Tips for using the nix package manager on macOS

nix-tips.md

Nix tips

Few tips from using the nix package manager on macOS.

Tip 0 Learn the fundamentals and benefits

https://www.youtube.com/watch?v=D5Gq2wkRXpU

Tip 1 Nix channel pinning

Reference the latest stable channel (18.09-darwin) here and pin to a specific version so we have reproducible builds. Only update if things appear broken or we need more recent updates from upstream.

let
  _pkgs = import <nixpkgs> {};
in
{ pkgs ?
	import (_pkgs.fetchFromGitHub { owner = "NixOS";
			repo = "nixpkgs-channels";
			rev = "e5eabccb00c5d18470c8eff1dca078f1d2365544";
            sha256 = "0mc7vx7afkd940b9lp9kk23kav7lf1z9wqzq4mmklw83ps09srj9";
	}) {}
}:

Tip 2 Ruby configuration

Build gems based on the bundler env by default, nix uses a pegged bundler version The default bundler loaded with bundlerEnv is basically a pass through, so don't use bundler as an input to nix, unless necessary. Bundler is used only as an installation means with bundix

  ruby_2_3 = pkgs.ruby_2_3;
  gems = pkgs.bundlerEnv {
    name = "my_bunlder_env";
    inherit ruby_2_3;
    gemfile = ./Gemfile;
    lockfile = ./Gemfile.lock;
    gemset = ./gemset.nix;
  };

gemset.nix is generated with Bundix and checked in.

Tip 3 Patching outputs by adding phases

this is a hack for xcode-install

  xcode-install = {
    dependencies = ["claide" "fastlane"];
    groups = ["default"];
    platforms = [];
    source = {
      remotes = ["https://rubygems.org"];
      sha256 = "11mhkhc2f6hlpmx9qzv7nz0sincqjp6fmiz0hm51vhic3v79jlg4";
      type = "gem";
    };
    version = "2.5.0";
    postPhases = [ "patchDownloadedGem" ];
    patchDownloadedGem = ''
    echo "Patching downloaded xcversion"
    source env-vars
    cd $out/lib/ruby/gems/2.5.0/gems/xcode-install-2.5.0
    patch -p0 <<'EOF'
diff --git lib/xcode/install.rb lib/xcode/install.rb
index a1c505b..2a87e85 100644
--- lib/xcode/install.rb
+++ lib/xcode/install.rb
@@ -366,8 +366,9 @@ HELP
         return path if path.exist?
       end
       if ENV.key?('XCODE_INSTALL_CACHE_DIR')
-        cache_path = Pathname.new(ENV['XCODE_INSTALL_CACHE_DIR']) + Pathname.new("xcode-#{version}.dmg")
-        return cache_path if cache_path.exist?
+        Pathname.glob(ENV['XCODE_INSTALL_CACHE_DIR'] + '/*').each do |fpath|
+          return fpath if /^xcode_#{version}\.dmg|xip$/ =~ fpath.basename.to_s
+        end
       end

       download(version, progress, url, progress_block)
EOF
    '';
  };

Tip 4 Basic stdenv usage

pkgs.stdenv.mkDerivation rec {
  name = "my_std_env";
  version = "0.1.0";
  buildInputs = [
    pkgs.ruby_2_3
    gems             # Defined above
    #...
  ];

  shellHook = ''
    # use a Gem directory within nix
    export GEM_HOME=$out/.gems
    export PATH=$out/.gems/bin:$PATH
  '';
}

Tip 5 macOS updates

Nix packages are be based on the current OS and may have OS level dependencies. If the OS is updated without updating/recompiling nix dependencies, packages may be broken.

On macOS updates, erase the entire cache of nix ( and likely Nix )

Reference: NixOS/nix#2444

Tip 6 CI <-> Developer Env consistency

Using Nix on the CI and locally makes it easy to ensure that the CI and the developer environment are always on the same packages transitively.