Fix deadlock at startup, if max_prepared_transactions is too small.

hlinnaka · hlinnaka · commit f73ebd766a49 · 2015-04-23T21:36:24.000+03:00
When the startup process recovers transactions by scanning pg_twophase directory, it should clear MyLockedGxact after it's done processing each transaction. Like we do during normal operation, at PREPARE TRANSACTION. Otherwise, if the startup process exits due to an error, it will try to clear the locking_backend field of the last recovered transaction. That's usually harmless, but if the error happens in MarkAsPreparing, while holding TwoPhaseStateLock, the shmem-exit hook will try to acquire TwoPhaseStateLock again, and deadlock with itself. This fixes bug #13128 reported by Grant McAlister. The bug was introduced by commit bb38fb0, so backpatch to all supported versions like that commit.
diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
@@ -2043,6 +2043,12 @@ RecoverPreparedTransactions(void)
 			if (InHotStandby)
 				StandbyReleaseLockTree(xid, hdr->nsubxacts, subxids);
 
+			/*
+			 * We're done with recovering this transaction. Clear MyLockedGxact,
+			 * like we do in PrepareTransaction() during normal operation.
+			 */
+			PostPrepare_Twophase();
+
 			pfree(buf);
 		}
 	}

Original file line number	Diff line number	Diff line change
`@@ -2043,6 +2043,12 @@ RecoverPreparedTransactions(void)`
`2043`	`2043`	`if (InHotStandby)`
`2044`	`2044`	`StandbyReleaseLockTree(xid, hdr->nsubxacts, subxids);`
`2045`	`2045`
	`2046`	`+ /*`
	`2047`	`+ * We're done with recovering this transaction. Clear MyLockedGxact,`
	`2048`	`+ * like we do in PrepareTransaction() during normal operation.`
	`2049`	`+ */`
	`2050`	`+ PostPrepare_Twophase();`
	`2051`	`+`
`2046`	`2052`	`pfree(buf);`
`2047`	`2053`	`}`
`2048`	`2054`	`}`