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

[Keith Packard]

Uli Schlachter <psychon at znc.in> writes:

> Hi again,
>
> TL;DR: I think that having to call get_socket_back() and waiting for
> c->out.writing to go to zero in a loop is the only issue we have due to the
> iolock getting dropped. The send_sync()s that were mentioned before
> seem safe to me.

Thanks for your careful analysis. I think you've correctly identified
all of the requirements for making sure we don't have inconsistencies in
the various paths related to writing requests.

The one thing that I think is still broken is the sequence number
requirements around wrapping and taking care of long periods without a
reply or event. In either case, we must atomically test the sequence
number condition and generate a request. That means we must not drop
iolock between the condition test and the request generation, and my
proposed patch of sticking the check for the socket inside send_request
would break that.

I think it's actually pretty easy to fix though. This patch creates a
new function, prepare_socket_request, which makes sure the current
thread can write a request, and then uses it in xcb_send_request and
_xcb_out_send_sync to fix the issues identified above. Then, it sticks
the sequence number checks in a loop and calls send_sync() *and*
prepare_socket_request() until both sequence conditions are
simultaneously true. At that point, it can call send_request knowing
that the socket is ready to send the request and the sequence number has
been cleaned up.

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-- 
keith.packard at intel.com
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