This bug appears as a hang in applications that wait for replies from
multiple threads, where one such thread has taken a user lock using
XLockDisplay.

Prior to this fix, the code could deadlock in this way: If thread 1 goes
to sleep waiting for a reply, and then thread 2 takes a user lock and
waits for a reply, then thread 2 will wait for thread 1 to process its
reply (because responses must be processed in order), but thread 1 will
wait for thread 2 to drop its user lock.

Fixed by making thread 1 not wait for thread 2 to drop its user lock.
This makes the semantics of user locks hard to define, but they were
already hard to define. The new behavior appears to be consistent with
the way Xlib worked historically, anyway.

Fixes: http://lists.freedesktop.org/archives/xcb/2011-March/006802.html



There was a similar potential for deadlock in _XReadEvents, fixed the
same way, with the same caveats about user-lock semantics.

Signed-off-by: Jamey Sharp <jamey@minilop.net>