[Xcb] xcb recursive write not protected in all cases?

[Josh Triplett]

On Mon, Dec 23, 2013 at 05:00:46PM -0800, Keith Packard wrote:
> I think we need to check out.writing closer to the actual use of the
> out.queue; perhaps it would suffice to check that down inside
> send_request itself? That would certainly offer the surest guarantee
> that iolock wasn't dropped between the check for out.writing and the use
> of out.queue.

Based on your analysis, that sounds like the best plan indeed: check
while holding the lock in send_request, where there's no way we can
subsequently drop it before sending the request.

- Josh Triplett