From: Tom Lane Date: Sun, 7 Aug 2016 18:36:02 +0000 (-0400) Subject: Avoid crashing in GetOldestSnapshot() if there are no known snapshots. X-Git-Tag: REL9_6_BETA4~23 X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=07a601eedab7c5fa4d62055fa3efacef2f38e446;p=postgresql Avoid crashing in GetOldestSnapshot() if there are no known snapshots. 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> --- diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index 4bddaed33b..1ec9f70f0e 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -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; }