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Fix theoretical torn page hazard.

The original report was concerned with a possible inconsistencybetween the heap and the visibility map, which I was unable toconfirm. The concern has been retracted.However, there did seem to be a torn page hazard when usingchecksums. By not setting the heap page LSN during redo, theprotections of minRecoveryPoint were bypassed. Fixed, along with amisleading comment.It may have been impossible to hit this problem in practice, becauseit would require a page tear between the checksum and the flags, so Iam marking this as a theoretical risk. But, as discussed, it didviolate expectations about the page LSN, so it may have otherconsequences.Backpatch to all supported versions.Reported-by: Konstantin KnizhnikReviewed-by: Konstantin KnizhnikDiscussion:https://postgr.es/m/fed17dac-8cb8-4f5b-d462-1bb4908c029e@garret.ruBackpatch-through: 11

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src/backend/access/heap
- heapam.c

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`‎src/backend/access/heap/heapam.c`

Lines changed: 4 additions & 2 deletions

Original file line number	Diff line number	Diff line change
`@@ -7997,8 +7997,7 @@ heap_xlog_visible(XLogReaderState *record)`
`7997`	`7997`	`/*`
`7998`	`7998`	`* We don't bump the LSN of the heap page when setting the visibility`
`7999`	`7999`	`* map bit (unless checksums or wal_hint_bits is enabled, in which`
`8000`		`- * case we must), because that would generate an unworkable volume of`
`8001`		`- * full-page writes. This exposes us to torn page hazards, but since`
	`8000`	`+ * case we must). This exposes us to torn page hazards, but since`
`8002`	`8001`	`* we're not inspecting the existing page contents in any way, we`
`8003`	`8002`	`* don't care.`
`8004`	`8003`	`*`
`@@ -8012,6 +8011,9 @@ heap_xlog_visible(XLogReaderState *record)`
`8012`	`8011`
`8013`	`8012`	`PageSetAllVisible(page);`
`8014`	`8013`
	`8014`	`+if (XLogHintBitIsNeeded())`
	`8015`	`+PageSetLSN(page,lsn);`
	`8016`	`+`
`8015`	`8017`	`MarkBufferDirty(buffer);`
`8016`	`8018`	`}`
`8017`	`8019`	`elseif (action==BLK_RESTORED)`

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Commit7dd39e9

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`‎src/backend/access/heap/heapam.c`

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