this for use by snapmgr.c.
*/
int
GetMaxSnapshotXidCount(void)
{
return procArray->maxProcs;
}
/*
* GetMaxSnapshotSubxidCount -- get max size for snapshot sub-XID array
*
* We have to export this for use by snapmgr.c.
*/
int
GetMaxSnapshotSubxidCount(void)
{
return TOTAL_MAX_CACHED_SUBXIDS;
}
/*
* GetSnapshotData -- returns information about running transactions.
*
@ -1187,14 +1209,14 @@ GetSnapshotData(Snapshot snapshot)
* we are in recovery, see later comments.
*/
snapshot->xip = (TransactionId *)
malloc(arrayP->maxProcs * sizeof(TransactionId));
malloc(GetMaxSnapshotXidCount() * sizeof(TransactionId));
if (snapshot->xip == NULL)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
errmsg("out of memory")));
Assert(snapshot->subxip == NULL);
snapshot->subxip = (TransactionId *)
malloc(TOTAL_MAX_CACHED_SUBXIDS * sizeof(TransactionId));
malloc(GetMaxSnapshotSubxidCount() * sizeof(TransactionId));
if (snapshot->subxip == NULL)
ereport(ERROR,
(errcode(ERRCODE_OUT_OF_MEMORY),
@ -1376,6 +1398,77 @@ GetSnapshotData(Snapshot snapshot)
return snapshot;
}
/*
* ProcArrayInstallImportedXmin -- install imported xmin into MyProc->xmin
*
* This is called when installing a snapshot imported from another
* transaction. To ensure that OldestXmin doesn't go backwards, we must
* check that the source transaction is still running, and we'd better do
* that atomically with installing the new xmin.
*
* Returns TRUE if successful, FALSE if source xact is no longer running.
*/
bool
ProcArrayInstallImportedXmin(TransactionId xmin, TransactionId sourcexid)
{
bool result = false;
ProcArrayStruct *arrayP = procArray;
int index;
Assert(TransactionIdIsNormal(xmin));
if (!TransactionIdIsNormal(sourcexid))
return false;
/* Get lock so source xact can't end while we're doing this */
LWLockAcquire(ProcArrayLock, LW_SHARED);
for (index = 0; index < arrayP->numProcs; index++)
{
volatile PGPROC *proc = arrayP->procs[index];
TransactionId xid;
/* Ignore procs running LAZY VACUUM */
if (proc->vacuumFlags & PROC_IN_VACUUM)
continue;
xid = proc->xid; /* fetch just once */
if (xid != sourcexid)
continue;
/*
* We check the transaction's database ID for paranoia's sake: if
* it's in another DB then its xmin does not cover us. Caller should
* have detected this already, so we just treat any funny cases as
* "transaction not found".
*/
if (proc->databaseId != MyDatabaseId)
continue;
/*
* Likewise, let's just make real sure its xmin does cover us.
*/
xid = proc->xmin; /* fetch just once */
if (!TransactionIdIsNormal(xid) ||
!TransactionIdPrecedesOrEquals(xid, xmin))
continue;
/*
* We're good. Install the new xmin. As in GetSnapshotData, set
* TransactionXmin too. (Note that because snapmgr.c called
* GetSnapshotData first, we'll be overwriting a valid xmin here,
* so we don't check that.)
*/
MyProc->xmin = TransactionXmin = xmin;
result = true;
break;
}
LWLockRelease(ProcArrayLock);
return result;
}
/*
* GetRunningTransactionData -- returns information about running transactions.
*