Prevent non-streaming replication connections from being selected sync slave

This prevents a pg_basebackup backup session that just does a base
backup (no xlog involved at all) from becoming the synchronous slave
and thus blocking all access while it runs.

Also fixes the problem when a higher priority slave shows up it would
become the sync standby before it has reached the STREAMING state, by
making sure we can only switch to a walsender that's actually STREAMING.

Fujii Masao

src/backend/replication/syncrep.c

@@ -397,6 +397,7 @@ SyncRepReleaseWaiters(void)
 		volatile WalSnd *walsnd = &walsndctl->walsnds[i];
 
 		if (walsnd->pid != 0 &&
+			walsnd->state == WALSNDSTATE_STREAMING &&
 			walsnd->sync_standby_priority > 0 &&
 			(priority == 0 ||
 			 priority > walsnd->sync_standby_priority))