wayland: how should dispatch the default wayland display queue?

[Daniel Stone]

Hi Eugen,

On 27 July 2014 22:16, Eugen Friedrich <friedrix at gmail.com> wrote:

> Hi Daniel,
>
> thanks, i understood we should add the wl_display_dispatch_pending call
> in the application
> there is currently no way to avoid this and basically it does not harm.
> I only wanted to understand if there is something missing.
>

Doing that is not enough. Firstly, it requires the application to do this,
or else EGL will break, which will make your stack appear broken with
different applications. Secondly, as an application may do this in any
thread, you could run into locking issues.

wl_display_sync() does the following (paraphrased):
  callback = wl_display_roundtrip(); /* creates wl_callback object on
default queue */
  while (!callback_signalled)
      wl_display_dispatch(); /* dispatch default queue until callback
processed */

This violates the rule that EGL must not intefere with the default queue -
meaning that the wl_callback object _cannot_ be placed on the main queue.

In order to do this correctly, you will see Mesa does:
  callback = wl_display_roundtrip();
  wl_callback_set_queue(callback, internal_egl_queue);
  while (!callback_signalled)
      wl_display_dispatch_queue(internal_egl_queue);

As with all objects created by your EGL implementation, the wl_callback
object must be moved to the queue immediately after it is created by
wl_display_roundtrip(). Doing this will ensure correctness and also not
require apps to be constantly dispatching the main queue. You can search
the Mesa codebase for uses of wl_display_roundtrip() to find some examples
of this pattern.

Again - any object created by your EGL implementation that is not
immediately moved to your internal event queue is a bug.

Cheers,
Daniel
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