home-manager: allow the tool to be installed in submodule setups

[musjj]

Description

Is there a reason why the standalone tool is not allowed to be installed when you are using home-manager as a submodule? I guess it's not necessary since you can just use nixos-rebuild instead. But if the user chooses to enable it anyways, IMO it should be properly installed.

Checklist

• Change is backwards compatible.

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Maintainer CC

@rycee

[rycee]

Yeah, it's been a long-standing wish of mine to install home-manager also when used as a NixOS module. The tool would need to be reduced quite a bit when used like that, though. For example, we cannot support sub-commands like option, build, instantiate, switch, etc.

They more or less work right now mostly as an implementation detail; basically that HM maintains its own profile even when used inside a NixOS profile. We'll probably eventually move to place the HM generation inside the NixOS profile and then the standalone and NixOS module versions no longer will be interoperable.

Pretty much the only sub-command that we could support would be things like packages and news.

[musjj]

I see, so you're not supposed to run home-manager switch, etc. in a submodule setup? I'm currently using it because it's slightly faster since the system configuration isn't processed.

I guess I'll close it for now, since it's not really ready yet.

[ncfavier]

I don't see why it couldn't be supported. nix build "$flakeAttr.config.home-manager.users.$user.home.activationPackage

[rycee]

@ncfavier Yeah, building and running the activation script would work but it would not have any persistent effect (similar to nixos-rebuild test). The user's "actual" current generation would show up inside the NixOS / nix-darwin profile directory. I.e., something like /run/current-system/home-manager/$user/….

So yeah, I guess we in principle could support home-manager test even in the NixOS module case 🙂

Ultimately I would like to have the current HM generation be activated through a systemd user service on login. Then it would be very natural to support home-manager login and home-manager test to repectively activate the generation on next login and immediately (but revert on next login).