Bug #651 If MASTER_POS_WAIT() is waiting, it does not stop if someone does STOP SLAVE

Description:
Currently, MASTER_POS_WAIT() is a loop like this:
while (!slave_SQL_thread_killed)
 {
  if position_we_wait_for<=current_position {we are done}
  /* else wait for some new data in the relay log */
  pthread_cond_wait(...)
 }
So if the SQL thread has read all relay logs, and the position is still not reached, MASTER_POS_WAIT() is waiting in pthread_cond_wait(),
and someone does STOP SLAVE, pthread_cond_wait() will not terminate
(it is never signaled), and so will never do the test in while(),
so will remain stuck (unless it was called with a timeout).

How to repeat:
Set up replication.
When it works and the master and slave are idle,
do 
SELECT master_pos_wait('the_current_master_binlog', a_too_big_position);
and in another connection do STOP SLAVE;

Suggested fix:
I'll fix that.

Fixed in 4.0 in changeset 1.1519.

This bug seems introduced again in 4.1. See also Bug #26622