[cairo-bugs] [Bug 98883] BadAccess errors in ShmAttach due to thread races with XNextRequest() usage in cairo-xlib-surface-shm.c

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=98883

--- Comment #18 from Daniel van Vugt <daniel.van.vugt at canonical.com> ---
> XShmAttach returns True to indicate it successfully queued the request.
> It does not fail to queue the request.
> 
> If the server fails to process the request successfully, then it sends an
> error response.
> 
> It would be helpful to understand why you are expecting an error response.

Essentially we have the Xorg server process having its privileges demoted and
re-promoted. If the XShmAttach request comes in while it is demoted then it
can't complete the request and returns BadAccess to the client. The client
initiated the request from _cairo_xlib_shm_pool_create but has since
(asynchronously) moved on to other code. So it crashes in an unpredictable
location when the BadAccess eventually comes in from the server.

I am happy with your explanation that "XShmAttach returns True to indicate it
successfully queued the request". However I don't believe the same applies to
_cairo_xlib_shm_pool_create, so I think any final fix would have to happen in
that function still.

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