From 66d3e97e2685a1b62f91e628eb6726993c034956 Mon Sep 17 00:00:00 2001
From: Arthur Zakirov <z-arthur@yandex.ru>
Date: Mon, 22 May 2017 14:22:20 +0300
Subject: [PATCH] Do non-exclusive backup for PostgreSQL 9.6+

---
 backup.c | 56 +++++++++++++++++++++++++++++++++++---------------------
 1 file changed, 35 insertions(+), 21 deletions(-)

diff --git a/backup.c b/backup.c
index 2ccdd728..70d5d791 100644
--- a/backup.c
+++ b/backup.c
@@ -53,6 +53,13 @@ static int is_ptrack_enable = false;
 /* Backup connection */
 static PGconn *backup_conn = NULL;
 
+/* PostgreSQL server version from "backup_conn" */
+static int server_version = 0;
+
+static bool exclusive_backup = false;
+/* Is pg_start_backup() was executed */
+static bool backup_in_progress = false;
+
 typedef struct
 {
 	const char *from_root;
@@ -159,7 +166,7 @@ do_backup_database(parray *backup_list)
 	 * If backup_label does not exist in $PGDATA, stop taking backup.
 	 * NOTE. We can check it only on master, though.
 	 */
-	if(!from_replica)
+	if (exclusive_backup)
 	{
 		char		label_path[MAXPGPATH];
 		join_path_components(label_path, pgdata, PG_BACKUP_LABEL_FILE);
@@ -399,13 +406,15 @@ do_backup(void)
 	backup_conn = pgut_connect(pgut_dbname);
 	pgut_atexit_push(backup_disconnect, NULL);
 
-	/* Confirm that this server version is supported */
-	check_server_version();
 	/* Confirm data block size and xlog block size are compatible */
 	confirm_block_size("block_size", BLCKSZ);
 	confirm_block_size("wal_block_size", XLOG_BLCKSZ);
 
 	from_replica = pg_is_in_recovery();
+
+	/* Confirm that this server version is supported */
+	check_server_version();
+
 	current.checksum_version = get_data_checksum_version(true);
 	current.stream = stream_wal;
 
@@ -489,7 +498,6 @@ do_backup(void)
 static void
 check_server_version(void)
 {
-	static int server_version = 0;
 	/* confirm server version */
 	server_version = PQserverVersion(backup_conn);
 
@@ -506,6 +514,9 @@ check_server_version(void)
 			 server_version / 10000,
 			 (server_version / 100) % 100,
 			 server_version % 100, "9.6");
+
+	/* Do exclusive backup only for PostgreSQL 9.5 */
+	exclusive_backup = server_version < 90600;
 }
 
 /*
@@ -581,7 +592,7 @@ pg_start_backup(const char *label, bool smooth, pgBackup *backup)
 
 	/* 2nd argument is 'fast'*/
 	params[1] = smooth ? "false" : "true";
-	if (from_replica)
+	if (!exclusive_backup)
 		res = pgut_execute(backup_conn,
 						   "SELECT pg_start_backup($1, $2, false)",
 						   2,
@@ -592,6 +603,8 @@ pg_start_backup(const char *label, bool smooth, pgBackup *backup)
 						   2,
 						   params);
 
+	backup_in_progress = true;
+
 	/* Extract timeline and LSN from results of pg_start_backup() */
 	XLogDataFromLSN(PQgetvalue(res, 0, 0), &xlogid, &xrecoff);
 	/* Calculate LSN */
@@ -629,13 +642,13 @@ pg_switch_wal(void)
 					   NULL);
 	PQclear(res);
 
-#if PG_VERSION_NUM >= 100000
-	res = pgut_execute(backup_conn, "SELECT * FROM pg_switch_wal()", 0,
-					   NULL);
-#else
-	res = pgut_execute(backup_conn, "SELECT * FROM pg_switch_xlog()", 0,
-					   NULL);
-#endif
+	if (server_version >= 100000)
+		res = pgut_execute(backup_conn, "SELECT * FROM pg_switch_wal()", 0,
+						   NULL);
+	else
+		res = pgut_execute(backup_conn, "SELECT * FROM pg_switch_xlog()", 0,
+						   NULL);
+
 	PQclear(res);
 }
 
@@ -891,12 +904,15 @@ pg_stop_backup(pgBackup *backup)
 	time_t		recovery_time;
 	TransactionId recovery_xid;
 
+	if (!backup_in_progress)
+		elog(FATAL, "backup is not in progress");
+
 	/* Remove annoying NOTICE messages generated by backend */
 	res = pgut_execute(backup_conn, "SET client_min_messages = warning;",
 					   0, NULL);
 	PQclear(res);
 
-	if (from_replica)
+	if (!exclusive_backup)
 		/*
 		 * Stop the non-exclusive backup. Besides stop_lsn it returns from
 		 * pg_stop_backup(false) copy of the backup label and tablespace map
@@ -914,13 +930,15 @@ pg_stop_backup(pgBackup *backup)
 						   " FROM pg_stop_backup()",
 						   0, NULL);
 
+	backup_in_progress = false;
+
 	/* Extract timeline and LSN from results of pg_stop_backup() */
 	XLogDataFromLSN(PQgetvalue(res, 0, 0), &xlogid, &xrecoff);
 	/* Calculate LSN */
 	stop_backup_lsn = (XLogRecPtr) ((uint64) xlogid << 32) | xrecoff;
 
 	/* Write backup_label and tablespace_map for backup from replica */
-	if (from_replica)
+	if (!exclusive_backup)
 	{
 		char		path[MAXPGPATH];
 		char		backup_label[MAXPGPATH];
@@ -1081,8 +1099,6 @@ fileExists(const char *path)
 static void
 backup_cleanup(bool fatal, void *userdata)
 {
-	char path[MAXPGPATH];
-
 	/*
 	 * Update status of backup in BACKUP_CONTROL_FILE to ERROR.
 	 * end_time != 0 means backup finished
@@ -1096,13 +1112,11 @@ backup_cleanup(bool fatal, void *userdata)
 	}
 
 	/*
-	 * If backup_label exists in $PGDATA, notify stop of backup to PostgreSQL
-	 * TODO Do we need this?
+	 * If backup is in progress, notify stop of backup to PostgreSQL
 	 */
-	join_path_components(path, pgdata, PG_BACKUP_LABEL_FILE);
-	if (fileExists(path))
+	if (backup_in_progress)
 	{
-		elog(LOG, "%s exists, stop backup", PG_BACKUP_LABEL_FILE);
+		elog(LOG, "backup in progress, stop backup");
 		pg_stop_backup(NULL);	/* don't care stop_lsn on error case */
 	}
 }