Fix race condition in TransactionGroupUpdateXidStatus().

Amit Kapila · Amit Kapila · commit b786304c2904 · 2021-06-28T09:29:38.000+05:30
When we cannot immediately acquire XactSLRULock in exclusive mode at commit time, we add ourselves to a list of processes that need their XIDs status update. We do this if the clog page where we need to update the current transaction status is the same as the group leader's clog page, otherwise, we allow the caller to clear it by itself. Now, when we can't add ourselves to any group, we were not clearing the current proc if it has already become a member of some group which was leading to an assertion failure when the same proc was assigned to another backend after the current backend exits. Reported-by: Alexander Lakhin Bug: 17072 Author: Amit Kapila Tested-By: Alexander Lakhin Backpatch-through: 11, where it was introduced Discussion: https://postgr.es/m/17072-2f8764857ef2c92a@postgresql.org
diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c
@@ -451,7 +451,12 @@ TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,
 		if (nextidx != INVALID_PGPROCNO &&
 			ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)
 		{
+			/*
+			 * Ensure that this proc is not a member of any clog group that
+			 * needs an XID status update.
+			 */
 			proc->clogGroupMember = false;
+			pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);
 			return false;
 		}
 

Original file line number	Diff line number	Diff line change
`@@ -451,7 +451,12 @@ TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status,`
`451`	`451`	`if (nextidx != INVALID_PGPROCNO &&`
`452`	`452`	`ProcGlobal->allProcs[nextidx].clogGroupMemberPage != proc->clogGroupMemberPage)`
`453`	`453`	`{`
	`454`	`+ /*`
	`455`	`+ * Ensure that this proc is not a member of any clog group that`
	`456`	`+ * needs an XID status update.`
	`457`	`+ */`
`454`	`458`	`proc->clogGroupMember = false;`
	`459`	`+ pg_atomic_write_u32(&proc->clogGroupNext, INVALID_PGPROCNO);`
`455`	`460`	`return false;`
`456`	`461`	`}`
`457`	`462`