MDEV-8193: UNTIL clause in START SLAVE is sporadically disobeyed by parallel replication

The code was using the wrong variable when comparing the binlog name
for the UNTIL position. This could cause the comparison to fail after
binlog rotation, in turn causing the UNTIL clause to not trigger slave
stop.

mysql-test/suite/rpl/t/rpl_mdev8193.test

@@ -0,0 +1,56 @@
+--source include/master-slave.inc
+--source include/have_binlog_format_statement.inc
+
+--connection slave
+
+--source include/stop_slave_sql.inc
+
+--connection master
+
+CALL mtr.add_suppression("Statement is unsafe because it uses a system function that may return a different value on the slave");
+create table t1 (i int);
+insert into t1 values (1),(2);
+insert into t1 values (3),(4);
+# This sleep() helps trigger the failure more reliably.
+insert into t1 select i+20+0*sleep(1) from t1 where i=1;
+flush logs;
+insert into t1 values (5),(6);
+insert into t1 values (7),(8);
+insert into t1 values (9),(10);
+
+--let $master_file = query_get_value(show master status,File,1)
+--let $master_pos = query_get_value(show master status,Position,1)
+
+insert into t1 values (11),(12);
+insert into t1 values (13),(14);
+insert into t1 values (15),(16);
+
+--connection slave
+
+set global slave_parallel_threads = 1;
+--replace_result $master_file MASTER_FILE $master_pos MASTER_POS
+eval start slave until master_log_file='$master_file', master_log_pos=$master_pos;
+
+--let $show_statement = SHOW SLAVE STATUS
+--let $field = Slave_SQL_Running
+--let $condition = = 'No'
+--let $wait_timeout = 10
+
+--source include/wait_show_condition.inc
+
+if (`select COUNT(*) <> 11 from t1`) 
+{
+	SELECT * FROM t1;
+	query_vertical show slave status;
+	die "Wrong number of rows in the table";
+}
+
+drop table t1;
+--source include/stop_slave_io.inc
+set global slave_parallel_threads = DEFAULT;
+
+--connection master
+drop table t1;
+
+--let $rpl_only_running_threads= 1
+--source include/rpl_end.inc