nixredo/builder.nix

buildArgs@{ lib ? import ./lib.nix {}, nixpkgs ? <nixpkgs>, pkgs ? import nixpkgs {}, root, filter ? [] }:
let
  inherit (lib) searchPath whichdo d1 d2;

  resolve = path: let
    builder = (whichdo path);
    in if builder == "" then throw "No rule to build ${path}"
       else runBuilder (d1 path builder) (d2 path builder) builder;

  runBuilder = d1: d2: builder:
    with pkgs.lib;
    with builtins;
    let
      # Funktionalise pkgs; our main entry point to resolve default*nix build scripts.
      funcPkgs = { __functor = self: arg: resolve arg; } // pkgs;

      _builder = let
        imported = (tryEval (let
          # Needs to be done as nix chokes on empty files rn.
          content = readFile builder;
        in assert content != "" && (! (hasPrefix "#!" content)); import builder));
      in if imported.success
         then imported.value
         else {
           src = filterSource
             (path: type: baseNameOf path != ".git" && baseNameOf path != ".envrc" && (all (x: path != x) filter))
             (/. + root);
           builder = let
             relPath = dirOf (removePrefix (root + "/") builder);
           in ''
             #!/bin/sh
             set -e
             pout=$(mktemp -u -p $PWD)
             cp -r --reflink=auto --no-preserve=mode "$src/" src
             cd src/${relPath}
             ${/. + builder} $d1 $d2 $out >$pout
             if [ -e $out -a -s $pout ]; then
               printf "Error: %s wrote to stdout and created \$3\n" "$d1" >&2
               exit 207
             elif [ -s $pout ]; then
               mv $pout $out
             fi
           '';
         };
      __builder = if isFunction _builder then _builder funcPkgs
                  else _builder;

      args = { stdenv = pkgs.stdenvNoCC; } // (if isString __builder
                                                then { builder = __builder; }
                                                else  __builder);

      drv = (args.deriver or args.stdenv.mkDerivation) ({
        name = pkgs.lib.strings.sanitizeDerivationName d1;
        buildCommand = args.builder;
        passAsFile = [ "buildCommand" ];
        preferLocalBuild = true;
        allowSubstitutes = false;
        outputs = [ "out" ];
        inherit d1 d2;
      } // (removeAttrs args [ "builder" "system" "deriver" "stdenv" "outputs" ]));
    in drv; # myself
in resolve