Test rpl.rpl_change_master_demote frequently fails on buildbot with "IO thread should not be running..."

CURRENT_TEST: rpl.rpl_change_master_demote

mysqltest: At line 292: "IO thread should not be running after START SLAVE UNTIL master_gtid_pos using a pre-existing GTID"

The result from queries just before the failure was:

< snip >

SELECT VARIABLE_NAME, GLOBAL_VALUE FROM INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE VARIABLE_NAME LIKE 'gtid_binlog_pos' OR VARIABLE_NAME LIKE 'gtid_slave_pos' ORDER BY VARIABLE_NAME ASC;

VARIABLE_NAME	GLOBAL_VALUE

GTID_BINLOG_POS	0-1-26,1-3-4,2-1-3,3-1-2,4-3-2

GTID_SLAVE_POS	0-2-24,1-3-4,2-1-3,3-1-2,4-3-2

CHANGE MASTER TO master_host='127.0.0.1', master_port=SLAVE_PORT, master_user='root', master_use_gtid=Slave_Pos, master_demote_to_slave=1;

SELECT VARIABLE_NAME, GLOBAL_VALUE FROM INFORMATION_SCHEMA.SYSTEM_VARIABLES WHERE VARIABLE_NAME LIKE 'gtid_binlog_pos' OR VARIABLE_NAME LIKE 'gtid_slave_pos' ORDER BY VARIABLE_NAME ASC;

VARIABLE_NAME	GLOBAL_VALUE

GTID_BINLOG_POS	0-1-26,1-3-4,2-1-3,3-1-2,4-3-2

GTID_SLAVE_POS	0-1-26,1-3-4,2-1-3,3-1-2,4-3-2

# GTID ssu_middle_binlog_pos should be considered in the past because

# gtid_slave_pos should be updated using the latest binlog gtids.

# The following call to sync_with_master_gtid.inc uses the latest

# binlog position and should still succeed despite the SSU stop

# position pointing to a previous event (because

# master_demote_to_slave=1 merges gtid_binlog_pos into gtid_slave_pos).

START SLAVE UNTIL master_gtid_pos="ssu_middle_binlog_pos";

Warnings:

Note	1278	It is recommended to use --skip-slave-start when doing step-by-step replication with START SLAVE UNTIL; otherwise, you will get problems if you get an unexpected slave's mariadbd restart

# Slave needs time to start and stop automatically

# Validating neither SQL nor IO threads are running..

More results from queries before failure can be found in /home/buildbot/amd64-debian-11-msan/build/mysql-test/var/13/log/rpl_change_master_demote.log