Logical replication

- Add PUBLICATION catalogs and DDL
- Add SUBSCRIPTION catalog and DDL
- Define logical replication protocol and output plugin
- Add logical replication workers

From: Petr Jelinek <petr@2ndquadrant.com>
Reviewed-by: Steve Singer <steve@ssinger.info>
Reviewed-by: Andres Freund <andres@anarazel.de>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>

src/backend/commands/dbcommands.c

@@ -37,6 +37,7 @@
 #include "catalog/pg_authid.h"
 #include "catalog/pg_database.h"
 #include "catalog/pg_db_role_setting.h"
+#include "catalog/pg_subscription.h"
 #include "catalog/pg_tablespace.h"
 #include "commands/comment.h"
 #include "commands/dbcommands.h"
@@ -790,6 +791,7 @@ dropdb(const char *dbname, bool missing_ok)
 	int			npreparedxacts;
 	int			nslots,
 				nslots_active;
+	int			nsubscriptions;
 
 	/*
 	 * Look up the target database's OID, and get exclusive lock on it. We
@@ -874,6 +876,21 @@ dropdb(const char *dbname, bool missing_ok)
 						dbname),
 				 errdetail_busy_db(notherbackends, npreparedxacts)));
 
+	/*
+	 * Check if there are subscriptions defined in the target database.
+	 *
+	 * We can't drop them automatically because they might be holding
+	 * resources in other databases/instances.
+	 */
+	if ((nsubscriptions = CountDBSubscriptions(db_id)) > 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_IN_USE),
+				 errmsg("database \"%s\" is being used by logical replication subscription",
+						dbname),
+				 errdetail_plural("There is %d subscription.",
+								  "There are %d subscriptions.",
+								  nsubscriptions, nsubscriptions)));
+
 	/*
 	 * Remove the database's tuple from pg_database.
 	 */