MDEV-30423 Deadlock on Replica during BACKUP STAGE BLOCK_COMMIT on XA transactions

The user XA commit execution branch was caught not have been covered
with MDEV-21953 fixes.

The XA involved deadlock is resolved now to apply the former fixes
pattern.
Along the fixes the following changes have been implemented.
- MDL lock attribute correction
- dissociation of the externally completed XA from the current
  thread's xid_state in the error branches
- cleanup_context() preseves the prepared XA
- wait_for_prior_commit() is relocated to satisfy both
  the binlog ON (log-slave-updates and skip-log-bin)
  and OFF slave execution branches.

mysql-test/suite/rpl/t/parallel_backup.test

@@ -12,6 +12,9 @@
 CREATE TABLE t1 (a INT PRIMARY KEY) ENGINE = innodb;
 
 --sync_slave_with_master
+call mtr.add_suppression("Deadlock found when trying to get lock");
+call mtr.add_suppression("Commit failed due to failure of an earlier commit");
+
 --source include/stop_slave.inc
 SET @old_parallel_threads= @@GLOBAL.slave_parallel_threads;
 SET @old_parallel_mode   = @@GLOBAL.slave_parallel_mode;
@@ -61,6 +64,32 @@ BACKUP STAGE END;
 --let $diff_tables= master:t1,slave:t1
 --source include/diff_tables.inc
 
+#
+--echo # MDEV-30423: dealock XA COMMIT vs BACKUP
+# Prove XA "COMPLETE" 'xid' does not dealock similary to the normal trx case.
+# The slave binlog group commit leader is blocked by a local trx like in
+# the above normal trx case.
+# [Notice a reuse of t1,aux_conn from above.]
+#
+--let $complete = COMMIT
+--source parallel_backup_xa.inc
+--let $complete = ROLLBACK
+--source parallel_backup_xa.inc
+
+--let $slave_ooo_error = 1
+--let $complete = COMMIT
+--source parallel_backup_xa.inc
+--connection slave
+--source include/start_slave.inc
+--source include/sync_with_master_gtid.inc
+
+--let $slave_ooo_error = 1
+--let $complete = ROLLBACK
+--source parallel_backup_xa.inc
+--connection slave
+--source include/start_slave.inc
+--source include/sync_with_master_gtid.inc
+
 
 # Clean up.
 --connection slave