[PATCH xserver] os: Call FlushClient() before sending FD-passing messages

[Keith Packard]

Alexander Volkov <a.volkov at rusbitech.ru> writes:

> Yes, it would be easier to fix this in libxcb, but I believe that it
> would be more correct to do this in the X server. At least I want to
> try to fix the X server.

Hrm.

The problem is that there are two streams of data here -- the stream of
fds and the stream of replies. xcb currently insists that they remain
aligned so that any fds are associated with the reply data received in
the same recvmsg operation. This allows an X server to send fds which
the client is not expecting, with the client can silently closing
them.

If we let the two streams run un-aligned, then there will be a queue of
received fds that should (unless there's a bug) eventually get
associated with the correct reply. The requirement here is looser -- we
just need to make sure the fds arrive no later than the reply data. This
seems much easier to achieve, and in fact the current X server code does
this today.

Changing xcb to allow early fds involves removing code that closes fds
received before the associated reply.

Changing the X server to ensure that fds are delivered exactly with the
associated reply data involves adding code to queue the fds and insert
additional flushes to make sure the kernel writes the fds with the start
of the reply, and then making sure that xcb doesn't discard fds received
with a partial reply.

Given these two possible solutions, I'd like to suggest that we might
prefer the simpler one.

-- 
-keith
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