Rearrange dropdb() to avoid errors after allowing other sessions to exit.

During Drop Database, it is better to error out before allowing other
sessions to exit and forcefully terminating autovacuum workers.  All the
other errors except for checking subscriptions are already done before.

Author: Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1+qhLkCYG2oy9xug9ur_j=G2wQNRYAyd+-kZfZ1z42pLw@mail.gmail.com

src/backend/commands/dbcommands.c

@@ -896,19 +896,6 @@ dropdb(const char *dbname, bool missing_ok)
 								  nslots_active, nslots_active)));
 	}
 
-	/*
-	 * Check for other backends in the target database.  (Because we hold the
-	 * database lock, no new ones can start after this.)
-	 *
-	 * As in CREATE DATABASE, check this after other error conditions.
-	 */
-	if (CountOtherDBBackends(db_id, &notherbackends, &npreparedxacts))
-		ereport(ERROR,
-				(errcode(ERRCODE_OBJECT_IN_USE),
-				 errmsg("database \"%s\" is being accessed by other users",
-						dbname),
-				 errdetail_busy_db(notherbackends, npreparedxacts)));
-
 	/*
 	 * Check if there are subscriptions defined in the target database.
 	 *
@@ -924,6 +911,19 @@ dropdb(const char *dbname, bool missing_ok)
 								  "There are %d subscriptions.",
 								  nsubscriptions, nsubscriptions)));
 
+	/*
+	 * Check for other backends in the target database.  (Because we hold the
+	 * database lock, no new ones can start after this.)
+	 *
+	 * As in CREATE DATABASE, check this after other error conditions.
+	 */
+	if (CountOtherDBBackends(db_id, &notherbackends, &npreparedxacts))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_IN_USE),
+				 errmsg("database \"%s\" is being accessed by other users",
+						dbname),
+				 errdetail_busy_db(notherbackends, npreparedxacts)));
+
 	/*
 	 * Remove the database's tuple from pg_database.
 	 */