Remove platform check from the systemd module

[midchildan]

Description

This is related to #2497 (comment).

The systemd module currently contains checks to make sure user services aren't defined on Darwin hosts. But with launchd support now in HM, this can result in feature gating boilerplate when trying to create modules that supports both systemd and launchd.

After this change, modules would be able to define both systemd services and launchd agents without the boilerplate. The systemd module would just ignore them on macOS and vice versa for launchd. I'm hoping this wouldn't be a big issue because most consumers of the systemd module already checks if it supports the target OS. I've also added checks for those that didn't.

For a more concrete example, here's how a module supporting both systemd and launchd might look before this change:

{ lib, pkgs, ... }:

let
  cfg = config.services.example;
  inherit (lib) mkIf mkMerge;
  inherit (pkgs.stdenv.hostPlatforms) isLinux isDarwin;
in {
  options.services.example.enable = mkEnableOption "example service";

  config = mkIf cfg.enable (mkMerge [
    (lib.mkIf isLinux {
      systemd.user.services.example = {
        Unit.Description = "example service";
        Service.ExecStart = "${pkgs.example}/bin/example";
        Install.WantedBy = [ "default.target" ];
      };
    })

    (lib.mkIf isDarwin {
      launchd.agents.example = {
        enable = true;
        config = {
          ProgramArguments = [ "${pkgs.example}/bin/example" ];
          ProcessType = "Background";
        };
      };
    })
  ]);
}

And here's how it'd look after this change:

{ lib, pkgs, ... }:

let
  cfg = config.services.example;
  inherit (lib) mkIf mkMerge;
in {
  options.services.example.enable = mkEnableOption "example service";

  config = mkIf cfg.enable {
    systemd.user.services.example = {
      Unit.Description = "example service";
      Service.ExecStart = "${pkgs.example}/bin/example";
      Install.WantedBy = [ "default.target" ];
    };

    launchd.agents.example = {
      enable = true;
      config = {
        ProgramArguments = [ "${pkgs.example}/bin/example" ];
        ProcessType = "Background";
      };
    };
  };
}

Checklist

• Change is backwards compatible.

• Code formatted with ./format.

• Code tested through 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.

  • Added myself and the module files to .github/CODEOWNERS.

[berbiche]

LGTM!

@rycee friendly ping

[stale]

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[berbiche]

@midchildan would you be interested in rebasing the PR?

@ncfavier @rycee I believe this PR is a decent QOL improvement

[midchildan]

I rebased this PR and added an additional platform check to the services.picom module.

[rycee]

Looks quite nice! 🙂 Merged to master now.