MDEV-33136: Properly BF-abort user transactions with explicit locks

User transactions may acquire explicit MDL locks from InnoDB level
when persistent statistics is re-read for a table.
If such a transaction would be subject to BF-abort, it was improperly
detected as a system transaction and wouldn't get aborted.

The fix: Check if a transaction holding explicit MDL locks is a user
transaction in the MDL conflict handling code.
Signed-off-by: Julius Goryavsky <julius.goryavsky@mariadb.com>

mysql-test/suite/galera/r/MDEV-33136.result

+connection node_2;
+connection node_1;
+connect node_1a,127.0.0.1,root,,test,$NODE_MYPORT_1;
+connection node_1;
+CREATE TABLE t1 (f1 INTEGER PRIMARY KEY) ENGINE=InnoDB;
+connection node_1a;
+TRUNCATE TABLE t1;
+SET SESSION wsrep_retry_autocommit = 0;
+SET DEBUG_SYNC = 'dict_stats_mdl_acquired SIGNAL may_toi WAIT_FOR bf_abort';
+INSERT INTO t1 VALUES (1);
+connection node_1;
+SET DEBUG_SYNC = 'now WAIT_FOR may_toi';
+TRUNCATE TABLE t1;
+connection node_1a;
+ERROR 40001: Deadlock found when trying to get lock; try restarting transaction
+connection node_1;
+SET DEBUG_SYNC = 'RESET';
+DROP TABLE t1;
+disconnect node_1a;
+disconnect node_2;
+disconnect node_1;

mysql-test/suite/galera/t/MDEV-33136.test

+#
+# MDEV-33136: Properly BF-abort user transactions with explicit locks
+#
+# User transactions may acquire explicit MDL locks from InnoDB level
+# when persistent statistics is re-read for a table.
+# If such a transaction would be subject to BF-abort, it was improperly
+# detected as a system transaction and wouldn't get aborted.
+#
+# The fix: Check if a transaction holding explicit MDL locks is a user
+# transaction in the MDL conflict handling code.
+
+--source include/galera_cluster.inc
+--source include/have_debug_sync.inc
+--source include/have_debug.inc
+
+--connect node_1a,127.0.0.1,root,,test,$NODE_MYPORT_1
+
+--connection node_1
+CREATE TABLE t1 (f1 INTEGER PRIMARY KEY) ENGINE=InnoDB;
+
+--connection node_1a
+TRUNCATE TABLE t1;
+# TRUNCATE forces the next statement to re-read statistics from persistent storage,
+# which will acquire MDL locks on the statistics tables in InnoDB.
+SET SESSION wsrep_retry_autocommit = 0;
+SET DEBUG_SYNC = 'dict_stats_mdl_acquired SIGNAL may_toi WAIT_FOR bf_abort';
+--send
+  INSERT INTO t1 VALUES (1);
+
+--connection node_1
+SET DEBUG_SYNC = 'now WAIT_FOR may_toi';
+TRUNCATE TABLE t1;
+
+--connection node_1a
+# Local INSERT gets aborted.
+--error ER_LOCK_DEADLOCK
+--reap
+
+# Cleanup
+--connection node_1
+SET DEBUG_SYNC = 'RESET';
+DROP TABLE t1;
+--disconnect node_1a
+--source include/galera_end.inc

sql/wsrep_mysqld.cc

@@ -3168,7 +3168,9 @@ void wsrep_handle_mdl_conflict(MDL_context *requestor_ctx,
       }
     }
     else if (granted_thd->lex->sql_command == SQLCOM_FLUSH ||
-             granted_thd->mdl_context.has_explicit_locks())
+            /* System transactions with explicit locks are BACKUP. */
+            (granted_thd->system_thread != NON_SYSTEM_THREAD &&
+             granted_thd->mdl_context.has_explicit_locks()))
     {
       WSREP_DEBUG("BF thread waiting for FLUSH");
       ticket->wsrep_report(wsrep_debug);

storage/innobase/dict/dict0stats.cc

@@ -3865,6 +3865,10 @@ dict_stats_fetch_from_ps(
 		goto release_and_exit;
 	}
 
+#ifdef ENABLED_DEBUG_SYNC
+	DEBUG_SYNC(thd, "dict_stats_mdl_acquired");
+#endif /* ENABLED_DEBUG_SYNC */
+
 	trx = trx_create();
 
 	trx_start_internal_read_only(trx);