[Impact]
Users upgrading to artful on a system that cannot run wayland may be logged into an unexpected session at the next boot.
[Steps to reproduce]
1. On a system with wayland capability, log into the "Ubuntu" session.
2. Install gnome-session if it wasn't already installed (this adds two sessions, "GNOME" and "GNOME on Xorg").
3. Disable wayland (uncomment WaylandEnable=false in /etc/gdm3/custom.conf) and reboot.
Expected result: at the greeter, the "Ubuntu on Xorg" session is selected by default (falling back from "Ubuntu" to "Ubuntu on Xorg").
Actual result: at the greeter, the "GNOME on Xorg" session is selected by default.
[Fix]
The fix consists in using the default fallback mechanism in gdm3, i.e. /usr/share/wayland-sessions/ubuntu.desktop falls back to /usr/share/xsessions/ubuntu.desktop, adding corresponding "-xorg.desktop" session files, and having gdm3 ignore them under wayland. This was discussed and approved upstream (https://bugzilla.gnome.org/show_bug.cgi?id=788552).
The fix involves a distro-patch in gdm3 (already merged upstream, will go away with the next release) and changes to the packaging of gnome-session (added symlinks).
[Test Case]
see [Steps to reproduce]
[Regression Potential]
The gdm3 patch has been thoroughly tested and merged upstream. Since this is C glib code, reviewers should carefully look for possible memory leaks, as strings are being allocated.
If a package installs /usr/share/xsessions/foo-xorg.desktop without a corresponding /usr/share/xsessions/foo.desktop, the session will be ignored if wayland is disabled. It is however considered a standard practice that foo-xorg is a fallback for foo, so it wouldn't make much sense to have the former without the latter.
In case of a partial upgrade:
- only gdm3 upgraded: nothing user-visible should happen, as gdm will ignore the foo-xorg session only if the corresponding foo session file exists
- only gnome-session upgraded: with wayland disabled, at the greeter the user might see two sessions that are in fact the same thing: "Ubuntu" and "Ubuntu on Xorg" (but logging into any of the two will work)
One minor gotcha with this patch: if, with wayland enabled, one intentionally selects ubuntu-xorg, then disables wayland, then reboots, in the login screen no session is selected in the dropdown (because gdm ignored ubuntu-xorg). However this will still launch the ubuntu-xorg session by default (which effectively is the same as the ubuntu session), so all is good (except for the fact that no session appears selected in the dropdown). This has the advantage of retaining the "ubuntu-xorg" session selected by the user if wayland is re-enabled later, though.
This is a regression in the sense that no session is visually selected in the dropdown, but it doesn't affect the default session selection.
We don't want to commit to fixing this in time for release. It would be nice to fix, but not a release blocker.