8000 Fix theoretical torn page hazard. · postgres/postgres@5eaf3e3 · GitHub
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Fix theoretical torn page hazard.
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 The concern has been retracted.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
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‎src/backend/access/heap/heapam.c

Lines changed: 4 additions & 2 deletions
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@@ -8437,8 +8437,7 @@ heap_xlog_visible(XLogReaderState *record)
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/*
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* We don't bump the LSN of the heap page when setting the visibility
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* map bit (unless checksums or wal_hint_bits is enabled, in which
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* case we must), because that would generate an unworkable volume of
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* full-page writes. This exposes us to torn page hazards, but since
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* case we must). This exposes us to torn page hazards, but since
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* we're not inspecting the existing page contents in any way, we
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* don't care.
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*
@@ -8452,6 +8451,9 @@ heap_xlog_visible(XLogReaderState *record)
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PageSetAllVisible(page);
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if (XLogHintBitIsNeeded())
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PageSetLSN(page, lsn);
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MarkBufferDirty(buffer);
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}
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else if (action == BLK_RESTORED)

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