Don't repeatedly register cache callbacks in pgoutput plugin.

Multiple cycles of starting up and shutting down the plugin within a
single session would eventually lead to "out of relcache_callback_list
slots", because pgoutput_startup blindly re-registered its cache
callbacks each time.  Fix it to register them only once, as all other
users of cache callbacks already take care to do.

This has been broken all along, so back-patch to all supported branches.

Shi Yu

Discussion: https://postgr.es/m/OSZPR01MB631004A78D743D68921FFAD3FDA79@OSZPR01MB6310.jpnprd01.prod.outlook.com

src/backend/replication/pgoutput/pgoutput.c

@@ -260,6 +260,7 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 				 bool is_init)
 {
 	PGOutputData *data = palloc0(sizeof(PGOutputData));
+	static bool publication_callback_registered = false;
 
 	/* Create our memory context for private allocations. */
 	data->context = AllocSetContextCreate(ctx->context,
@@ -323,9 +324,18 @@ pgoutput_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 		/* Init publication state. */
 		data->publications = NIL;
 		publications_valid = false;
+
+		/*
+		 * Register callback for pg_publication if we didn't already do that
+		 * during some previous call in this process.
+		 */
+		if (!publication_callback_registered)
+		{
 			CacheRegisterSyscacheCallback(PUBLICATIONOID,
 										  publication_invalidation_cb,
 										  (Datum) 0);
+			publication_callback_registered = true;
+		}
 
 		/* Initialize relation schema cache. */
 		init_rel_sync_cache(CacheMemoryContext);
@@ -948,7 +958,9 @@ static void
 init_rel_sync_cache(MemoryContext cachectx)
 {
 	HASHCTL		ctl;
+	static bool relation_callbacks_registered = false;
 
+	/* Nothing to do if hash table already exists */
 	if (RelationSyncCache != NULL)
 		return;
 
@@ -963,10 +975,16 @@ init_rel_sync_cache(MemoryContext cachectx)
 
 	Assert(RelationSyncCache != NULL);
 
+	/* No more to do if we already registered callbacks */
+	if (relation_callbacks_registered)
+		return;
+
 	CacheRegisterRelcacheCallback(rel_sync_cache_relation_cb, (Datum) 0);
 	CacheRegisterSyscacheCallback(PUBLICATIONRELMAP,
 								  rel_sync_cache_publication_cb,
 								  (Datum) 0);
+
+	relation_callbacks_registered = true;
 }
 
 /*