Improve messaging during logical replication worker startup

In case the subscription is removed before the worker is fully started,
give a specific error message instead of the generic "cache lookup"
error.

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

src/backend/replication/logical/worker.c

@@ -1553,14 +1553,20 @@ ApplyWorkerMain(Datum main_arg)
 										 ALLOCSET_DEFAULT_SIZES);
 	StartTransactionCommand();
 	oldctx = MemoryContextSwitchTo(ApplyContext);
-	MySubscription = GetSubscription(MyLogicalRepWorker->subid, false);
+
+	MySubscription = GetSubscription(MyLogicalRepWorker->subid, true);
+	if (!MySubscription)
+	{
+		ereport(LOG,
+				(errmsg("logical replication apply worker for subscription %u will not "
+						"start because the subscription was removed during startup",
+						MyLogicalRepWorker->subid)));
+		proc_exit(0);
+	}
+
 	MySubscriptionValid = true;
 	MemoryContextSwitchTo(oldctx);
 
-	/* Setup synchronous commit according to the user's wishes */
-	SetConfigOption("synchronous_commit", MySubscription->synccommit,
-					PGC_BACKEND, PGC_S_OVERRIDE);
-
 	if (!MySubscription->enabled)
 	{
 		ereport(LOG,
@@ -1571,6 +1577,10 @@ ApplyWorkerMain(Datum main_arg)
 		proc_exit(0);
 	}
 
+	/* Setup synchronous commit according to the user's wishes */
+	SetConfigOption("synchronous_commit", MySubscription->synccommit,
+					PGC_BACKEND, PGC_S_OVERRIDE);
+
 	/* Keep us informed about subscription changes. */
 	CacheRegisterSyscacheCallback(SUBSCRIPTIONOID,
 								  subscription_change_cb,