pg_createsubscriber: Fix an unpredictable recovery wait time.

Amit Kapila · Amit Kapila · commit 03b08c8f5f3e · 2024-07-30T14:01:01.000+05:30
The problem is that the tool is using the LSN returned by pg_create_logical_replication_slot() as recovery_target_lsn. This LSN is ahead of the current WAL position and the recovery waits until the publisher writes a WAL record to reach the target and ends the recovery. On idle systems, this wait time is unpredictable and could lead to failure in promoting the subscriber. To avoid that, insert a harmless WAL record. Reported-by: Alexander Lakhin and Tom Lane Diagnosed-by: Hayato Kuroda Author: Euler Taveira Reviewed-by: Hayato Kuroda, Amit Kapila Backpatch-through: 17 Discussion: https://postgr.es/m/2377319.1719766794%40sss.pgh.pa.us Discussion: https://postgr.es/m/CA+TgmoYcY+Wb67NAwaHT7MvxCSeV86oSc+va9hHKaasE42ukyw@mail.gmail.com
diff --git a/src/bin/pg_basebackup/pg_createsubscriber.c b/src/bin/pg_basebackup/pg_createsubscriber.c
@@ -778,6 +778,28 @@ setup_publisher(struct LogicalRepInfo *dbinfo)
 		else
 			exit(1);
 
+		/*
+		 * Since we are using the LSN returned by the last replication slot as
+		 * recovery_target_lsn, this LSN is ahead of the current WAL position
+		 * and the recovery waits until the publisher writes a WAL record to
+		 * reach the target and ends the recovery. On idle systems, this wait
+		 * time is unpredictable and could lead to failure in promoting the
+		 * subscriber. To avoid that, insert a harmless WAL record.
+		 */
+		if (i == num_dbs - 1 && !dry_run)
+		{
+			PGresult   *res;
+
+			res = PQexec(conn, "SELECT pg_log_standby_snapshot()");
+			if (PQresultStatus(res) != PGRES_TUPLES_OK)
+			{
+				pg_log_error("could not write an additional WAL record: %s",
+							 PQresultErrorMessage(res));
+				disconnect_database(conn, true);
+			}
+			PQclear(res);
+		}
+
 		disconnect_database(conn, false);
 	}
 
