Re: Rework the way multixact truncations work

On 2015-12-10 08:55:54 -0500, Robert Haas wrote:
> Maybe. But I think we could use a little more vigorous discussion of
> that issue, since Andres doesn't seem to be very convinced by your
> analysis, and I don't currently understand what you've fixed because I
> can't, as mentioned several times, follow your patch stack.

The issue at hand is that the following block
oldestOffsetKnown =
find_multixact_start(oldestMultiXactId, &oldestOffset);

...
else if (prevOldestOffsetKnown)
{
/*
* 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.
*/
oldestOffset = prevOldestOffset;
oldestOffsetKnown = true;
}
in SetOffsetVacuumLimit() fails to restore offsetStopLimit, which then
is set in shared memory:
/* Install the computed values */
LWLockAcquire(MultiXactGenLock, LW_EXCLUSIVE);
MultiXactState->oldestOffset = oldestOffset;
MultiXactState->oldestOffsetKnown = oldestOffsetKnown;
MultiXactState->offsetStopLimit = offsetStopLimit;
LWLockRelease(MultiXactGenLock);

so, if find_multixact_start() failed - a "should never happen" occurance
- we install a wrong stop limit. It does get 'repaired' upon the next
suceeding find_multixact_start() in SetOffsetVacuumLimit() or a restart
though.

Adding a 'prevOffsetStopLimit' and using it seems like a ~5 line patch.