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Fix two bugs in setting the vm bit of empty pages.

Use a critical section when setting the all-visible flag on an empty page,and WAL-logging it. log_newpage_buffer() contains an assertion that itmust be called inside a critical section, and it's the right thing to dowhen modifying a buffer anyway.Also, the page should be marked dirty before calling log_newpage_buffer(),per the comment in log_newpage_buffer() and src/backend/access/transam/README.Patch by Andres Freund, in response to my report. Backpatch to 9.2, likethe patch that introduced these bugs (a6370fd).

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src/backend/commands
- vacuumlazy.c

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`‎src/backend/commands/vacuumlazy.c`

Lines changed: 6 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -663,6 +663,11 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,`
`663`	`663`	`/* empty pages are always all-visible */`
`664`	`664`	`if (!PageIsAllVisible(page))`
`665`	`665`	`{`
	`666`	`+START_CRIT_SECTION();`
	`667`	`+`
	`668`	`+/* mark buffer dirty before writing a WAL record */`
	`669`	`+MarkBufferDirty(buf);`
	`670`	`+`
`666`	`671`	`/*`
`667`	`672`	`* It's possible that another backend has extended the heap,`
`668`	`673`	`* initialized the page, and then failed to WAL-log the page`
`@@ -682,9 +687,9 @@ lazy_scan_heap(Relation onerel, LVRelStats *vacrelstats,`
`682`	`687`	`log_newpage_buffer(buf);`
`683`	`688`
`684`	`689`	`PageSetAllVisible(page);`
`685`		`-MarkBufferDirty(buf);`
`686`	`690`	`visibilitymap_set(onerel,blkno,buf,InvalidXLogRecPtr,`
`687`	`691`	`vmbuffer,InvalidTransactionId);`
	`692`	`+END_CRIT_SECTION();`
`688`	`693`	`}`
`689`	`694`
`690`	`695`	`UnlockReleaseBuffer(buf);`

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Commit83eb540

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`‎src/backend/commands/vacuumlazy.c`

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