innodb.alter_large_dml failed in buildbot with debug sync point wait timeout

innodb.alter_large_dml 'innodb'          w2 [ fail ]

        Test ended at 2019-07-20 01:00:41

CURRENT_TEST: innodb.alter_large_dml

--- /mnt/buildbot/build/mariadb-10.3.17/mysql-test/suite/innodb/r/alter_large_dml.result	2019-07-19 05:37:57.000000000 -0400

+++ /mnt/buildbot/build/mariadb-10.3.17/mysql-test/suite/innodb/r/alter_large_dml.reject	2019-07-20 01:00:40.703730940 -0400

@@ -28,8 +28,12 @@

 SET DEBUG_SYNC = 'now SIGNAL dml_done';

 connect con2, localhost,root,,test;

 SET DEBUG_SYNC = 'now WAIT_FOR ddl_start';

+Warnings:

+Warning	1639	debug sync point wait timed out

 CREATE TABLE t2(f1 INT NOT NULL)ENGINE=InnoDB;

 connection default;

+Warnings:

+Warning	1639	debug sync point wait timed out

 SHOW CREATE TABLE t1;

 Table	Create Table

 t1	CREATE TABLE `t1` (

mysqltest: Result length mismatch