From ccde00b9b97ed8b7307e39f95bd48922bf955bb2 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Mon, 14 Dec 2015 11:34:16 +0100 Subject: [PATCH] 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 --- src/backend/access/transam/multixact.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index c886ec3228..c730fc49c5 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -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 */