[RFC wayland] Add wl_proxy destruction callbacks

Miguel Angel Vico mvicomoya at nvidia.com
Mon May 30 11:10:42 UTC 2016

Hi all,

A few days ago, I had a little chat over IRC with Pekka about addition
of proxy objects destruction callbacks to the wayland client protocol.

Summing up, we recently ran into some applications where native objects
(wl_surface, wl_egl_window, wl_display) used by EGL are destroyed/made
invalid before destroying the corresponding EGL objects. This sometimes
causes crashes of the EGL driver, which is not nice. We have seen this
with the NVIDIA EGL implementation, but I assume the Mesa EGL
implementation is similarly exposed.

I agree this is in fact an application bug, but the EGL spec states that
functions such as makeCurrent or swapBuffers should return error (not
crash) if the native objects become invalid. I also agree the spec
should have been clearer and probably allowed "undefined behavior", but
it is not the case.

Having an objects destruction notification mechanism such as destruction
callbacks would allow us to satisfy the spec.

Also, such functionality would also make life way easier under certain
circumstances. I'm basically thinking about multi-threaded applications,
where several threads make use of the same native objects, and for some
reason one of the threads has to destroy one or more of them due to some
sort of error happening.

Of course, this can still be considered an application bug, and the
application could still make sure none of the threads is going to use
the native objects before destroying them, but again, specs allow users
to do many non-recommended things.

I think we should try to do our best to gracefully handle those
non-desirable API usages, and avoid crashes whenever is possible.

Pekka did not see this as something crazy to have, but wanted to hear
from some of the toolkits guys before making the decision of whether
changing the wl_proxy ABI is a good idea.

As an alternative, destruction callbacks could be hung off of
wl_egl_window. In a similar way we support wl_egl_window_resize
callbacks, we could support wl_egl_window_destroy callbacks.

However, this isn't as foolproof as adding wl_proxy destruction
callbacks, since destruction of wl_surface or wl_display objects before
wl_egl_window would lead to same issues.

I really think adding destruction callbacks to wl_proxy would be an
improvement worth making, but others' thoughts must be heard first.



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