read_only=1 and wsrep_read_only_option race condition

static my_bool wsrep_read_only_option(THD *thd, TABLE_LIST *all_tables)
{
 int opt_readonly_saved = opt_readonly;
 ulong flag_saved = (ulong)(thd->security_context()->master_access() & SUPER_ACL);
opt_readonly = 0;
 DEBUG_SYNC(thd, "read_only_cleared"); // added for test

--source include/galera_cluster.inc
--source include/have_innodb.inc
CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
--connection node_2
SET GLOBAL read_only=TRUE;
CREATE USER foo@localhost;
SET DEBUG_SYNC= 'read_only_cleared SIGNAL not_ro WAIT_FOR ro_checked';
send INSERT INTO t1 VALUES (1);
--echo # Open connection to node 2 using 'foo' user.
--let $port_2= \$NODE_MYPORT_2
--connect(foo_node_2,127.0.0.1,foo,,test,$port_2,)
--connection node_1
--echo
--echo # Connect with foo_node_2
--connection foo_node_2
--error ER_OPTION_PREVENTS_STATEMENT
INSERT INTO t1 VALUES (2);

worker[1] Using MTR_BUILD_THREAD 300, with reserved ports 13000..13009
galera.galera_read_only [ fail ]
 Test ended at 2018-09-14 16:16:38
CURRENT_TEST: galera.galera_read_only
mysqltest: At line 32: query 'INSERT INTO t1 VALUES (2)' succeeded - should have failed with errno 1290...
The result from queries just before the failure was:
CREATE TABLE t1 (id INT PRIMARY KEY) ENGINE=InnoDB;
SET GLOBAL read_only=TRUE;
CREATE USER foo@localhost;
SET DEBUG_SYNC= 'read_only_cleared SIGNAL not_ro WAIT_FOR ro_checked';
INSERT INTO t1 VALUES (1);
# Open connection to node 2 using 'foo' user.
# Connect with foo_node_2
safe_process[406]: Child process: 408, exit: 1