jlesquembre / simple-flake

Share code example

This is an example for NixOS/nixpkgs#217679

The goal is to reduce duplication between Nixpkgs and Nix files in upstream projects (like this project).

To do that, the idea is to commit the shared nix expressions to Nixpkgs. Following some convetions, we can reuse the nix expressions in the upstream project.

For a full derivation example, see https://github.com/jlesquembre/nixpkgs/blob/share-nixpkgs/pkgs/applications/misc/simple-pkg/default.nix

1. (upstream) Define a package (in this example, in the flake.nix) like this:

   packages.default = pkgs.callPackage "${nixpkgs}/pkgs/applications/misc/simple-pkg/default.nix" {
     # Use this as src, overriding the value on nixpkgs
     projectInfo = {
       src = ./.;
       version = "DEV";
     };
   };

2. (Nixpkgs) Our expression is defined like this:

   {
     # ...
     nixpkgsUpdater # Optional
   , srcFromJSON
   
     # projectInfo is overloaded, it can be:
     # - Path to an "info.json" file (That's the case in nixpkgs)
     # - Attribute set with values to use in mkDerivation
   , projectInfo ? srcFromJSON ./info.json
   }:
   
   stdenv.mkDerivation {
     pname = "simple";
     inherit (projectInfo) src version;
   
     # Function to be executed by r-ryantm bot
     # nixpkgsUpdater is optional
     passthru.updateScript = nixpkgsUpdater {
       fetcher = "fetchFromGitHub";
       fetcherArgs = {
         owner = "github-username";
         repo = "simple-flake";
       };
     };
   }

3. (Nixpkgs) Notice that we have a info.json file in the same directory that the default.nix file:

   {
     "fetcher": "fetchFromGitHub",
     "fetcherArgs": {
       "hash": "sha256-x9BrU5hj6HsnRudt7a7qEJOTX2WU1UdAxiPK40NLvew=",
       "owner": "jlesquembre",
       "repo": "simple-flake",
       "rev": "v1.0"
     },
     "version": "1.0"
   }

   info.json has to provide 2 fields, fetcher and fetcherArgs. fetcher is one of the fetchers defined in Nixpkgs manual: Fetcher. fetcherArgs are the arguments to that function. Any other fields would be passed to mkDerivation, in the projectInfo attribute set.

   We can create info.json manually or with the nixpkgsUpdater helper function: nix build /path/to/local/nixpkgs#simple-pkg.updateScript; ./result

Update shared nix code

In our repository, nixpkgs is an input:

inputs = {
  nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
};

To do changes, we can point nixpkgs to our local copy:

inputs = {
  nixpkgs.url = "/path/to/my/local/nixpkgs";
};