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Fix DetermineSafeOldestOffset for the case where there are no mxacts.

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Commit b69bf30b9b failed to take into
account the possibility that there might be no multixacts in existence
at all.

Report by Thomas Munro; patch by me.
This commit is contained in:
Robert Haas 2015-05-10 21:34:26 -04:00
parent 1a8a4e5cde
commit 312747c224
1 changed files with 17 additions and 6 deletions
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23
src/backend/access/transam/multixact.c
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@ -2492,13 +2492,24 @@ DetermineSafeOldestOffset(MultiXactId oldestMXact)
return;
/*
* We determine the safe upper bound for offsets of new xacts by reading
* the offset of the oldest multixact, and going back one segment. This
* way, the sequence of multixact member segments will always have a
* one-segment hole at a minimum. We start spewing warnings a few
* complete segments before that.
* Determine the offset of the oldest multixact. Normally, we can read
* the offset from the multixact itself, but there's an important special
* case: if there are no multixacts in existence at all, oldestMXact
* obviously can't point to one. It will instead point to the multixact
* ID that will be assigned the next time one is needed.
*/
oldestOffset = find_multixact_start(oldestMXact);
LWLockAcquire(MultiXactGenLock, LW_SHARED);
if (MultiXactState->nextMXact == oldestMXact)
{
oldestOffset = MultiXactState->nextOffset;
LWLockRelease(MultiXactGenLock);
}
else
{
LWLockRelease(MultiXactGenLock);
oldestOffset = find_multixact_start(oldestMXact);
}
/* move back to start of the corresponding segment */
oldestOffset -= oldestOffset %
(MULTIXACT_MEMBERS_PER_PAGE * SLRU_PAGES_PER_SEGMENT);