Bug#1035301: gdm3/bullseye: logging in to Plasma and then Xfce fails

[Simon McVittie]

Package: gdm3,plasma-desktop,xfce4-session
Tags: bullseye
Control: found -1 gdm3/3.38.2.1-1
Control: fixed -1 gdm3/43.0-3
Control: found -1 plasma-desktop/4:5.20.5-4
Control: fixed -1 plasma-desktop/4:5.27.2-1
Control: found -1 xfce4-session/4.16.0-1
Control: fixed -1 xfce4-session/4.18.1-1
Severity: normal

While we were testing install media for the Debian 11.7 point release,
Emyr Williams found an issue with installing multiple desktop environments.
This appears to have been fixed, worked around or otherwise avoided in
bookworm, but still affects bullseye. It's not yet clear whether this is a
result of a bug in gdm3, Plasma or Xfce.

Original steps to reproduce:

* Install Debian 11.7 on real or virtual hardware
* At the tasksel stage, choose to install every desktop environment
* If you are asked to choose a display manager, choose GNOME's gdm3
  (this was the default when I tried it)
* Finish the installation and boot the installed system
* At the gdm3 login prompt, choose the (single) user account
* Do not enter the password yet
* In the cog wheel menu in the bottom right corner, choose Plasma
* Enter the password and log in
* Log out from Plasma but do not reboot or shut down
* Back at the gdm3 login prompt, choose the user account again
* In the cog wheel menu in the bottom right corner, choose Xfce
* Enter the password and log in

Slightly simpler steps to reproduce:

* Install Debian 11 in a VM (I used autopkgtest-build-qemu)
* Log in as root
* Create a user account with a non-empty password, or set a non-empty
  password on an existing user account
* apt install gdm3 plasma-desktop xfce4-session
* Reboot
* At the gdm3 login prompt, choose the user account
* Continue as above

Parts of this that work as expected:

* gdm3's greeter runs on tty1. It's a Wayland compositor (gnome-shell in
  a special mode), with Xwayland on a high-numbered display, normally :1024.
* When you log in to Plasma (in X11 mode), gdm starts a new Xorg server
  on the next available virtual console, normally tty2, normally with
  X11 display :0. It works as you'd hope.
* When you log out from Plasma, you're taken back to tty1 as before,
  again with a Wayland compositor and a high-numbered Xwayland display.

Expected result:

* When you log in to Xfce, gdm starts a new Xorg server on tty2 or maybe
  tty3, with X11 display :0 or maybe :1. Xfce works correctly.

Actual result:

* Xfce doesn't start, and after a brief flash of black screen and/or old
  framebuffer contents from Plasma, you're sent back to the gdm greeter.

Workarounds:

* After leaving Plasma, reboot the system. Xfce will work correctly in a
  clean boot.
* Or, use a different pair of desktop environments (all others seemed OK,
  it seemed to be specifically Plasma -> Xfce that is the problem case).

Possible workarounds (not yet tested, these are the next things to try):

* Choosing a different display manager like lightdm
* KillUserProcesses=yes in /etc/systemd/logind.conf

Journal log attached from following the simpler steps, from reboot onwards,
with annotations added via `logger -t NOTE "about to log in to Plasma"` and
similar commands from a root ssh session.
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