[PATCH wayland 4/4] client: Fix wl_display_roundtrip_queue() race condition

[Pekka Paalanen]

On Wed, 27 Apr 2016 15:37:41 +0800
Jonas Ådahl <jadahl at gmail.com> wrote:

> Without this commit, wl_display_roundtrip_queue() is vulnerable to a
> race condition, causing the callback to be dispatched on the wrong
> queue.
> 
> The race condition happens if some non-main thread calls
> wl_display_roundtrip_queue() with its thread local queue, and the main
> thread reads and dispatches the callback event from the
> wl_display_sync() call before the thread local queue is set.
> 
> The issue is fixed by using a proxy wrapper, making the initialization
> of the callback proxy atomic, effectively making it no longer possible
> for some other thread to dispatch the proxy before the correct thread
> local queue is set.
> 
> Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
> ---
>  src/wayland-client.c | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)
> 
> diff --git a/src/wayland-client.c b/src/wayland-client.c
> index 6de91cd..cb5213b 100644
> --- a/src/wayland-client.c
> +++ b/src/wayland-client.c
> @@ -1106,14 +1106,20 @@ static const struct wl_callback_listener sync_listener = {
>  WL_EXPORT int
>  wl_display_roundtrip_queue(struct wl_display *display, struct wl_event_queue *queue)
>  {
> +	struct wl_display *display_wrapper;
>  	struct wl_callback *callback;
>  	int done, ret = 0;
>  
>  	done = 0;
> -	callback = wl_display_sync(display);
> +
> +	display_wrapper = wl_proxy_create_wrapper(display);

Check display_wrapper.

> +	wl_proxy_set_queue((struct wl_proxy *) display_wrapper, queue);
> +	callback = wl_display_sync(display_wrapper);
> +	wl_proxy_wrapper_destroy(display_wrapper);
> +
>  	if (callback == NULL)
>  		return -1;
> -	wl_proxy_set_queue((struct wl_proxy *) callback, queue);
> +
>  	wl_callback_add_listener(callback, &sync_listener, &done);
>  	while (!done && ret >= 0)
>  		ret = wl_display_dispatch_queue(display, queue);

Then:
Reviewed-by: Pekka Paalanen <pekka.paalanen at collabora.co.uk>


Thanks,
pq
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