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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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`‎src/backend/access/transam/multixact.c`

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Original file line number	Diff line number	Diff line change
`@@ -2661,6 +2661,18 @@ SetOffsetVacuumLimit(bool finish_setup)`
`2661`	`2661`	`(errmsg("oldest MultiXactId member offset unknown")));`
`2662`	`2662`	`}`
`2663`	`2663`
	`2664`	`+/*`
	`2665`	`+ * If we failed to get the oldest offset this time, but we have a value`
	`2666`	`+ * from a previous pass through this function, assess the need for`
	`2667`	`+ * autovacuum based on that old value rather than automatically forcing`
	`2668`	`+ * it.`
	`2669`	`+ */`
	`2670`	`+if (prevOldestOffsetKnown&& !oldestOffsetKnown)`
	`2671`	`+{`
	`2672`	`+oldestOffset=prevOldestOffset;`
	`2673`	`+oldestOffsetKnown= true;`
	`2674`	`+}`
	`2675`	`+`
`2664`	`2676`	`/*`
`2665`	`2677`	`* Do we need an emergency autovacuum? If we're not sure, assume yes.`
`2666`	`2678`	`*/`

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Commited16f73

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`‎src/backend/access/transam/multixact.c`

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