Packaging a go app for NixOS

Alex Kretzschmar -

The imposter syndrome around almost anything to do with NixOS is as strong as the pull to use it in the first place.

However, when I found that one of the apps I use daily on my servers was not available in nixpkgs, I thought I'd take a stab at packaging it. What follows is really just a polished up version of my personal notes - it is not intended to replace the NixOS documentation.

I'm not doing this with a Flake because I'm new to packaging and this build is quite simple. If you're curious, here's my ever-evolving and probably flawed nix-config repo.

Here are a few useful links:

The app

The application I wanted to package was arsham/figurine. I recently made a YouTube video about this app if you'd like more information but essentially it's purpose in my workflow is to print out a beautified textual banner at login on a Linux box.

It's written in Go but it does not exist in nixpkgs. So let's try and fix that.

Building the app

The first step was to fork the nixpkgs repo. Next, was finding the correct place to place the code. In the end I settled for putting along apps like cowsay and lolcat which modify text output to the terminal. I have no idea if this was the right place but it seems logical to me.

nixpkgs/pkgs/tools/misc/figurine

This is the relative path from the root of the nixpkgs repo. This repo is a gigantic monolith containing 1000s of build instructions for all the packges in nixpkgs.

Once I'd created the figurine directory, I added the following contents to a file named default.nix.

{ lib
, stdenv
, fetchFromGitHub
, buildGoModule
}:

let
  version = "1.3.0";
in
buildGoModule {
  pname = "figurine";
  inherit version;

  src = fetchFromGitHub {
    owner = "arsham";
    repo = "figurine";
    rev = "v${version}";
    hash = "sha256-1q6Y7oEntd823nWosMcKXi6c3iWsBTxPnSH4tR6+XYs=";
  };

  vendorSha256 = "sha256-mLdAaYkQH2RHcZft27rDW1AoFCWKiUZhh2F0DpqZELw=";

  meta = with lib; {
    homepage = "https://github.com/arsham/figurine";
    description = "Print your name in style";
    license = licenses.asl20;
    maintainers = with maintainers; [ ironicbadger ];
  };
}

I largely copied from other go packages and got to the point that I could try doing a build relatively quickly. And so I ran nix-build default.nix from within the figurine directory. I was greeted with an error.

error: cannot evaluate a function that has an argument without a value ('lib')
       Nix attempted to evaluate a function as a top level expression; in
       this case it must have its arguments supplied either by default
       values, or passed explicitly with '--arg' or '--argstr'. See
       https://nixos.org/manual/nix/stable/language/constructs.html#functions.

       at /home/alex/git/ib/nixpkgs/pkgs/tools/text/figurine/default.nix:1:3:

            1| { lib
             |   ^
            2| , stdenv

Whilst I don't pretend to understand half of what the error messages are saying when dealing with Nix, a kind stranger helped me out and explained to me that I need to pass in an existing nixpkgs context to the build environment for any dependencies to be obtained. That command looks like this.

nix-build -E 'with import <nixpkgs> {}; callPackage ./default.nix {}'

Next I was met with another error, this time relating to the vendorSha256 hash. In order to obtain this hash, I used the following, did a build, and updated the value accordingly.

vendorSha256 = lib.fakeSha256

Which will generate an output something like this (this is faked below but should give you an idea):

	hash mismatch in fixed-output derivation '/nix/store/q4cfnancqqvkc3v18r48zfq0vig6v6y9-pgmetrics-1.6.2-go-modules':
  wanted: sha256:0000000000000000000000000000000000000000000000000000
  got:    sha256:0llbx2sgcx95ym2q4l3334rdj3nkgr9z5jyp8406cp3k1ixi7gdb

In order to get the sha256 hash for fetchFromGithub, I used the following:

nix flake prefetch github:<owner><repo>/<rev>

###
nix flake prefetch github:arsham/figurine/v1.3.0
Downloaded 'github:arsham/figurine/d51c2458c6c429240feca33e999cfb54f418df48' to '/nix/store/pah8nb4pp0b11yj5lgd8izn46zxw0zmr-source' (hash 'sha256-1q6Y7oEntd823nWosMcKXi6c3iWsBTxPnSH4tR6+XYs=').

Doing the build

Everything just kinda worked after that. The build took a minute or so, and the completed package was deposited in ./result/bin/ where I could run it and test it.

It worked!

So the final step was to submit a PR to nixpkgs. At the time of writing that PR is still open, and I have no idea if it will be accepted or not but I feel like I climed a small hillock today. Huzzah!