Bug 7825

Created attachment 2529 [details]
Testcase of the problem (source and binary)

When a Wayland client hides a window by attaching a nil buffer to the window's surface, the Wayland compositor will not hide the window, but display the last frame that was rendered.

Attaching a nil buffer is how Wayland clients written in Qt hide their window, which means every Qt application that hides a window and uses Wayland is affected.

This can be reproduced with the attached testcase. The testcase contains the source and a binary build on fido/imx6qsabresd. In the testcase, the window is hidden after 4 seconds, yet Weston still displays the window.

I reproduced this on Yocto dizzy and on Yocto fido, using core-image-weston on an imx6qsabresd.

This happens with Weston and with Qt-based compositors, the bug is in the eglSwapBuffers() implementation of the Vivante driver: The Vivante driver sends wl_surface::commit() messages to the compositor *after* the last eglSwapBuffers() call, as seen in the following log (WAYLAND_DEBUG=client):

Hiding window...
    Attaching nil buffer to window.
[2081588.396]  -> wl_surface@3.attach(nil, 0, 0)
[2081588.798]  -> wl_surface@3.commit()
Done hiding window...
[2081590.690] wl_callback@12.done(2506047518)
[2081590.946]  -> wl_surface@3.frame(new id wl_callback@12)
[2081591.200]  -> wl_surface@3.attach(wl_buffer@8, 0, 0)
[2081591.568]  -> wl_surface@3.damage(0, 0, 640, 480)
[2081592.106]  -> wl_surface@3.commit()

eglSwapBuffers() should not return before having sent the wl_surface::commit() message to the compositor. While I can't find any official documentation for that behavior, it is mentioned in multiple places:

http://www.jlekstrand.net/jason/projects/wayland/wayland-android/ - "eglSwapBuffers [..] must call wl_display.attach, wl_display.damage, and wl_display.commit before it returns."

https://www.mail-archive.com/wayland-devel@lists.freedesktop.org/msg12706.html: "eglSwapBuffers should be calling wl_surface.commit before it returns."

The QtWayland developers also said that this would be the expected behavior from eglSwapBuffers().