Cure existing systemd units

[cole-h]

Description

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

Validating with install.determinate.systems

If a maintainer has added the upload to s3 label to this PR, it will become available for installation via install.determinate.systems:

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix/pr/$PR_NUMBER | sh -s -- install

[cole-h]

Switched to symlink directly because we know where it is and where it should be.

[Hoverbear]

I am fairly sure we couldn't do this with the steam deck? Might be worth checking.

[cole-h]

I validated and this works just fine, it seems.

Installed, ran nix run nixpkgs#hello | nix run nixpkgs#ponysay, rebooted, ran nix run nixpkgs#hello | nix run nixpkgs#ponysay again, worked fine.

[Hoverbear]

Interesting, for some reason I thought this would give issues with cross mount symlinks...

[cole-h]

AFAIK, this is just a simplification of what systemctl link does anyways:

https://github.com/systemd/systemd-stable/blob/8d1db692194f7db407754e85961d82d99117ddab/src/systemctl/systemctl-enable.c#L119 -> https://github.com/systemd/systemd-stable/blob/8d1db692194f7db407754e85961d82d99117ddab/src/shared/install.c#L2407 -> https://github.com/systemd/systemd-stable/blob/8d1db692194f7db407754e85961d82d99117ddab/src/shared/install.c#L2488 -> https://github.com/systemd/systemd-stable/blob/8d1db692194f7db407754e85961d82d99117ddab/src/shared/install.c#L559

[cole-h]

I don't think we are currently structured to be able to have some interactivity within the plans... We do have a prompt function but that's part of the CLI and it would feel weird putting that in here...

[Hoverbear]

We have discussed adding some UX for curing to prompt users, but as of right now -- no support.

[cole-h]

I also explicitly do not consider unit overrides, since that would be a huge nightmare to handle (AFAIK, the only way to know about overrides is to do something like systemctl cat the unit and look at the comments that tell you what came from which file).

If there was an easy way to detect, we could at least warn or error about it, but I don't see an easy way to do that...

systemctl status can kinda show this, but I'd be very much against trying to parse that output:

● nix-daemon.service - Nix Daemon
     Loaded: loaded (/etc/systemd/system/nix-daemon.service; linked; preset: enabled)
    Drop-In: /etc/systemd/system.control/nix-daemon.service.d
             └─50-CPUWeight.conf
             /nix/store/z19lfwkg4bcrvq4llrqyv18n4nkjvlw1-system-units/nix-daemon.service.d
             └─overrides.conf

[cole-h]

re: overrides -- apparently there's a dbus API we could use, but for now I'm going with "best effort" and checking the place where the overrides are likely to appear.