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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 * 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 Snapshot
GetOldestSnapshot(void) GetOldestSnapshot(void)
{ {
Snapshot OldestRegisteredSnapshot = NULL; Snapshot OldestRegisteredSnapshot = NULL;
XLogRecPtr RegisteredLSN = InvalidXLogRecPtr; XLogRecPtr RegisteredLSN = InvalidXLogRecPtr;
XLogRecPtr ActiveLSN = InvalidXLogRecPtr;
if (!pairingheap_is_empty(&RegisteredSnapshots)) if (!pairingheap_is_empty(&RegisteredSnapshots))
{ {
@ -416,10 +416,12 @@ GetOldestSnapshot(void)
} }
if (OldestActiveSnapshot != NULL) if (OldestActiveSnapshot != NULL)
ActiveLSN = OldestActiveSnapshot->as_snap->lsn; {
XLogRecPtr ActiveLSN = OldestActiveSnapshot->as_snap->lsn;
if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN) if (XLogRecPtrIsInvalid(RegisteredLSN) || RegisteredLSN > ActiveLSN)
return OldestActiveSnapshot->as_snap; return OldestActiveSnapshot->as_snap;
}
return OldestRegisteredSnapshot; return OldestRegisteredSnapshot;
} }