Dummy display manager that allows running X as a normal user.
The X server is started manually from a vt using `startx`.
Session startup commands must be provided by the user
in ~/.xinitrc, which is NOT automatically generated.
This is necessary when system-wide dconf settings must be configured, i.e. to
disable GDM's auto-suspending of the machine when no user is logged in.
Related to https://github.com/NixOS/nixpkgs/issues/42053.
The switch from slim to lightdm in #30890 broke some nixos tests
because lightdm by default doesn't permit auto-login for root.
Override /etc/pam.d/lightdm-autologin to allow it.
Switch from slim to lightdm as the display-manager.
If plasma5 is used as desktop-manager use sdddm.
If gnome3 is used as desktop-manager use gdm.
Based on #12516
The wallpaper used is *structurally compatible* with the other one,
meaning that the logo is at the same location, and not bigger.
It has one drawback: the logo is brighter, which clashes with the grub
usage. This is to be fixed with new options in grub.
The default session might be found in `extraSessionFilePackages`, but it's not
viable to detect at evaluation time, so emit a warning.
In LightDM instead of checking `defaultSessionName` against
`displayManager.session.names` we rely on the assertions in
`desktopManager` and `windowMananger` and just check that there's at least one
default set. The second assertion could never actually be triggered.
This makes it easier to support a wider variety of .desktop session files. In
particular this makes it possible to use both the «legacy» sessions and upstream
session files.
We separate `xsession` into two parts, `xsessionWrapper` and `xsession`.
`xsessionWrapper` sets up the correct environment and then lauches the session's
Exec command (from the .desktop file), falling back to launching the default
window/desktopManager through the `xsession` script (required by at least some
nixos tests).
`xsession` then _only_ handles launching desktop-managers/window-managers defined
through `services.xserver.desktopManager.session`.
Previously, the mkDesktops function produced a flat package containing
session files in the top level. As a preparation for introduction of
Wayland sessions, the files will now be placed to $out/share/xsessions.
