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/

1 files changed, 22 insertions, 0 deletions

diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index b99ded5df67..58790e0e96e 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -504,6 +504,12 @@ typedef enum ExclusiveBackupState
 } 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.
  */
 typedef struct XLogCtlInsert
@@ -10566,13 +10572,17 @@ 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.
@@ -10628,6 +10638,15 @@ pg_stop_backup_callback(int code, Datum arg)
 }
 
 /*
+ * 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.
  *
@@ -10794,6 +10813,9 @@ 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).