Fix backup canceling

Assert-enabled build crashes but without asserts it works by wrong way:
it may not reset forcing full page write and preventing from starting
exclusive backup with the same name as cancelled.
Patch replaces pair of booleans
nonexclusive_backup_running/exclusive_backup_running to single enum to
correctly describe backup state.

Backpatch to 9.6 where bug was introduced

Reported-by: David Steele
Authors: Michael Paquier, David Steele
Reviewed-by: Anastasia Lubennikova

https://commitfest.postgresql.org/13/1068/

Author: feodor

src/backend/access/transam/xlog.c

@@ -482,6 +482,12 @@ typedef enum ExclusiveBackupState
 	EXCLUSIVE_BACKUP_STOPPING
 } ExclusiveBackupState;
 
+/*
+ * Session status of running backup, used for sanity checks in SQL-callable
+ * functions to start and stop backups.
+ */
+static SessionBackupState sessionBackupState = SESSION_BACKUP_NONE;
+
 /*
  * Shared state data for WAL insertion.
  */
@@ -10249,13 +10255,17 @@ do_pg_start_backup(const char *backupidstr, bool fast, TimeLineID *starttli_p,
 
 	/*
 	 * Mark that start phase has correctly finished for an exclusive backup.
+	 * Session-level locks are updated as well to reflect that state.
 	 */
 	if (exclusive)
 	{
 		WALInsertLockAcquireExclusive();
 		XLogCtl->Insert.exclusiveBackupState = EXCLUSIVE_BACKUP_IN_PROGRESS;
 		WALInsertLockRelease();
+		sessionBackupState = SESSION_BACKUP_EXCLUSIVE;
 	}
+	else
+		sessionBackupState = SESSION_BACKUP_NON_EXCLUSIVE;
 
 	/*
 	 * We're done.  As a convenience, return the starting WAL location.
@@ -10310,6 +10320,15 @@ pg_stop_backup_callback(int code, Datum arg)
 	WALInsertLockRelease();
 }
 
+/*
+ * Utility routine to fetch the session-level status of a backup running.
+ */
+SessionBackupState
+get_backup_status(void)
+{
+	return sessionBackupState;
+}
+
 /*
  * do_pg_stop_backup is the workhorse of the user-visible pg_stop_backup()
  * function.
@@ -10477,6 +10496,9 @@ do_pg_stop_backup(char *labelfile, bool waitforarchive, TimeLineID *stoptli_p)
 	}
 	WALInsertLockRelease();
 
+	/* Clean up session-level lock */
+	sessionBackupState = SESSION_BACKUP_NONE;
+
 	/*
 	 * Read and parse the START WAL LOCATION line (this code is pretty crude,
 	 * but we are not expecting any variability in the file format).

src/backend/access/transam/xlogfuncs.c

@@ -43,8 +43,6 @@
  */
 static StringInfo label_file;
 static StringInfo tblspc_map_file;
-static bool exclusive_backup_running = false;
-static bool nonexclusive_backup_running = false;
 
 /*
  * Called when the backend exits with a running non-exclusive base backup,
@@ -79,10 +77,11 @@ pg_start_backup(PG_FUNCTION_ARGS)
 	char	   *backupidstr;
 	XLogRecPtr	startpoint;
 	DIR		   *dir;
+	SessionBackupState status = get_backup_status();
 
 	backupidstr = text_to_cstring(backupid);
 
-	if (exclusive_backup_running || nonexclusive_backup_running)
+	if (status == SESSION_BACKUP_NON_EXCLUSIVE)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("a backup is already in progress in this session")));
@@ -97,7 +96,6 @@ pg_start_backup(PG_FUNCTION_ARGS)
 	{
 		startpoint = do_pg_start_backup(backupidstr, fast, NULL, NULL,
 										dir, NULL, NULL, false, true);
-		exclusive_backup_running = true;
 	}
 	else
 	{
@@ -114,7 +112,6 @@ pg_start_backup(PG_FUNCTION_ARGS)
 
 		startpoint = do_pg_start_backup(backupidstr, fast, NULL, label_file,
 									dir, NULL, tblspc_map_file, false, true);
-		nonexclusive_backup_running = true;
 
 		before_shmem_exit(nonexclusive_base_backup_cleanup, (Datum) 0);
 	}
@@ -148,23 +145,22 @@ Datum
 pg_stop_backup(PG_FUNCTION_ARGS)
 {
 	XLogRecPtr	stoppoint;
+	SessionBackupState status = get_backup_status();
 
-	if (nonexclusive_backup_running)
+	if (status == SESSION_BACKUP_NON_EXCLUSIVE)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("non-exclusive backup in progress"),
 				 errhint("Did you mean to use pg_stop_backup('f')?")));
 
 	/*
 	 * Exclusive backups were typically started in a different connection, so
-	 * don't try to verify that exclusive_backup_running is set in this one.
-	 * Actual verification that an exclusive backup is in fact running is
-	 * handled inside do_pg_stop_backup.
+	 * don't try to verify that status of backup is set to
+	 * SESSION_BACKUP_EXCLUSIVE in this function. Actual verification that an
+	 * exclusive backup is in fact running is handled inside do_pg_stop_backup.
 	 */
 	stoppoint = do_pg_stop_backup(NULL, true, NULL);
 
-	exclusive_backup_running = false;
-
 	PG_RETURN_LSN(stoppoint);
 }
 
@@ -192,6 +188,7 @@ pg_stop_backup_v2(PG_FUNCTION_ARGS)
 
 	bool		exclusive = PG_GETARG_BOOL(0);
 	XLogRecPtr	stoppoint;
+	SessionBackupState status = get_backup_status();
 
 	/* check to see if caller supports us returning a tuplestore */
 	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
@@ -223,7 +220,7 @@ pg_stop_backup_v2(PG_FUNCTION_ARGS)
 
 	if (exclusive)
 	{
-		if (nonexclusive_backup_running)
+		if (status == SESSION_BACKUP_NON_EXCLUSIVE)
 			ereport(ERROR,
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 errmsg("non-exclusive backup in progress"),
@@ -234,14 +231,13 @@ pg_stop_backup_v2(PG_FUNCTION_ARGS)
 		 * return NULL for both backup_label and tablespace_map.
 		 */
 		stoppoint = do_pg_stop_backup(NULL, true, NULL);
-		exclusive_backup_running = false;
 
 		nulls[1] = true;
 		nulls[2] = true;
 	}
 	else
 	{
-		if (!nonexclusive_backup_running)
+		if (status != SESSION_BACKUP_NON_EXCLUSIVE)
 			ereport(ERROR,
 					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 					 errmsg("non-exclusive backup is not in progress"),
@@ -252,7 +248,6 @@ pg_stop_backup_v2(PG_FUNCTION_ARGS)
 		 * and tablespace map so they can be written to disk by the caller.
 		 */
 		stoppoint = do_pg_stop_backup(label_file->data, true, NULL);
-		nonexclusive_backup_running = false;
 		cancel_before_shmem_exit(nonexclusive_base_backup_cleanup, (Datum) 0);
 
 		values[1] = CStringGetTextDatum(label_file->data);

src/include/access/xlog.h

@@ -274,15 +274,34 @@ extern void assign_max_wal_size(int newval, void *extra);
 extern void assign_checkpoint_completion_target(double newval, void *extra);
 
 /*
- * Starting/stopping a base backup
+ * Routines to start, stop, and get status of a base backup.
  */
+
+/*
+ * Session-level status of base backups
+ *
+ * This is used in parallel with the shared memory status to control parallel
+ * execution of base backup functions for a given session, be it a backend
+ * dedicated to replication or a normal backend connected to a database. The
+ * update of the session-level status happens at the same time as the shared
+ * memory counters to keep a consistent global and local state of the backups
+ * running.
+ */
+typedef enum SessionBackupState
+{
+	SESSION_BACKUP_NONE,
+	SESSION_BACKUP_EXCLUSIVE,
+	SESSION_BACKUP_NON_EXCLUSIVE
+} SessionBackupState;
+
 extern XLogRecPtr do_pg_start_backup(const char *backupidstr, bool fast,
 				TimeLineID *starttli_p, StringInfo labelfile, DIR *tblspcdir,
 			  List **tablespaces, StringInfo tblspcmapfile, bool infotbssize,
 				   bool needtblspcmapfile);
 extern XLogRecPtr do_pg_stop_backup(char *labelfile, bool waitforarchive,
 				  TimeLineID *stoptli_p);
 extern void do_pg_abort_backup(void);
+extern SessionBackupState get_backup_status(void);
 
 /* File path names (all relative to $PGDATA) */
 #define BACKUP_LABEL_FILE		"backup_label"