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Fix bug in SetOffsetVacuumLimit() triggered by find_multixact_start() failure. 

Previously, if find_multixact_start() failed, SetOffsetVacuumLimit() would
install 0 into MultiXactState->offsetStopLimit if it previously succeeded.
Luckily, there are no known cases where find_multixact_start() will return
an error in 9.5 and above. But if it were to happen, for example due to
filesystem permission issues, it'd be somewhat bad: GetNewMultiXactId()
could continue allocating mxids even if close to a wraparound, or it could
erroneously stop allocating mxids, even if no wraparound is looming.  The
wrong value would be corrected the next time SetOffsetVacuumLimit() is
called, or by a restart.

Reported-By: Noah Misch, although this is not his preferred fix
Discussion: 20151210140450.GA22278@alap3.anarazel.de
Backpatch: 9.5, where the bug was introduced as part of 4f627f
This commit is contained in:
Andres Freund 2015-12-14 11:34:16 +01:00
parent 2a3544960e
commit cca705a5d9
1 changed files with 6 additions and 2 deletions
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8
src/backend/access/transam/multixact.c
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@ -2552,6 +2552,7 @@ SetOffsetVacuumLimit(void)
bool oldestOffsetKnown = false;
bool prevOldestOffsetKnown;
MultiXactOffset offsetStopLimit = 0;
MultiXactOffset prevOffsetStopLimit;
/*
* NB: Have to prevent concurrent truncation, we might otherwise try to
@ -2566,6 +2567,7 @@ SetOffsetVacuumLimit(void)
nextOffset = MultiXactState->nextOffset;
prevOldestOffsetKnown = MultiXactState->oldestOffsetKnown;
prevOldestOffset = MultiXactState->oldestOffset;
prevOffsetStopLimit = MultiXactState->offsetStopLimit;
Assert(MultiXactState->finishedStartup);
LWLockRelease(MultiXactGenLock);
@ -2633,11 +2635,13 @@ SetOffsetVacuumLimit(void)
{
/*
* If we failed to get the oldest offset this time, but we have a
* value from a previous pass through this function, use the old value
* rather than automatically forcing it.
* value from a previous pass through this function, use the old
* values rather than automatically forcing an emergency autovacuum
* cycle again.
*/
oldestOffset = prevOldestOffset;
oldestOffsetKnown = true;
offsetStopLimit = prevOffsetStopLimit;
}
/* Install the computed values */