[Wayland-bugs] [Bug 91750] Wayland may have interface bug when multi-threads programing.

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=91750

--- Comment #8 from Jonas Ådahl <jadahl at gmail.com> ---
(In reply to Yujie Shen from comment #7)
> (In reply to Jonas Ådahl from comment #6)
> > Yujie: as indirectly stated by Pekka, my example was incomplete. You would
> > also need to dispatch pending events before the
> > wl_display_prepare_read(display) call. This makes it, ignoring error
> > handling, more or less:
> > 
> > 	wl_surface_attach();
> > 	while (wl_display_prepare_read_queue(display, queue) < 0)
> > 		wl_display_dispatch_queue_pending(display, queue);
> > 	window->callback = wl_surface_frame(window->surface);
> > 	wl_proxy_set_queue(window->callback,queue2);
> > 	wl_callback_add_listener(window->callback, &frame_listener, window);
> > 	wl_display_cancel_read(display);
> > 	wl_surface_commit(window->surface);
> > 
> > This has the side effect that events already in the queue will be always
> > dispatched before the frame is requested. This may have unwanted effects, so
> > please consider whether this is Ok in your architecture. If it is not, you
> > need to make sure that the above code is done after all events on the queue
> > has been dispatched.
> 
> Thanks for your kindly help.
> By adding 
> >while (wl_display_prepare_read_queue(display, queue2) < 0)
> >		wl_display_dispatch_queue_pending(display, queue2);
> >wl_surface_commit(window->surface);
> it works in my case.
> but, I still don't understand why it requires the wl_event_queue to be
> empty??
> I'm trying to summary the restrictions of wayland when multi-thread
> programming.
> May I know if you have any idea about it.

It is a requirement by wl_display_prepare_read_queue() that to succeed the
queue has to be empty and it needs to succeed in order to block the other
threads from dispatching. You should put it before wl_surface_frame() though,
because it is between wl_surface_frame() and wl_proxy_set_queue() the race can
happen, and don't forget to call wl_display_cancel_read().

Earlier today I wrote a summary about the known threading issues in the API.
Please see bug 91769.

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