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Commit6199b1f

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Fix corner case in autovacuum-forcing logic for multixact wraparound.
Since find_multixact_start() relies on SimpleLruDoesPhysicalPageExist(),and that function looks only at the on-disk state, it's possible for itto fail to find a page that exists in the in-memory SLRU that has notbeen written yet. If that happens, SetOffsetVacuumLimit() willerroneously decide to force emergency autovacuuming immediately.We should probably fix find_multixact_start() to consider the datacached in memory as well as on the on-disk state, but that's no excusefor SetOffsetVacuumLimit() to be stupid about the case where it canno longer read the value after having previously succeeded in doing so.Report by Andres Freund.
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‎src/backend/access/transam/multixact.c

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@@ -2680,6 +2680,18 @@ SetOffsetVacuumLimit(bool finish_setup)
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(errmsg("oldest MultiXactId member offset unknown")));
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}
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/*
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* If we failed to get the oldest offset this time, but we have a value
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* from a previous pass through this function, assess the need for
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* autovacuum based on that old value rather than automatically forcing
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* it.
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*/
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if (prevOldestOffsetKnown&& !oldestOffsetKnown)
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{
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oldestOffset=prevOldestOffset;
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oldestOffsetKnown= true;
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}
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/*
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* Do we need an emergency autovacuum? If we're not sure, assume yes.
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*/

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