generic-linux-gpu: add support for GNU Shepherd

[gs-101]

Opening up support for other init systems.

Description

This PR outlines GPU driver support for Linux distros featuring an init system other than systemd.

The first supported init system is GNU Shepherd, as it's the default init system on GNU Guix.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

• Code tested through nix run .#tests -- test-all or
  nix-shell --pure tests -A run.all.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.
  • Generate a news entry. See News
  • Basic tests added. See Tests

• If this PR adds an exciting new feature or contains a breaking change.

  • Generate a news entry. See News

[gs-101]

Some things of note:

1. I didn't install the Shepherd file because Shepherd doesn't really have a "services"
   directory, it has files¹ (viz. /etc/shepherd.scm and $XDG_CONFIG_HOME/shepherd/init.scm). So we don't really have a standard place to install the file (do we place it under a /etc/shepherd/? or under $XDG_CONFIG_HOME/shepherd/init.d?).
2. I added a iniSystem option as recommended², but I'm not sure of how we should handle it. Lazyness took me over and I just started copying files all over (even renaming non-nixos-gpu-setup to non-nixos-gpu-setup-systemd!) but that seems impractical. Just too much duplication.

Footnotes

1. https://shepherding.services/manual/html_node/Invoking-shepherd.html ↩

2. https://github.com/nix-community/home-manager/pull/8057#issuecomment-3681399116 ↩

[gs-101]

Changed "shepherd" to "GNU Shepherd" and reworded the long description of the
news commit.

[exzombie]

1. I didn't install the Shepherd file because doesn't really have a "services"
   directory, it has files1 (viz. /etc/shepherd.scm and $XDG_CONFIG_HOME/shepherd/init.scm). So we don't really have a standard place to install the file (do we place it under a /etc/shepherd/? or under $XDG_CONFIG_HOME/shepherd/init.d?).

Not being a user of Guix, I can't tell you where to symlink the service definition. But you must have a symlink somewhere so that you can create a gcroot. Without it, Nix will garbage-collect the drivers at some point, and you will be left with a broken system after reboot.

Perhaps the setup script can add the link somewhere in /etc, then add the service to the Guix configuration. Or instruct the user how to do add it. And to run sudo guix system reconfigure on update, or something like that.

Perhaps Guix experts can provide some ideas on how to do this in an automated manner?

[gs-101]

Perhaps the setup script can add the link somewhere in /etc [...]

Makes more sense, since it's a service closer to the system. /etc/shepherd/non-nixos-gpu.scm.

[...] how to do this in an automated manner?

Not sure on how to automate this, but i'll try this setup for non-nixos-gpu-setup on my system:

#!/bin/bash

set -e

# Install the Shepherd service file and ensure that the store path won't be
# garbage-collected as long as it's installed.
unit_path=/etc/shepherd/non-nixos-gpu.scm
ln -sf @@resources@@/non-nixos-gpu.scm "$unit_path"
ln -sf "$unit_path" "@@statedir@@"/gcroots/non-nixos-gpu.scm

herd load root /etc/shepherd/non-nixos-gpu.scm

I didn't run any "update" commands because just by running this herd load the files were added to /run/opengl-driver.

I'll experiment with home-manager on a test branch.

[gs-101]

I got the service to install the drivers at /run/opengl-driver but I couldn't make it update them.
I tried to delete (rm -rf) the /run/opengl-driver, but that just nuked my graphical environment.