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Commitcca705a

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Fix bug in SetOffsetVacuumLimit() triggered by find_multixact_start() failure.
Previously, if find_multixact_start() failed, SetOffsetVacuumLimit() wouldinstall 0 into MultiXactState->offsetStopLimit if it previously succeeded.Luckily, there are no known cases where find_multixact_start() will returnan error in 9.5 and above. But if it were to happen, for example due tofilesystem permission issues, it'd be somewhat bad: GetNewMultiXactId()could continue allocating mxids even if close to a wraparound, or it coulderroneously stop allocating mxids, even if no wraparound is looming. Thewrong value would be corrected the next time SetOffsetVacuumLimit() iscalled, or by a restart.Reported-By: Noah Misch, although this is not his preferred fixDiscussion: 20151210140450.GA22278@alap3.anarazel.deBackpatch: 9.5, where the bug was introduced as part of 4f627f
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‎src/backend/access/transam/multixact.c

Lines changed: 6 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -2552,6 +2552,7 @@ SetOffsetVacuumLimit(void)
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boololdestOffsetKnown= false;
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boolprevOldestOffsetKnown;
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MultiXactOffsetoffsetStopLimit=0;
2555+
MultiXactOffsetprevOffsetStopLimit;
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/*
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* NB: Have to prevent concurrent truncation, we might otherwise try to
@@ -2566,6 +2567,7 @@ SetOffsetVacuumLimit(void)
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nextOffset=MultiXactState->nextOffset;
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prevOldestOffsetKnown=MultiXactState->oldestOffsetKnown;
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prevOldestOffset=MultiXactState->oldestOffset;
2570+
prevOffsetStopLimit=MultiXactState->offsetStopLimit;
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Assert(MultiXactState->finishedStartup);
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LWLockRelease(MultiXactGenLock);
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@@ -2633,11 +2635,13 @@ SetOffsetVacuumLimit(void)
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{
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/*
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* If we failed to get the oldest offset this time, but we have a
2636-
* value from a previous pass through this function, use the old value
2637-
* rather than automatically forcing it.
2638+
* value from a previous pass through this function, use the old
2639+
* values rather than automatically forcing an emergency autovacuum
2640+
* cycle again.
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*/
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oldestOffset=prevOldestOffset;
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oldestOffsetKnown= true;
2644+
offsetStopLimit=prevOffsetStopLimit;
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}
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/* Install the computed values */

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