Implement backup API functions for non-exclusive backups

Previously non-exclusive backups had to be done using the replication protocol
and pg_basebackup. With this commit it's now possible to make them using
pg_start_backup/pg_stop_backup as well, as long as the backup program can
maintain a persistent connection to the database.

Doing this, backup_label and tablespace_map are returned as results from
pg_stop_backup() instead of being written to the data directory. This makes
the server safe from a crash during an ongoing backup, which can be a problem
with exclusive backups.

The old syntax of the functions remain and work exactly as before, but since the
new syntax is safer this should eventually be deprecated and removed.

Only reference documentation is included. The main section on backup still needs
to be rewritten to cover this, but since that is already scheduled for a separate
large rewrite, it's not included in this patch.

Reviewed by David Steele and Amit Kapila

1 files changed, 31 insertions, 24 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 00140ba8f40..f9644db0c35 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -9768,8 +9768,8 @@ XLogFileNameP(TimeLineID tli, XLogSegNo segno)
  */
 XLogRecPtr
 do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
-				   char **labelfile, DIR *tblspcdir, List **tablespaces,
-				   char **tblspcmapfile, bool infotbssize,
+				   StringInfo labelfile, DIR *tblspcdir, List **tablespaces,
+				   StringInfo tblspcmapfile, bool infotbssize,
 				   bool needtblspcmapfile)
 {
 	bool		exclusive = (labelfile == NULL);
@@ -9783,8 +9783,6 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 	XLogSegNo	_logSegNo;
 	struct stat stat_buf;
 	FILE	   *fp;
-	StringInfoData labelfbuf;
-	StringInfoData tblspc_mapfbuf;
 
 	backup_started_in_recovery = RecoveryInProgress();
 
@@ -9981,7 +9979,8 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 		/*
 		 * Construct tablespace_map file
 		 */
-		initStringInfo(&tblspc_mapfbuf);
+		if (exclusive)
+			tblspcmapfile = makeStringInfo();
 
 		datadirpathlen = strlen(DataDir);
 
@@ -10054,7 +10053,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 			if (tablespaces)
 				*tablespaces = lappend(*tablespaces, ti);
 
-			appendStringInfo(&tblspc_mapfbuf, "%s %s\n", ti->oid, ti->path);
+			appendStringInfo(tblspcmapfile, "%s %s\n", ti->oid, ti->path);
 
 			pfree(buflinkpath.data);
 #else
@@ -10073,23 +10072,24 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 		/*
 		 * Construct backup label file
 		 */
-		initStringInfo(&labelfbuf);
+		if (exclusive)
+			labelfile = makeStringInfo();
 
 		/* Use the log timezone here, not the session timezone */
 		stamp_time = (pg_time_t) time(NULL);
 		pg_strftime(strfbuf, sizeof(strfbuf),
 					"%Y-%m-%d %H:%M:%S %Z",
 					pg_localtime(&stamp_time, log_timezone));
-		appendStringInfo(&labelfbuf, "START WAL LOCATION: %X/%X (file %s)\n",
+		appendStringInfo(labelfile, "START WAL LOCATION: %X/%X (file %s)\n",
 			 (uint32) (startpoint >> 32), (uint32) startpoint, xlogfilename);
-		appendStringInfo(&labelfbuf, "CHECKPOINT LOCATION: %X/%X\n",
+		appendStringInfo(labelfile, "CHECKPOINT LOCATION: %X/%X\n",
 					 (uint32) (checkpointloc >> 32), (uint32) checkpointloc);
-		appendStringInfo(&labelfbuf, "BACKUP METHOD: %s\n",
+		appendStringInfo(labelfile, "BACKUP METHOD: %s\n",
 						 exclusive ? "pg_start_backup" : "streamed");
-		appendStringInfo(&labelfbuf, "BACKUP FROM: %s\n",
+		appendStringInfo(labelfile, "BACKUP FROM: %s\n",
 						 backup_started_in_recovery ? "standby" : "master");
-		appendStringInfo(&labelfbuf, "START TIME: %s\n", strfbuf);
-		appendStringInfo(&labelfbuf, "LABEL: %s\n", backupidstr);
+		appendStringInfo(labelfile, "START TIME: %s\n", strfbuf);
+		appendStringInfo(labelfile, "LABEL: %s\n", backupidstr);
 
 		/*
 		 * Okay, write the file, or return its contents to caller.
@@ -10123,7 +10123,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 						(errcode_for_file_access(),
 						 errmsg("could not create file \"%s\": %m",
 								BACKUP_LABEL_FILE)));
-			if (fwrite(labelfbuf.data, labelfbuf.len, 1, fp) != 1 ||
+			if (fwrite(labelfile->data, labelfile->len, 1, fp) != 1 ||
 				fflush(fp) != 0 ||
 				pg_fsync(fileno(fp)) != 0 ||
 				ferror(fp) ||
@@ -10132,10 +10132,12 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 						(errcode_for_file_access(),
 						 errmsg("could not write file \"%s\": %m",
 								BACKUP_LABEL_FILE)));
-			pfree(labelfbuf.data);
+			/* Allocated locally for exclusive backups, so free separately */
+			pfree(labelfile->data);
+			pfree(labelfile);
 
 			/* Write backup tablespace_map file. */
-			if (tblspc_mapfbuf.len > 0)
+			if (tblspcmapfile->len > 0)
 			{
 				if (stat(TABLESPACE_MAP, &stat_buf) != 0)
 				{
@@ -10159,7 +10161,7 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 							(errcode_for_file_access(),
 							 errmsg("could not create file \"%s\": %m",
 									TABLESPACE_MAP)));
-				if (fwrite(tblspc_mapfbuf.data, tblspc_mapfbuf.len, 1, fp) != 1 ||
+				if (fwrite(tblspcmapfile->data, tblspcmapfile->len, 1, fp) != 1 ||
 					fflush(fp) != 0 ||
 					pg_fsync(fileno(fp)) != 0 ||
 					ferror(fp) ||
@@ -10170,13 +10172,9 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 									TABLESPACE_MAP)));
 			}
 
-			pfree(tblspc_mapfbuf.data);
-		}
-		else
-		{
-			*labelfile = labelfbuf.data;
-			if (tblspc_mapfbuf.len > 0)
-				*tblspcmapfile = tblspc_mapfbuf.data;
+			/* Allocated locally for exclusive backups, so free separately */
+			pfree(tblspcmapfile->data);
+			pfree(tblspcmapfile);
 		}
 	}
 	PG_END_ENSURE_ERROR_CLEANUP(pg_start_backup_callback, (Datum) BoolGetDatum(exclusive));
@@ -10283,7 +10281,16 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 	 */
 	WALInsertLockAcquireExclusive();
 	if (exclusive)
+	{
+		if (!XLogCtl->Insert.exclusiveBackup)
+		{
+			WALInsertLockRelease();
+			ereport(ERROR,
+					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					 errmsg("exclusive backup not in progress")));
+		}
 		XLogCtl->Insert.exclusiveBackup = false;
+	}
 	else
 	{
 		/*