f6a6c46d7f
Commit b69bf30b9bfacafc733a9ba77c9587cf54d06c0c advanced the stop point at vacuum time, but this has subsequently been shown to be unsafe as a result of analysis by myself and Thomas Munro and testing by Thomas Munro. The crux of the problem is that the SLRU deletion logic may get confused about what to remove if, at exactly the right time during the checkpoint process, the head of the SLRU crosses what used to be the tail. This patch, by me, fixes the problem by advancing the stop point only following a checkpoint. This has the additional advantage of making the removal logic work during recovery more like the way it works during normal running, which is probably good. At least one of the calls to DetermineSafeOldestOffset which this patch removes was already dead, because MultiXactAdvanceOldest is called only during recovery and DetermineSafeOldestOffset was set up to do nothing during recovery. That, however, is inconsistent with the principle that recovery and normal running should work similarly, and was confusing to boot. Along the way, fix some comments that previous patches in this area neglected to update. It's not clear to me whether there's any concrete basis for the decision to use only half of the multixact ID space, but it's neither necessary nor sufficient to prevent multixact member wraparound, so the comments should not say otherwise.
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