PGPRO-2040: Use checkpoint LSN to retreive start_lsn for a replica backup

Author: za-arthur

src/backup.c

@@ -1142,10 +1142,11 @@ pg_switch_wal(PGconn *conn)
 	res = pgut_execute(conn, "SET client_min_messages = warning;", 0, NULL);
 	PQclear(res);
 
-	if (server_version >= 100000)
-		res = pgut_execute(conn, "SELECT * FROM pg_catalog.pg_switch_wal()", 0, NULL);
-	else
-		res = pgut_execute(conn, "SELECT * FROM pg_catalog.pg_switch_xlog()", 0, NULL);
+#if PG_VERSION_NUM >= 100000
+	res = pgut_execute(conn, "SELECT * FROM pg_catalog.pg_switch_wal()", 0, NULL);
+#else
+	res = pgut_execute(conn, "SELECT * FROM pg_catalog.pg_switch_xlog()", 0, NULL);
+#endif
 
 	PQclear(res);
 }
@@ -1584,9 +1585,6 @@ wait_replica_wal_lsn(XLogRecPtr lsn, bool is_start_backup)
 
 	while (true)
 	{
-		PGresult   *res;
-		uint32		lsn_hi;
-		uint32		lsn_lo;
 		XLogRecPtr	replica_lsn;
 
 		/*
@@ -1595,32 +1593,32 @@ wait_replica_wal_lsn(XLogRecPtr lsn, bool is_start_backup)
 		 */
 		if (is_start_backup)
 		{
-			if (server_version >= 100000)
-				res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_wal_replay_lsn()",
-								   0, NULL);
-			else
-				res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_xlog_replay_location()",
-								   0, NULL);
+			replica_lsn = get_checkpoint_location(backup_conn);
 		}
 		/*
 		 * For lsn from pg_stop_backup() we need it only to be received by
 		 * replica and fsync()'ed on WAL segment.
 		 */
 		else
 		{
-			if (server_version >= 100000)
-				res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_wal_receive_lsn()",
-								   0, NULL);
-			else
-				res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_xlog_receive_location()",
-								   0, NULL);
-		}
+			PGresult   *res;
+			uint32		lsn_hi;
+			uint32		lsn_lo;
 
-		/* Extract timeline and LSN from result */
-		XLogDataFromLSN(PQgetvalue(res, 0, 0), &lsn_hi, &lsn_lo);
-		/* Calculate LSN */
-		replica_lsn = ((uint64) lsn_hi) << 32 | lsn_lo;
-		PQclear(res);
+#if PG_VERSION_NUM >= 100000
+			res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_wal_receive_lsn()",
+							   0, NULL);
+#else
+			res = pgut_execute(backup_conn, "SELECT pg_catalog.pg_last_xlog_receive_location()",
+							   0, NULL);
+#endif
+
+			/* Extract LSN from result */
+			XLogDataFromLSN(PQgetvalue(res, 0, 0), &lsn_hi, &lsn_lo);
+			/* Calculate LSN */
+			replica_lsn = ((uint64) lsn_hi) << 32 | lsn_lo;
+			PQclear(res);
+		}
 
 		/* target lsn was replicated */
 		if (replica_lsn >= lsn)

src/pg_probackup.h

@@ -560,17 +560,19 @@ extern bool wal_contains_lsn(const char *archivedir, XLogRecPtr target_lsn,
 
 /* in util.c */
 extern TimeLineID get_current_timeline(bool safe);
+extern XLogRecPtr get_checkpoint_location(PGconn *conn);
+extern uint64 get_system_identifier(char *pgdata);
+extern uint64 get_remote_system_identifier(PGconn *conn);
+extern uint32 get_data_checksum_version(bool safe);
+
 extern void sanityChecks(void);
 extern void time2iso(char *buf, size_t len, time_t time);
 extern const char *status2str(BackupStatus status);
 extern void remove_trailing_space(char *buf, int comment_mark);
 extern void remove_not_digit(char *buf, size_t len, const char *str);
-extern uint32 get_data_checksum_version(bool safe);
 extern const char *base36enc(long unsigned int value);
 extern char *base36enc_dup(long unsigned int value);
 extern long unsigned int base36dec(const char *text);
-extern uint64 get_system_identifier(char *pgdata);
-extern uint64 get_remote_system_identifier(PGconn *conn);
 extern pg_time_t timestamptz_to_time_t(TimestampTz t);
 extern int parse_server_version(char *server_version_str);
 

src/util.c

@@ -122,6 +122,40 @@ get_current_timeline(bool safe)
 	return ControlFile.checkPointCopy.ThisTimeLineID;
 }
 
+/*
+ * Get last check point record ptr from pg_tonrol.
+ */
+XLogRecPtr
+get_checkpoint_location(PGconn *conn)
+{
+#if PG_VERSION_NUM >= 90600
+	PGresult   *res;
+	uint32		lsn_hi;
+	uint32		lsn_lo;
+	XLogRecPtr	lsn;
+
+	res = pgut_execute(conn,
+					   "SELECT checkpoint_location FROM pg_catalog.pg_control_checkpoint()",
+					   0, NULL);
+	XLogDataFromLSN(PQgetvalue(res, 0, 0), &lsn_hi, &lsn_lo);
+	PQclear(res);
+	/* Calculate LSN */
+	lsn = ((uint64) lsn_hi) << 32 | lsn_lo;
+
+	return lsn;
+#else
+	char	   *buffer;
+	size_t		size;
+	ControlFileData ControlFile;
+
+	buffer = fetchFile(conn, "global/pg_control", &size);
+	digestControlFile(&ControlFile, buffer, size);
+	pg_free(buffer);
+
+	return ControlFile.checkPoint;
+#endif
+}
+
 uint64
 get_system_identifier(char *pgdata_path)
 {