Possible fix for stray deadlock kill after parallel replication of XA PREPARE,...

Possible fix for stray deadlock kill after parallel replication of XA PREPARE, using somewhat undesirable global mutex in trx_disconnect_prepared()
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

storage/innobase/trx/trx0trx.cc

@@ -554,7 +554,16 @@ void trx_disconnect_prepared(trx_t *trx)
   trx->read_view.close();
   trx_sys.trx_list.freeze();
   trx->is_recovered= true;
+  /*
+    Take the lock_sys.wait_mutex around releasing the THD associated with the
+    prepared transaction. This way, lock_wait() will see a consistent picture
+    while processing locking state: either it runs before clearing the
+    trx->mysql_thd, and sees the lock owned by a specific THD; or it runs after
+    and sees a lock not associated with THD (but not some mixture).
+  */
+  mysql_mutex_lock(&lock_sys.wait_mutex);
   trx->mysql_thd= NULL;
+  mysql_mutex_unlock(&lock_sys.wait_mutex);
   trx_sys.trx_list.unfreeze();
   /* todo/fixme: suggest to do it at innodb prepare */
   trx->will_lock= false;