Xsessions rework

[poscat0x04]

Motivation for this change

Note: xcfg := config.services.xserver

A summary of how xsession files are currently generated and used:

1. Enabled desktop managers and window managers append their startup scripts to xcfg.desktopManager.session and xcfg.windowManager.session resepctively.
2. One final startup script is generated for every possible pair of desktop managers and window managers by concatenating their startup scripts together and is used to further generate the xsession files. (source) The generated packages are stored in xcfg.displayManager.sessionPackages.
3. The packages are then combined into a single package (xcfg.displayManager.sessionData) which can be consumed by different display managers (sddm, gdm, etc.)

This generation method is less than ideal because:

1. It will generate xsession files that start window managers for desktop managers that don't support external window managers (such as gnome)
2. The generated startup script is overly simplistic: it simply starts the window manager before starting the desktop manager. This ignores the fact that some desktop managers need special treatments when used together with an external window manager. For example, KDE requires the environment variable KDEWM to be set. This also means that the window manager will not have its environment varibles set by the desktop manager, which causes issues like plasma5 + i3 ignore qt theme settings #94482.

Summary of this change

An option supportExternalWM which defaults to false is added for all desktop managers. Desktop managers that support external window managers should set this option to true and provide a function genStart that accepts a package (the binary/startup script of the WM) or null (when the DM is used without an external WM) and returns a string (the final startup script for the DM). The generation method is also changed to adapt to this option.

Note that #100057 will have to change if this pr is merged.

This pr will close #82074, close #94482

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.

[taku0]

Code-wise LGTM. Please rebase on master.
One concern is renaming option from config.services.xserver.displayManager.session to config.services.xserver.displayManager.dmSession, which may break existing customizations. I'm not sure about compatibility policy of NixOS configuration file.

[poscat0x04]

rebased.

[poscat0x04]

We probably need some more testing. It worked when it was introduced, but I'm not sure if it still works now.

[xaverdh]

shouldn't this be wm.start instead of wm.bin?

[xaverdh]

my system fails to start from this pr (rebased on master) because the waitPID thing fails (the additional indirection introduced when calling the resulting script, hides the variable from the caller). If I replace this with wm.start as it used to be in the old code, the wm startup script gets included verbatim and it works again.

[taku0]

@xaverdh could you provide a reproducing configuration?

[xaverdh]

This should do (tested in vm)

{ pkgs, ... }:
{
  users.users.bob.isNormalUser = true;
  services.xserver = {
    enable = true;
    windowManager.xmonad.enable = true;
    displayManager = {
      autoLogin = {
        enable = true;
        user = "bob";
      };
      lightdm = {
        enable = true;
        greeter.enable = false;
      };
    };
  };
}

You can leave out the auto login stuff and log in manually as bob if you want.

[taku0]

The configuration doesn't work on master (without this PR) on QEMU:

rm -f nixos.qcow2 && nixos-rebuild build-vm -I nixos-config=./configuration.nix -I nixpkgs=. && ./result/bin/run-nixos-vm

[xaverdh]

It works for me from master. What error do you get / in what way does it fail?
Note: If you just get a black screen, then everything is working (xmonad is a rather minimal window manager).

[taku0]

I got a black screen. OK, I proceed to testing the PR.

[taku0]

The current implementation generates the following script:

#! /nix/store/cwnwyy82wrq53820z6yg7869z8dl5s7g-bash-4.4-p23/bin/bash

# Legacy session script used to construct .desktop files from
# `services.xserver.displayManager.session` entries. Called from
# `sessionWrapper`.

# Start the window manager.
systemd-cat -t xmonad -- /nix/store/4nl2kcgi927xckk4p8lkxpxi4sxd65wy-xmonad-with-packages-8.10.4/bin/xmonad  &
waitPID=$!


# Start the desktop manager.
if [ -e $HOME/.background-image ]; then
  /nix/store/zv8gfabp44k5f1zxl3dkq7pvx6kqvpls-feh-3.6.3/bin/feh --bg-scale  $HOME/.background-image
fi




test -n "$waitPID" && wait "$waitPID"

/run/current-system/systemd/bin/systemctl --user stop graphical-session.target

exit 0

The new implementation generates the following script:

#!/nix/store/cwnwyy82wrq53820z6yg7869z8dl5s7g-bash-4.4-p23/bin/bash
# Legacy session script used to construct .desktop files from
# `services.xserver.displayManager.session` entries. Called from
# `sessionWrapper`.

# Run the startup script.
/nix/store/71ql8j8sdcnc5dpnn35syvy1h4gjwpv4-run-xmonad
if [ -e $HOME/.background-image ]; then
  /nix/store/zv8gfabp44k5f1zxl3dkq7pvx6kqvpls-feh-3.6.3/bin/feh --bg-scale  $HOME/.background-image
fi




test -n "$waitPID" && wait "$waitPID"

/run/current-system/systemd/bin/systemctl --user stop graphical-session.target

exit 0

where /nix/store/71ql8j8sdcnc5dpnn35syvy1h4gjwpv4-run-xmonad is the following.

#!/nix/store/cwnwyy82wrq53820z6yg7869z8dl5s7g-bash-4.4-p23/bin/bash
systemd-cat -t xmonad -- /nix/store/4nl2kcgi927xckk4p8lkxpxi4sxd65wy-xmonad-with-packages-8.10.4/bin/xmonad  &
waitPID=$!

The script sets waitPID but it is not propagated to the parent script.

So, using wm.start (raw script text) instead of wm.bin (script file name), as @xaverdh states, will fix this.

@poscat0x04 could you fix this?

[xaverdh]

Apart from the issue mentioned above, this works fine for me.
Using none+xmonad here.

[nrdxp]

@poscat0x04, what's the status on this? Could it be made ready for the next release?

[stale]

I marked this as stale due to inactivity. → More info