FULL
Bug#21753696: MAKE SHOW SLAVE STATUS NON BLOCKING IF IO
THREAD WAITS FOR DISK SPACE

Problem:
========
Currently SHOW SLAVE STATUS blocks if IO thread waits for
disk space. This makes automation tools verifying
server health block on taking relevant action. Finally this
will create SHOW SLAVE STATUS piles.

Analysis:
=========
SHOW SLAVE STATUS hangs on mi->data_lock if relay log write
is waiting for free disk space while holding mi->data_lock.
mi->data_lock is needed to protect the format description
event (mi->format_description_event) which is accessed by
the clients running FLUSH LOGS and slave IO thread. Note
relay log writes don't need to be protected by
mi->data_lock, LOCK_log is used to protect relay log between
IO and SQL thread (see MYSQL_BIN_LOG::append_event). The
code takes mi->data_lock to protect
mi->format_description_event during relay log rotate which
might get triggered right after relay log write.

Fix:
====
Release the data_lock just for the duration of writing into
relay log.

Made change to ensure the following lock order is maintained
to avoid deadlocks.

data_lock, LOCK_log

data_lock is held during relay log rotations to protect
the description event.