MDEV-33331: IO Thread Relay Log Inconsistent Statistics After MDEV-32551

After MDEV-32551, in a master/slave setup, if the replica's IO thread
quickly and successively reconnects (i.e quickly running
STOP SLAVE IO_THREAD followed by START SLAVE IO_THREAD), the relay log
rotation behavior changes. That is, MDEV-32551 changed the logic of the
binlog_dump_thread on the primary, such that it can stop itself before
sending any events if it sees a new connection has been created to a
replica with the same server_id. Pre MDEV-32551, the connection would
establish and it would send a "fake" rotate event to populate the
log name. Post MDEV-32551, the connection stops itself, and a rotate
event is not sent.

This made the test rpl.rpl_mariadb_slave_capability unstable because
it is reliant on the name of the relay logs (which is dependent on the
number of rotates); and the pre-amble of the test would quickly
start/stop the IO thread. There a binlog dump thread could end itself
before sending a rotate event to the replica, thereby changing the name
of the relay log.

This patch fixes this by adding in a synchronization in-between IO thread
restarts, such that it waits for the primary's binlog dump threads to
sync up with the state of the replica.

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

@@ -6,6 +6,11 @@ connection slave;
 include/stop_slave.inc
 CHANGE MASTER TO MASTER_USE_GTID=NO;
 include/start_slave.inc
+connection master;
+# Ensure only the new binlog dump thread is alive (wait for the old one
+# to complete its kill)
+# And that it has already sent its fake rotate
+connection slave;
 include/stop_slave.inc
 # Test slave with no capability gets dummy event, which is ignored.
 set @old_dbug= @@global.debug_dbug;

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

@@ -18,6 +18,18 @@ connection slave;
 CHANGE MASTER TO MASTER_USE_GTID=NO;
 --source include/start_slave.inc
 
+--connection master
+--echo # Ensure only the new binlog dump thread is alive (wait for the old one
+--echo # to complete its kill)
+--let $wait_condition= select count(*)=1 from information_schema.processlist where command='Binlog Dump'
+--source include/wait_condition.inc
+
+--echo # And that it has already sent its fake rotate
+--let $wait_condition= select count(*)=1 from information_schema.processlist where state LIKE '%Master has sent all binlog to slave%' and command='Binlog Dump'
+--source include/wait_condition.inc
+
+
+--connection slave
 --source include/stop_slave.inc
 --echo # Test slave with no capability gets dummy event, which is ignored.
 set @old_dbug= @@global.debug_dbug;