nixos/xfce: use sessionPackages

[worldofpeace]

Motivation for this change

Use upstream session in nixpkgs.
Required a little fixup in the package.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[worldofpeace]

What problem does it solve?
As a disadvantage: it becomes difficult to understand the boot sequence and so less hackable

It's simply the canonical way to do things. I'm not sure how it makes the boot sequence difficult, in the end the display manager will have a session file.

[jtojnar]

Did not review https://git.xfce.org/xfce/xfce4-session/tree/scripts/startxfce4.in?h=xfce-4.14.0 but we were running xinitrc before too.

[jtojnar]

@volth The main benefit is reducing the NixOS modules in favour of upstream solutions, just like we do with tmpfiles.d, systemd services and so on.

[davidak]

@volth i use Pantheon for some time now :)

[romildo]

LGTM

[worldofpeace]

From IRC with @jtojnar

worldofpeace https://github.com/NixOS/nixpkgs/pull/78421 should probably be mentioned in release notes as it disallows using Xfce as a DE with a different WM

worldofpeace: We use it as such in the example `defaultSession = "xfce+icewm";` in the release highlights 🤣️

Jan Tojnar: oh no, that's actually no good. I know a lot of people in NixOS use xfce with a different wm and they totally wouldn't be happy with that

Ideally the Xfce session would use sessionPackages and users can still construct + wm sessions.
@jtojnar can we even do that? If not, I guess we can revert this on master and release.

[jtojnar]

We could provide both upstream session and our old one but dm=xfce with wm=none would clash with it.

We could provide option for switching between the two but that would be ugly.

Finally, we could add new option desktopManager.mixNmatchableDesktops and have:

services.xserver.desktopManager.session = map (desktop: {
      name = "mm-${desktop}";
      bgSupport = true;
      start = ''
        ${pkgs.dex}/bin/dex "${desktop}" &
        waitPID=$!
      '';
    }) cfg.desktopManager.mixNmatchableDesktops;

and then add desktopManager.mixNmatchableDesktops = "xfce"; to this module.

[worldofpeace]

We could provide both upstream session and our old one but dm=xfce with wm=none would clash with it.

We could provide option for switching between the two but that would be ugly.

Finally, we could add new option desktopManager.mixNmatchableDesktops and have:

services.xserver.desktopManager.session = map (desktop: {
      name = "mm-${desktop}";
      bgSupport = true;
      start = ''
        ${pkgs.dex}/bin/dex "${desktop}" &
        waitPID=$!
      '';
    }) cfg.desktopManager.mixNmatchableDesktops;

and then add desktopManager.mixNmatchableDesktops = "xfce"; to this module.

It seems we have more features to implement 😄
I'm going to revert this one for now.