gnome-session not fully systemd managed

[jtojnar]

I believe we are not using the --builtin session for GNOME Shell but it looks like at least the GDM greeter is still relying on that, as evidenced by the logs:

gnome-session-binary[962]: GnomeDesktop-DEBUG(+): Not systemd managed, will not move PID 1193 into transient scope

Debug config

```nix services.xserver = { enable = true; displayManager.gdm = { enable = true; debug = true; }; desktopManager.gnome = { enable = true; debug = true; }; }; ```

cc @NixOS/gnome

Related:

• non-systemd support might be removed from gnome-sesion – although --builtin currently remains
• Pantheon’s Gala is still using --builtin: https://github.com/NixOS/nixpkgs/pull/140429
• So is flashback cc @chpatrick

[bobby285271]

Sounds like --builtin is now gone in that MR and e.g. Budgie also uses --builtin.

[bobby285271]

Flashback should be systemd managed now.

Budgie will probably fork gnome-session (this might also helps if "remove x11 session code" is happening before Budgie 11 I imagine).

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I filed https://github.com/BuddiesOfBudgie/budgie-desktop/issues/505

Misc notes on budgie-session:

• It disables systemd support by default so it does not install gnome-session-ctl / budgie-session-ctl
• The test will need to updated to budgie-session-binary