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Avoid crashing in GetOldestSnapshot() if there are no known snapshots.

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The sole caller expects NULL to be returned in such a case, so make
it so and document it.

Per reports from Andreas Seltenreich and Regina Obe.  This doesn't
really fix their problem, as now their RETURNING queries will say
"ERROR: no known snapshots", but in any case this function should
not dump core in a reasonably-foreseeable situation.

Report: <87vazemeda.fsf@credativ.de>
Report: <20160807051854.1427.32414@wrigleys.postgresql.org>
This commit is contained in:
Tom Lane 2016-08-07 14:36:02 -04:00
parent bcbecbce2f
commit 07a601eeda
1 changed files with 7 additions and 5 deletions
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12
src/backend/utils/time/snapmgr.c
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@ -399,14 +399,14 @@ GetLatestSnapshot(void)
/*
* GetOldestSnapshot
*
* Get the oldest known snapshot, as judged by the LSN.
* 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;
XLogRecPtr ActiveLSN = InvalidXLogRecPtr;
if (!pairingheap_is_empty(&RegisteredSnapshots))
{
@ -416,10 +416,12 @@ GetOldestSnapshot(void)
}
if (OldestActiveSnapshot != NULL)
ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
{
XLogRecPtr ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
return OldestActiveSnapshot->as_snap;
if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
return OldestActiveSnapshot->as_snap;
}
return OldestRegisteredSnapshot;
}