Employ new documentation.nixos.extraModuleSources option

[bb010g]

Deterministic manual builds with documentation.nixos.includeAllModules! The friendly extension to the asset path is so you'll see "morph/options.nix" instead of "options.nix" when the manual mentions where morph's options are from.

Note that this depends on the option's presence in NixOS, and it probably won't be commonly available for a little while after merge (PR NixOS/nixpkgs#81557). I'm hoping for a 20.03 backport so this can be solved for the next stable.

To work around this in the meantime, a per-machine nixpkgs arg can be provided along the lines of:

let
  # for pkgs.applyPatches
  # (if you're using niv this has already been evaluated)
  bootstrapPkgs = import <nixpkgs> { };

  nixpkgsArgs = {
    config = { allowUnfree = false; };
  };
  # https://github.com/NixOS/nixpkgs/pull/81557
  nixpkgsExtraSourcesPatch = bootstrapPkgs.fetchPatch {
    url = https://github.com/NixOS/nixpkgs/commit/7535c42ebf14618e6c32577a7e2150f0cd522803.patch;
    sha256 = "0ph6xzg7j89lcjpxklhvgkvp3qmqhplhnq6p4l84b06h3krcb876";
  };
  importNixpkgs = nixpkgs: let
    nixpkgs' = bootstrapPkgs.applyPatches {
      src = nixpkgs.outPath or nixpkgs;
      patches = [ nixpkgsExtraSourcesPatch ];
    };      
  in import nixpkgs' nixpkgsArgs;
in {
  # replace <nixpkgs> with your preferred Nixpkgs source
  network.pkgs = importNixpkgs <nixpkgs>;
}

If you're already using niv, though, this can be shortened by wrapping nix/sources.nix with approximately:

let
  # ...
in sources // rec {
  pkgs = importNixpkgs sources.nixpkgs;
  # if you have a nixpkgs-unstable source:
  pkgs-unstable = importNixpkgs sources.nixpkgs-unstable;
}

and then just inheriting pkgs where desired generally (network = { inherit pkgs; }; in this particular case). This also makes sure you don't miss patching when changing Nixpkgs providers.

---

This change is 

[bb010g]

Bump upon landing in 20.03.

[bb010g]

Travis failed on a missing go-bindata, but nix-build --show-trace -k -E '(import <nixos-20.03> { }).callPackage ./nix-packaging/default.nix { } succeeded just fine.

[johanot]

Travis is just Travis I guess :) Thanks for the PR!