[RFC wayland] Add wl_proxy destruction callbacks

Daniel Vetter daniel at ffwll.ch
Mon May 30 15:44:58 UTC 2016

On Mon, May 30, 2016 at 01:10:42PM +0200, Miguel Angel Vico wrote:
> 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.

Just a quick comment: I guess a piglit testcase to demonstrate the failure
(or well, just any minimal test) would be awesome. That way folks can
quickly figure out what all goes wrong without this.
Daniel Vetter
Software Engineer, Intel Corporation

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