Help with proxy queues please

[John Cox]

On Mon, 11 Sept 2023 at 15:52, Pekka Paalanen <ppaalanen at gmail.com> wrote:
>
> On Mon, 11 Sep 2023 12:53:15 +0100
> John Cox <jc at kynesim.co.uk> wrote:
>
> > Hi
> > I am clearly deeply confused by event queues and proxies - I have this
> > snippet of code (from some VLC work I'm doing):
> >
> > sys->eventq = wl_display_create_queue(video_display(sys));
> > if (sys->eventq == NULL) {
> >     msg_Err(vd, "Failed to create event Q");
> >     goto error;
> > }
> > if ((sys->wrapped_display =
> > wl_proxy_create_wrapper(video_display(sys))) == NULL)
> > {
> >     msg_Err(vd, "Failed to create wrapper");
> >     goto error;
> > }
> > wl_proxy_set_queue((struct wl_proxy *)sys->wrapped_display, sys->eventq);
> >
> > struct wl_callback * cb;
> > struct wl_registry *const registry =
> > wl_display_get_registry(sys->wrapped_display);
> > if (registry == NULL) {
> >     msg_Err(vd, "Cannot get registry for display");
> >     goto error;
> > }
> >
> > wl_registry_add_listener(registry, &registry_cbs, vd);
> > cb = wl_display_sync(sys->wrapped_display);
> > wl_callback_add_listener(cb, &reg_done_sync_listener, vd);
> >
> > msg_Info(vd, "Roundtrip start");
> > wl_display_roundtrip_queue(sys->wrapped_display, sys->eventq);
>
> The problem is probably here.
>
> wrapped_display is just a shell of a wl_proxy, but
> wl_display_roundtrip_queue() needs the original wl_display, just like
> wl_display_dispatch_queue() it calls.
>
> Thanks to elinor in IRC for pointing this out, when I first went on a
> complete wild goose chase wondering if a wl_display can even be
> wrapped, but of course it must be wrappable.
>
> > msg_Info(vd, "Roundtrip done");
> > wl_registry_destroy(registry);
> >
> > video_display() returns the display we are rendering to. My
> > expectation was that the registry & sync listeners would be called
> > somewhere between the roundtrip start/end debug lines, but what seems
> > to happen is that the roundtrip just returns immediately.
> >
> > I have clearly misunderstood something at a fundamental level. I
> > thought that the wrapped display would set the Q that the listeners
> > would send events on and th eroundtrip would wait for that. The
> > wl_proxy_create_wrapper docn on
> > https://wayland.freedesktop.org/docs/html/apb.html#Client-classwl__proxy
> > also seems to imply this.
> >
> > Any hints would be gratefully received - thanks
>
> Yeah, it's a bit awkward to have wl_display both as a proxy and a
> Wayland connection. Some APIs need only a wl_proxy, others need the
> connection.
>
> That's something to add in the docs somehow.
>
> As a rule of thumb, I'd say that anything that explicitly takes a
> 'struct wl_display *' as an argument in wayland-client-core.h needs the
> original wl_display and does not work with a proxy-wrapper.
>
> Another exception is that you cannot change the queue of the original
> wl_display (or at least it's buggy), while a proxy-wrapped wl_display
> you can.
>
>
> Thanks,
> pq
Many thanks for that. That makes it all (a little) clearer.

John Cox