sharedModules: add pkgs to module args

[nrdxp]

Fixes #1878.

Description

See the linked issue for context. Opting for a minimal solution.

Todo

• fix for NixOS module
• consider removing useGlobalPackages altogether and always opt for NixOS systems pkgs.
• check nix-darwin module for similar issue and fix if found

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}
  
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[berbiche]

Looking at:

home-manager/nixos/default.nix

Lines 19 to 23 in 2a08899

	imports = import ../modules/modules.nix {
	inherit pkgs;
	lib = extendedLib;
	useNixpkgsModule = !cfg.useGlobalPkgs;
	};

I wonder whether we could rewrite it to imports = (import ....) ++ cfg.sharedModules;

[nrdxp]

That was my first attempt at a solution, unfortunately it didn't work.

[berbiche]

Sorry, I talked too soon :(

[Pacman99]

@berbiche I thought that would work(earlier comment) too. Looking at the submoduleWith implementation it seems like the error might come up, because the first set of modules gets evaluated first or at least collected(I think). But then again, most of the implementation is very cryptic to me. And considering that adding sharedModules to imports doesn't help, it seems like its something else.

[berbiche]

@Pacman99 I think paths are treated differently from functions unfortunately.

If the sharedModule is a file then pkgs is available, otherwise it isn't and specialArgs needs to be set.

[berbiche]

https://github.com/NixOS/nixpkgs/blob/d600f006643e074c2ef1d72e462e218b647a096c/lib/modules.nix#L284

Yup, functions are eagerly checked whereas files and paths are not.

[nrdxp]

Moreover, it looks like they need to be in the current implementation or infinite recursion is inevitable, so I now feel more confident that this is the proper and least invasive fix.

[Pacman99]

No idea if this would work, but could a modules argument be added to modules/modules.nix and these modules can be passed to that.
Also would this PR fix the issue if useGlobalPkgs is false, or does the issue even exist then?

[nrdxp]

Since pkgs is generated based on the value of config.nixpkgs fixing it for useGlobalPackages = false may not even be possible without causing infinite recursion. Perhaps we should make a stronger move and always use system pkgs when using the home-manager NixOS module, removing useGlobalPackages as an option entirely. A similar problem may exist for the darwin module as well.

[Pacman99]

I think this is an issue of using the wrong type for shared modules. See my comment on the issue.

[nrdxp]

Duplicate of #1880