Receive invalidation messages correctly in tablesync worker

We didn't accept any invalidation messages until the whole sync process
had finished (because it flattens all the remote transactions in the
single one).  So the sync worker didn't learn about subscription
changes/drop until it has finished.  This could lead to "orphaned" sync
workers.

Author: Petr Jelinek <petr.jelinek@2ndquadrant.com>
Reported-by: Masahiko Sawada <sawada.mshk@gmail.com>

src/backend/replication/logical/worker.c

@@ -118,7 +118,7 @@ static void send_feedback(XLogRecPtr recvpos, bool force, bool requestReply);
 
 static void store_flush_position(XLogRecPtr remote_lsn);
 
-static void reread_subscription(void);
+static void maybe_reread_subscription(void);
 
 /* Flags set by signal handlers */
 static volatile sig_atomic_t got_SIGHUP = false;
@@ -165,8 +165,7 @@ ensure_transaction(void)
 
 	StartTransactionCommand();
 
-	if (!MySubscriptionValid)
-		reread_subscription();
+	maybe_reread_subscription();
 
 	MemoryContextSwitchTo(ApplyMessageContext);
 	return true;
@@ -463,6 +462,12 @@ apply_handle_commit(StringInfo s)
 
 		store_flush_position(commit_data.end_lsn);
 	}
+	else
+	{
+		/* Process any invalidation messages that might have accumulated. */
+		AcceptInvalidationMessages();
+		maybe_reread_subscription();
+	}
 
 	in_remote_transaction = false;
 
@@ -1119,8 +1124,7 @@ LogicalRepApplyLoop(XLogRecPtr last_received)
 			 * now.
 			 */
 			AcceptInvalidationMessages();
-			if (!MySubscriptionValid)
-				reread_subscription();
+			maybe_reread_subscription();
 
 			/* Process any table synchronization changes. */
 			process_syncing_tables(last_received);
@@ -1302,17 +1306,20 @@ send_feedback(XLogRecPtr recvpos, bool force, bool requestReply)
 		last_flushpos = flushpos;
 }
 
-
 /*
- * Reread subscription info and exit on change.
+ * Reread subscription info if needed. Most changes will be exit.
  */
 static void
-reread_subscription(void)
+maybe_reread_subscription(void)
 {
 	MemoryContext oldctx;
 	Subscription *newsub;
 	bool		started_tx = false;
 
+	/* When cache state is valid there is nothing to do here. */
+	if (MySubscriptionValid)
+		return;
+
 	/* This function might be called inside or outside of transaction. */
 	if (!IsTransactionState())
 	{