SLAVE STATUS

Problem:

If a client thread on an slave does FLUSH TABLES WITH READ
LOCK; then master does some updates, SHOW SLAVE STATUS in
the same client will be blocked.

Analysis:

Execute FLUSH TABLES WITH READ LOCK on slave and at the same
time execute a DML on the master. Then the DML should be
made to stop at a state "Waiting for commit lock". This
state means that sql thread is holding rli->data_lock and
waiting for MDL_COMMIT lock. Now in the same client session
where FLUSH TABLES WITH READ LOCK was executed issue SHOW
SLAVE STATUS command. This command will be blocked waiting
for rli->data_lock causing a dead lock. Once this happens
it will not be possible to release the global read lock as
"UNLOCK TABLES" command has to be issued in the same client
where global read lock was acquired. This causes the dead
lock.

Fix:

Existing code holds the rli->data_lock for the whole
duration of commit operation. Instead of holding the lock
for entire commit duration the code has been restructured in
such a way that the lock is held only during the period when
rli object is being updated.