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Fix RBM_ZERO_AND_LOCK mode to not acquire lock on local buffers.

Commit81c4508 introduced a new RBM_ZERO_AND_LOCK mode to ReadBuffer, whichtakes a lock on the buffer before zeroing it. However, you cannot take alock on a local buffer, and you got a segfault instead. The version of thatpatch committed to master included a check for !isLocalBuf, and thereforedidn't crash, but oddly I missed that in the back-patched versions. Thispatch adds that check to the back-branches too.RBM_ZERO_AND_LOCK mode is only used during WAL replay, and in hash indexes.WAL replay only deals with shared buffers, so the only way to trigger thebug is with a temporary hash index.Reported by Artem Ignatyev, analysis by Tom Lane.

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src/backend/storage/buffer
- bufmgr.c

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`‎src/backend/storage/buffer/bufmgr.c`

Lines changed: 2 additions & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -494,7 +494,8 @@ ReadBuffer_common(SMgrRelation smgr, char relpersistence, ForkNumber forkNum,`
`494`	`494`	`* (Note that we cannot use LockBuffer() of LockBufferForCleanup() here,`
`495`	`495`	`* because they assert that the buffer is already valid.)`
`496`	`496`	`*/`
`497`		`-if (mode==RBM_ZERO_AND_LOCK\|\|mode==RBM_ZERO_AND_CLEANUP_LOCK)`
	`497`	`+if ((mode==RBM_ZERO_AND_LOCK\|\|mode==RBM_ZERO_AND_CLEANUP_LOCK)&&`
	`498`	`+!isLocalBuf)`
`498`	`499`	`LWLockAcquire(bufHdr->content_lock,LW_EXCLUSIVE);`
`499`	`500`
`500`	`501`	`if (isLocalBuf)`

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Commitf6c4a86

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`‎src/backend/storage/buffer/bufmgr.c`

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