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     *
valuefrom 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.