nixos/displayManager: introduce defaultSession

[hedning]

Motivation for this change

Integrate services.xserver.displayManager.sessionPackages better with the existing session system.

resolves #54089

services.xserver.displayManager.defaultSession

There's two ways of providing graphical sessions now:

1. displayManager.session via. desktopManager.session and
   windowManager.session
2. displayManager.sessionPackages

sessionPackages doesn't make a distinction between desktop and window
managers. This makes selecting a session provided by a package using
desktopManager.default nonsensical.

We therefor introduce displayManager.defaultSession which can select a session
from either displayManager.session or displayManager.sessionPackages.

It will default to desktopManager.default + windowManager.default as before.
If the dm default is "none" it will select the first provided session from
sessionPackages.

services.xserver.displayManager.sessionPackages

Note extraSessionFilePackages is renamed to the less verbose sessionPackages.

When adding a package, foo, to sessionPackages session names are now required to be specified in foo.passthru.providedSessions, this is used to validate defaultSession at evaluation time.

At build time mkDesktops will check that the sessions provided by a package is actually present, bailing out if not, hopefully preventing typos etc.

closes #39871

Things done

Ran these tests succesfully:

• gnome3
• sddm
• plasma5
• lightdm
• xfce

---

After default options deprecation:

• gdm+xfce+openbox
• lightdm+xfce+openbox
• lightdm+gnome-flashback-metacity

[jtojnar]

Should we perhaps deprecate desktopManager.default and windowManager.default?

[hedning]

Hmm, perhaps, but I'd imagine they're both used quite heavily. Also recreating the the desktopManager.default+windowManager.default manually also seems like chore, though I'm not certain how common it us to mix those in reality.

[hedning]

We should probably add some documentation too.

[hedning]

Added share/wayland-sessions to lightdm's sessions-directory, this won't work with gnome due to duplicate names, but is necessary when having a default session which is only found there. It should also be useful for sway et. al. Slim doesn't seem to support default session.

[aanderse]

ping (triage)

[worldofpeace]

Last time I checked @hedning was rethinking this approach.

[hedning]

So it's possible to reuse desktopManager.default for selecting sessions from sessionPackages. But we still need to construct a defaultSession for lightdm/sddm which require the same logic as in displayManager.defaultSession (ie. checking if the name cannot be found in displayManager.session.names it's available in displayManager.sessionPackages).

So thinking on this some more I think it might be better to stick with an explicit defaultSession that the various display managers can use without trouble. The downside would be users who've set desktopManager.default = "gnome" which now would fail with an unhelpful error message. And somewhat a proliferation of similar options.

So I guess the question is this:
Do we want to select default sessions using desktopManager.default (and windowManager.default) or add a unified option that can override these?

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/wayland-sessions/4315/6

[jtojnar]

Yeah, I noticed that too, though on unstable it retains lightdm background for me for some reason.

[jtojnar]

Rebased onto the sway changes and squashed the commits. Is there anything remaining to do? I successfully ran gnome-flashback-metacity from GDM, and none+xmonad and pantheon from LightDM, and successfully ran all the DE/DM tests.

[worldofpeace]

It's good with me @jtojnar, and mostly because I don't think I can look over it again differently 😄

I'd say, maybe one more reviewer with a different set of eyes.
If waiting doesn't help we can just merge.

[hedning]

Went over the new code and it looks good to me. 👍 on deprecating the old default options and implementing legacy sessions through sessionPackages. Thanks for bringing this to the finish line :)

[worldofpeace]

@Profpatsch Agreed to QA this by using it.

[grahamc]

✔️ I upgraded from 19.09 to nixos-unstable, and added this to my configuration:

services.xserver = {
      enable = true;
      libinput.enable = true;
      displayManager.sddm.enable = true;
};

After deleting an old ~/.config/i3/config, sway started just fine out of the box. After this change my gpg pinentry windows changed to be more plasma-ey, but I'm assuming that isn't unsurprising. I did have to select "sway" in the drop-down by hand each boot, but since I erase my / on every boot I'm assuming that is normal too :).

[hedning]

@grahamc: with this pr LightDM will launch wayland sessions (just double checked that it works with sway), and you'll be able to use displayManager.defaultSession to force LightDM to select the sway session by default :)

[worldofpeace]

@grahamc: with this pr LightDM will launch wayland sessions (just double checked that it works with sway), and you'll be able to use displayManager.defaultSession to force LightDM to select the sway session by default :)

This reminds me I have a branch with canonical/lightdm@03f2189 on top of this. Will PR this after, I'm not sure how it works for sway without the commit though (perhaps it uses the key in the session that forces it)

[xaverdh]

Just tried this with my xmonad + lightdm autologin setup. Works perfectly. I was a bit surprised to get an xterm window though, when not explicitly setting defaultSession to "none+xmonad"
(it chose "xterm+xmonad" otherwise) even though I set services.xserver.desktopManager.xterm.enable to false. I guess xterm somehow always winds up in the sessions?

[xaverdh]

Ah sorry services.xserver.desktopManager.xterm.enable was not set to false (explicitly). The defaultText here is misleading. When stateVersion is older then "19.09" it defaults to xSessionEnabled, not false..
EDIT: The issue with the code is that the manual gets generated on the hydra builders, not locally, so the stateVersion used is the one from the builder. This yields an incorrect defaultText.

[worldofpeace]

@xaverdh Autologin requries a defaultSession to be selected. So I believe it choose head cfg.displayManager.sessionData.sessionNames when you didn't set one. I believe the option description describe this well. As for #53843 (comment), we hope to eliminate this in 20.03 (default to false) #68371.

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/setting-up-urxvt-i3/7847/2