Bug#954783: Non-functioning gnome-apps after mate-session due to surviving CLUTTER_BACKEND env var

[Dimitri Schwarz]

Package: libclutter-gtk-1.0-0
Version: 1.8.4-4

Package: gnome-session-common
Version: 3.30.1-2


In short: When running a mate session the environment-variable CLUTTER_BACKEND=x11 is set. This variable survives a logout and relogin into the GNOME wayland-session, which doesn't override it. The 
result is a bunch of not launchable clutter-gtk applications.

To reproduce the issue:
  * Install debian 10.3.0 with gnome and mate environment
  * Optional:
      * Login into GNOME (wayland) Session
          * Run `env | grep -Ei 'x11|wayland'`
             XDG_SESSION_TYPE=wayland
             WAYLAND_DISPLAY=wayland-0
          * Note: CLUTTER_BACKEND is not set
          * Launch `gnome-control-center` via graphical environment or console
             * Works fine
          * Logout
  * Login into MATE session
      * `env | grep -Ei 'x11|wayland'`
         CLUTTER_BACKEND=x11
         XDG_SESSION_TYPE=x11
      * `sudo grep -Ri CLUTTER_BACKEND /etc/`
         /etc/X11/Xsession.d/99mate-environment:    export CLUTTER_BACKEND=x11
      * Logout
  * Login into GNOME (wayland) session
      * `env | grep -Ei 'x11|wayland'`
         CLUTTER_BACKEND=x11 # <- survived!
         XDG_SESSION_TYPE=wayland
         WAYLAND_DISPLAY=wayland-0
      * Launch `gnome-control-center` via graphical environment or console
      * Observe it doesn't startup
         (gnome-control-center:4399): Clutter-Gtk-ERROR **: 12:23:42.914: *** Unsupported backend.
      * In fact all of the following apps terminate the same way
          * `compgen -c | while read cmd; do [ -n "$(which $cmd)" ] && ldd $(which $cmd) | grep -q clutter-gtk && echo $cmd; done`
             cheese
             totem
             evolution
             gnome-control-center
             gnome-contacts
             cheese
             totem
             evolution
             gnome-control-center
             gnome-contacts
             gnome-nibbles
             swell-foop
             lightsoff
             quadrapassel
      * Now launch `CLUTTER_BACKEND=wayland gnome-control-center`
         * Works fine

So I suppose the surviving of session-variables is the core issue here. In case of it beeng rather a feature than a bug, I suppose gnome should set CLUTTER_BACKEND explicitly aswell. Or maybe 
clutter-gtk shall respect this constellation of CLUTTER_BACKEND and GDK_WAYLAND_DISPLAY?