Fix ordering of operations in SyncRepWakeQueue to avoid assertion failure.

Commit 14e8803f removed the locking in SyncRepWaitForLSN, but that
introduced a race condition, where SyncRepWaitForLSN might see
syncRepState already set to SYNC_REP_WAIT_COMPLETE, but the process was
not yet removed from the queue. That tripped the assertion, that the
process should no longer be in the uqeue. Reorder the operations in
SyncRepWakeQueue to remove the process from the queue first, and update
syncRepState only after that, and add a memory barrier in between to make
sure the operations are made visible to other processes in that order.

Fixes bug #14721 reported by Const Zhang. Analysis and fix by Thomas Munro.
Backpatch down to 9.5, where the locking was removed.

Discussion: https://www.postgresql.org/message-id/20170629023623.1480.26508%40wrigleys.postgresql.org

src/backend/replication/syncrep.c

@@ -293,8 +293,11 @@ SyncRepWaitForLSN(XLogRecPtr lsn, bool commit)
 	 * WalSender has checked our LSN and has removed us from queue. Clean up
 	 * state and leave.  It's OK to reset these shared memory fields without
 	 * holding SyncRepLock, because any walsenders will ignore us anyway when
-	 * we're not on the queue.
+	 * we're not on the queue.  We need a read barrier to make sure we see
+	 * the changes to the queue link (this might be unnecessary without
+	 * assertions, but better safe than sorry).
 	 */
+	pg_read_barrier();
 	Assert(SHMQueueIsDetached(&(MyProc->syncRepLinks)));
 	MyProc->syncRepState = SYNC_REP_NOT_WAITING;
 	MyProc->waitLSN = 0;
@@ -1022,15 +1025,22 @@ SyncRepWakeQueue(bool all, int mode)
 									   offsetof(PGPROC, syncRepLinks));
 
 		/*
-		 * Set state to complete; see SyncRepWaitForLSN() for discussion of
-		 * the various states.
+		 * Remove thisproc from queue.
 		 */
-		thisproc->syncRepState = SYNC_REP_WAIT_COMPLETE;
+		SHMQueueDelete(&(thisproc->syncRepLinks));
 
 		/*
-		 * Remove thisproc from queue.
+		 * SyncRepWaitForLSN() reads syncRepState without holding the lock, so
+		 * make sure that it sees the queue link being removed before the
+		 * syncRepState change.
 		 */
-		SHMQueueDelete(&(thisproc->syncRepLinks));
+		pg_write_barrier();
+
+		/*
+		 * Set state to complete; see SyncRepWaitForLSN() for discussion of
+		 * the various states.
+		 */
+		thisproc->syncRepState = SYNC_REP_WAIT_COMPLETE;
 
 		/*
 		 * Wake only when we have set state and removed from queue.