Issue #20. Check system_identifier for PostgreSQL9.5.

System identifier is fetched via pg_read_binary_file() for Pg 9.5.

src/backup.c

@@ -915,32 +915,15 @@ check_server_version(void)
 static void
 check_system_identifiers(void)
 {
-	PGresult   *res;
 	uint64		system_id_conn;
 	uint64		system_id_pgdata;
-	char	   *val;
 
 	system_id_pgdata = get_system_identifier(pgdata);
+	system_id_conn = get_remote_system_identifier(backup_conn);
 
-	if (server_version < 90600) {
-		// Skip match system_identifier between backup data directory and DB connection as
-		// pg_control_system() exists only in 9.6 onwards
-	} else {
-		res = pgut_execute(backup_conn,
-					   "SELECT system_identifier FROM pg_control_system()",
-					   0, NULL, true);
-		val = PQgetvalue(res, 0, 0);
-		if (!parse_uint64(val, &system_id_conn, 0))
-		{
-			PQclear(res);
-			elog(ERROR, "%s is not system_identifier", val);
-		}
-		PQclear(res);
-
-		if (system_id_conn != system_identifier)
-			elog(ERROR, "Backup data directory was initialized for system id %ld, but connected instance system id is %ld",
-				 system_identifier, system_id_conn);
-	}
+	if (system_id_conn != system_identifier)
+		elog(ERROR, "Backup data directory was initialized for system id %ld, but connected instance system id is %ld",
+			 system_identifier, system_id_conn);
 	if (system_id_pgdata != system_identifier)
 		elog(ERROR, "Backup data directory was initialized for system id %ld, but target backup directory system id is %ld",
 			 system_identifier, system_id_pgdata);

src/fetch.c

@@ -82,3 +82,35 @@ slurpFile(const char *datadir, const char *path, size_t *filesize, bool safe)
 		*filesize = len;
 	return buffer;
 }
+
+/*
+ * Receive a single file as a malloc'd buffer.
+ */
+char *
+fetchFile(PGconn *conn, const char *filename, size_t *filesize)
+{
+	PGresult   *res;
+	char	   *result;
+	const char *params[1];
+	int			len;
+
+	params[0] = filename;
+	res = pgut_execute(conn, "SELECT pg_read_binary_file($1)",
+					   1, params, false);
+
+	/* sanity check the result set */
+	if (PQntuples(res) != 1 || PQgetisnull(res, 0, 0))
+		elog(ERROR, "unexpected result set while fetching remote file \"%s\"",
+			 filename);
+
+	/* Read result to local variables */
+	len = PQgetlength(res, 0, 0);
+	result = pg_malloc(len + 1);
+	memcpy(result, PQgetvalue(res, 0, 0), len);
+	result[len] = '\0';
+
+	PQclear(res);
+	*filesize = len;
+
+	return result;
+}

src/pg_probackup.h

@@ -367,6 +367,7 @@ extern char *slurpFile(const char *datadir,
 					   const char *path,
 					   size_t *filesize,
 					   bool safe);
+extern char *fetchFile(PGconn *conn, const char *filename, size_t *filesize);
 
 /* in help.c */
 extern void help_pg_probackup(void);
@@ -458,6 +459,7 @@ extern uint32 get_data_checksum_version(bool safe);
 extern char *base36enc(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 void pgBackup_init(pgBackup *backup);
 

src/util.c

@@ -120,6 +120,39 @@ get_system_identifier(char *pgdata_path)
 	return ControlFile.system_identifier;
 }
 
+uint64
+get_remote_system_identifier(PGconn *conn)
+{
+#if PG_VERSION_NUM >= 90600
+	PGresult   *res;
+	uint64		system_id_conn;
+	char	   *val;
+
+	res = pgut_execute(conn,
+					   "SELECT system_identifier FROM pg_control_system()",
+					   0, NULL, true);
+	val = PQgetvalue(res, 0, 0);
+	if (!parse_uint64(val, &system_id_conn, 0))
+	{
+		PQclear(res);
+		elog(ERROR, "%s is not system_identifier", val);
+	}
+	PQclear(res);
+
+	return system_id_conn;
+#else
+	char	   *buffer;
+	size_t		size;
+	ControlFileData ControlFile;
+
+	buffer = fetchFile(conn, "global/pg_control", &size);
+	digestControlFile(&ControlFile, buffer, size);
+	pg_free(buffer);
+
+	return ControlFile.system_identifier;
+#endif
+}
+
 uint32
 get_data_checksum_version(bool safe)
 {

src/utils/pgut.c

@@ -1498,7 +1498,7 @@ pgut_set_port(const char *new_port)
 
 PGresult *
 pgut_execute(PGconn* conn, const char *query, int nParams, const char **params,
-			 bool exit_on_error)
+			 bool text_result)
 {
 	PGresult   *res;
 
@@ -1528,22 +1528,24 @@ pgut_execute(PGconn* conn, const char *query, int nParams, const char **params,
 	if (nParams == 0)
 		res = PQexec(conn, query);
 	else
-		res = PQexecParams(conn, query, nParams, NULL, params, NULL, NULL, 0);
+		res = PQexecParams(conn, query, nParams, NULL, params, NULL, NULL,
+						   /*
+							* Specify zero to obtain results in text format,
+							* or one to obtain results in binary format.
+							*/
+						   (text_result) ? 0 : 1);
 	on_after_exec();
 
-	if (exit_on_error)
+	switch (PQresultStatus(res))
 	{
-		switch (PQresultStatus(res))
-		{
-			case PGRES_TUPLES_OK:
-			case PGRES_COMMAND_OK:
-			case PGRES_COPY_IN:
-				break;
-			default:
-				elog(ERROR, "query failed: %squery was: %s",
-					 PQerrorMessage(conn), query);
-				break;
-		}
+		case PGRES_TUPLES_OK:
+		case PGRES_COMMAND_OK:
+		case PGRES_COPY_IN:
+			break;
+		default:
+			elog(ERROR, "query failed: %squery was: %s",
+				 PQerrorMessage(conn), query);
+			break;
 	}
 
 	return res;

src/utils/pgut.h

@@ -125,7 +125,8 @@ extern PGconn *pgut_connect_replication_extended(const char *pghost, const char
 									 const char *dbname, const char *login,
 									 const char *pwd);
 extern void pgut_disconnect(PGconn *conn);
-extern PGresult *pgut_execute(PGconn* conn, const char *query, int nParams, const char **params, bool exit_on_error);
+extern PGresult *pgut_execute(PGconn* conn, const char *query, int nParams,
+							  const char **params, bool text_result);
 extern bool pgut_send(PGconn* conn, const char *query, int nParams, const char **params, int elevel);
 extern void pgut_cancel(PGconn* conn);
 extern int pgut_wait(int num, PGconn *connections[], struct timeval *timeout);