[Xcb] callback-based reply handling

[Barton C Massey]

In message <20050411082453.GJ24279 at minilop.net> you wrote:
> > Some reason not to keep the internal queue as a PQ on
> > request sequence number?  Probably :-).
> 
> Nope, no reason not to. :-) Doesn't help though. At the moment I'm using
> a sorted linked list for easy implementation, which (I should hope) is
> equivalent.
> 
> The problem is that an item could be added to the list while the
> reply-handling thread is blocked inside XCB, waiting for a later reply.
> I dunno how to wake it up and make it come back to the earlier one, or
> how to efficiently ensure that XCB never blocks.

Ahh.  I think I know how to hack XCB to help with this, but
you don't want to.  It would be ugly.  

I thought of a simpler kludge just now, but it wouldn't work
:-).  Let me think more.

    Bart