MDEV-8113: Parallel slave: slave hangs on ALTER TABLE (or other DDL) as the...

MDEV-8113: Parallel slave: slave hangs on ALTER TABLE (or other DDL) as the first event after slave start

In optimistic parallel replication, it is not safe to try to run a following
transaction in parallel with a DDL statement, and there is code to prevent
this.

However, the code was missing the case where the DDL is the very first event
after slave start. In this case, following transactions could run in
parallel with the DDL, which can cause the slave to hang or even corrupt
slave in unlucky cases.

mysql-test/suite/rpl/r/rpl_parallel_optimistic.result

@@ -455,6 +455,47 @@ a	b
 include/stop_slave.inc
 SET GLOBAL debug_dbug= @old_debug;
 include/start_slave.inc
+*** MDEV-8113: ALTER TABLE causes slave hang in optimistic parallel replication ***
+include/stop_slave.inc
+ALTER TABLE t2 ADD c INT;
+INSERT INTO t2 (a,b) VALUES (50, 0);
+INSERT INTO t2 (a,b) VALUES (51, 1);
+INSERT INTO t2 (a,b) VALUES (52, 2);
+INSERT INTO t2 (a,b) VALUES (53, 3);
+INSERT INTO t2 (a,b) VALUES (54, 4);
+INSERT INTO t2 (a,b) VALUES (55, 5);
+INSERT INTO t2 (a,b) VALUES (56, 6);
+INSERT INTO t2 (a,b) VALUES (57, 7);
+INSERT INTO t2 (a,b) VALUES (58, 8);
+INSERT INTO t2 (a,b) VALUES (59, 9);
+ALTER TABLE t2 DROP COLUMN c;
+SELECT * FROM t2 WHERE a >= 50 ORDER BY a;
+a	b
+50	0
+51	1
+52	2
+53	3
+54	4
+55	5
+56	6
+57	7
+58	8
+59	9
+include/save_master_gtid.inc
+include/start_slave.inc
+include/sync_with_master_gtid.inc
+SELECT * FROM t2 WHERE a >= 50 ORDER BY a;
+a	b
+50	0
+51	1
+52	2
+53	3
+54	4
+55	5
+56	6
+57	7
+58	8
+59	9
 include/stop_slave.inc
 SET GLOBAL slave_parallel_mode=@old_parallel_mode;
 SET GLOBAL slave_parallel_threads=@old_parallel_threads;

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

@@ -429,6 +429,33 @@ SET GLOBAL debug_dbug= @old_debug;
 --source include/start_slave.inc
 
 
+--echo *** MDEV-8113: ALTER TABLE causes slave hang in optimistic parallel replication ***
+
+--connection server_2
+--source include/stop_slave.inc
+
+--connection server_1
+ALTER TABLE t2 ADD c INT;
+INSERT INTO t2 (a,b) VALUES (50, 0);
+INSERT INTO t2 (a,b) VALUES (51, 1);
+INSERT INTO t2 (a,b) VALUES (52, 2);
+INSERT INTO t2 (a,b) VALUES (53, 3);
+INSERT INTO t2 (a,b) VALUES (54, 4);
+INSERT INTO t2 (a,b) VALUES (55, 5);
+INSERT INTO t2 (a,b) VALUES (56, 6);
+INSERT INTO t2 (a,b) VALUES (57, 7);
+INSERT INTO t2 (a,b) VALUES (58, 8);
+INSERT INTO t2 (a,b) VALUES (59, 9);
+ALTER TABLE t2 DROP COLUMN c;
+SELECT * FROM t2 WHERE a >= 50 ORDER BY a;
+--source include/save_master_gtid.inc
+
+--connection server_2
+--source include/start_slave.inc
+--source include/sync_with_master_gtid.inc
+SELECT * FROM t2 WHERE a >= 50 ORDER BY a;
+
+
 # Clean up.
 
 --connection server_2

sql/rpl_parallel.cc

@@ -2299,6 +2299,11 @@ rpl_parallel::do_event(rpl_group_info *serial_rgi, Log_event *ev,
       }
       gco->flags= flags;
     }
+    else
+    {
+      if (gtid_flags & Gtid_log_event::FL_DDL)
+        force_switch_flag= group_commit_orderer::FORCE_SWITCH;
+    }
     rgi->speculation= speculation;
 
     if (gtid_flags & Gtid_log_event::FL_GROUP_COMMIT_ID)