flake/lib.nixosSystem: add _file-keys for error-location if needed

[Ma27]

Motivation for this change

⚠️ Caution: this is a late-night idea and I'm not sure if it's a good one! Before anybody considers to merge this, a review from our module-system experts is needed :)

---

When inlining a module with a problematic declaration, you usually get
get a not-so helpful error like this:

$ cat flake.nix
{
  description = "A very basic flake";
  inputs.nixpkgs.url = path:../.;
  outputs = { self, nixpkgs }: {
    nixosConfigurations.foo = nixpkgs.lib.nixosSystem {
      system = "x86_64-linux";
      modules = [
        ({ lib, ... }: { services.wrong = 2; })
        { services.nginx.enable = true; }
      ];
    };
  };
}
$ nixos-rebuild build --flake .#foo -L
error: The option `services.wrong' does not exist. Definition values:
       - In `<unknown-file>': 2

While it's certainly possible to guess where this comes from, this is
IMHO fairly confusing for beginners (and kinda reminds me of the
infamous "infinite recursion at undefined position"-error).

The module-system determines the position of a declaration using the
_file-key: this is either toString path if path is e.g. a value
from imports = [ ./foo.nix ] or the file used as NIXOS_CONFIG in
<nixpkgs/nixos>.

However such a mechanism doesn't exist (yet) for inlined flake modules,
so I tried to implement this in a fairly basic way:

• modules = [ ./path.nix ] in lib.nixosSystem doesn't require any
  additional steps, ./path.nix is used as position for all
  declarations inside ./path.nix.

• For non-path declarations, the position of modules inside the
  flake.nix which declares these modules is determined by doing
  unsafeGetAttrPos on the modules-argument of lib.nixosSystem.

  So the flake.nix from above would now raise the following
  error-message:

    $ nixos-rebuild build --flake .#foo -L
    error: The option `services.wrong' does not exist. Definition values:
           - In `/nix/store/4vi3nhqjyma73ygs4f93q38qjkhkaxw8-source/flake.nix': 2
  

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Relase notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

[roberth]

Did you try unsafeGetAttrPos on nixosSystem's args.modules?

[Ma27]

Good point! Gonna try it out, this should simplify this by a lot!

[Ma27]

@roberth thanks a lot, very good point! Updated the PR accordingly! :)

[infinisil]

Haven't tested it myself, but I think this should be fine.

[roberth]

I've added a tiny refactor to fix the formatting and add an inline doc.

[roberth]

Can be merged afaic.

[Ma27]

Squashed this together to have a nicer commit-message and re-checked this. Would merge in a minute as several folks are fine with this in the current state :)

[ncfavier]

Is there a reason for having this in flake.nix rather than nixos/lib/eval-config.nix? It seems to work the same.

[Ma27]

I don't recall the details anymore, but I think this was a flake-specific issue. Can you reproduce the overall problem with a non-flake NixOS configuration? If that's the case, please share your code with me and if needed, we should move the change to nixos/lib/eval-config.nix indeed.

[ncfavier]

The reason I'm asking is that I'm working on a PR to move much of the logic of lib.nixosSystem out of flake.nix, so if it can be done for this it's good news.

I didn't run into the overall problem, but it's easy to imagine a situation where it would arise: basically any time eval-config.nix is invoked directly with inline modules that have errors in them.

Now I'm actually wondering if this could be moved directly to lib.evalModules... I think falling back to unsafeGetAttrPos for inline modules makes sense for any use of the module system. EDIT: #126769 (comment)