patching sources in nixpkgs

A lot of software has to be patched for Nix since it breaks some fundamental assumptions about how software should be deployed.

To do that, people use patches which are usually the text format of diff files from git commits.

To generate the patch we can manually clone the repo and create some changes, commit it into git and generate the patch using git format-patch -1 HEAD. This generates a file which we can move into our Nix repo.

We can use it by adding a reference to the file in the patches attribute of mkDerivation.

Note the important part : $src references the source from GitHub UNPATCHED. The patch is applied after and the source is copied to $PWD when you build. Hence the cp -rf * $out instead of $src/*.

{ pkgs, ...}:

pkgs.stdenv.mkDerivation {
  name = "papermod";
  src = pkgs.fetchFromGitHub {
    owner = "adityatelange";
    repo = "hugo-PaperMod";

    # Current master
    rev = "3f50861";
    hash = "sha256-NpO7VEXApOpOXznvhbSWWdxBrzn+xn81OGyUGe4/Hzc=";
  };
  patches = [ ./0001-Fix-span-somehow-broken.patch ];
  buildPhase = ''
    mkdir $out
    cp -rf * $out
  '';
}

Cheers!