No way to instantiate systemd template units

[hmenke]

Describe the bug

Systemd has the notion of template units, e.g. [email protected] is such a unit. These template units contain common configuration that is suppose to be shared among all instances of this template. For example on a typical Linux distribution it would be possible to instantiate the aforementioned unit for a particular machine

[root@arch] # systemctl enable [email protected]
Created symlink /etc/systemd/system/machines.target.wants/[email protected] → /etc/systemd/system/[email protected].

Something similar is currently not possible on NixOS. Trying to use

{
  systemd.services."systemd-nspawn@archlinux".enable = true;
}

will create an entirely new unit from scratch, completely ignoring the existence of the template [email protected]. Even worse, because /etc/systemd/system links into the read-only Nix store, it's not even possible to instantiate the template in a stateful manner:

[root@nixos] # systemctl enable [email protected]
Failed to enable unit: Unit file /etc/systemd/system/machines.target.wants/[email protected] does not exis.

To Reproduce
Steps to reproduce the behavior:

1. Add systemd.services."systemd-nspawn@archlinux".enable = true; to /etc/nixos/configuration.nix.
2. Rebuild
3. Compare the output of systemctl cat [email protected] and systemctl cat [email protected] and see that they have nothing to do with each other.

Expected behavior
Template units are a powerful concept of systemd and it would be beneficial if NixOS users could take advantage of it.

Screenshots
If applicable, add screenshots to help explain your problem.

Additional context
Add any other context about the problem here.

Notify maintainers

The systemd module doesn't have a maintainer field, so I'm pinging the systemd package maintainers: @andir @eelco @flokli

Metadata
Please run nix-shell -p nix-info --run "nix-info -m" and paste the result.

• system: "x86_64-linux"
• host os: Linux 5.9.16, NixOS, 20.09.2405.e065200fc90 (Nightingale)
• multi-user?: yes
• sandbox: yes
• version: nix-env (Nix) 2.4pre20201201_5a6ddb3
• nixpkgs: /etc/channels/nixos

Maintainer information:

# a list of nixpkgs attributes affected by the problem
attribute:
# a list of nixos modules affected by the problem
module: systemd

[hmenke]

Possible duplicate of #80933

[flokli]

yes, dup of #80933.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/declaring-instances-of-a-generic-systemd-service-e-g-my-service-foo/11738/2

[hmenke]

Here is a workaround for those that are interested:

{ config, lib, pkgs, ... }: {

  systemd.packages = [
    (pkgs.runCommandNoCC "machines" {
      preferLocalBuild = true;
      allowSubstitutes = false;
    } ''
      mkdir -p $out/etc/systemd/system/
      ln -s /etc/systemd/system/[email protected] $out/etc/systemd/system/[email protected]
    '')
  ];

  systemd.services."systemd-nspawn@archlinux".wantedBy = [ "machines.target" ];

}