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Fix theoretical torn page hazard.

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The original report was concerned with a possible inconsistency
between the heap and the visibility map, which I was unable to
confirm. The concern has been retracted.

However, there did seem to be a torn page hazard when using
checksums. By not setting the heap page LSN during redo, the
protections of minRecoveryPoint were bypassed. Fixed, along with a
misleading comment.

It may have been impossible to hit this problem in practice, because
it would require a page tear between the checksum and the flags, so I
am marking this as a theoretical risk. But, as discussed, it did
violate expectations about the page LSN, so it may have other
consequences.

Backpatch to all supported versions.

Reported-by: Konstantin Knizhnik
Reviewed-by: Konstantin Knizhnik
Discussion: https://postgr.es/m/fed17dac-8cb8-4f5b-d462-1bb4908c029e@garret.ru
Backpatch-through: 11
This commit is contained in:
Jeff Davis 2022-11-10 14:46:30 -08:00
parent 9c1a4fc891
commit 7bf713dd2d
1 changed files with 4 additions and 2 deletions
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6
src/backend/access/heap/heapam.c
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@ -8823,8 +8823,7 @@ heap_xlog_visible(XLogReaderState *record)
/*
* We don't bump the LSN of the heap page when setting the visibility
* map bit (unless checksums or wal_hint_bits is enabled, in which
* case we must), because that would generate an unworkable volume of
* full-page writes. This exposes us to torn page hazards, but since
* case we must). This exposes us to torn page hazards, but since
* we're not inspecting the existing page contents in any way, we
* don't care.
*
@ -8838,6 +8837,9 @@ heap_xlog_visible(XLogReaderState *record)
PageSetAllVisible(page);
if (XLogHintBitIsNeeded())
PageSetLSN(page, lsn);
MarkBufferDirty(buffer);
}
else if (action == BLK_RESTORED)