build-hook.nix

{
  busybox,
  contentAddressed ? false,
}:

with import ./config.nix;

let

  caArgs =
    if contentAddressed then
      {
        outputHashMode = "recursive";
        outputHashAlgo = "sha256";
        __contentAddressed = true;
      }
    else
      { };

  mkDerivation =
    args:
    derivation (
      {
        inherit system;
        builder = busybox;
        args = [
          "sh"
          "-e"
          args.builder or (builtins.toFile "builder-${args.name}.sh" ''
            if [ -e "$NIX_ATTRS_SH_FILE" ]; then source $NIX_ATTRS_SH_FILE; fi;
            eval "$buildCommand"
          '')
        ];
      }
      // removeAttrs args [
        "builder"
        "meta"
        "passthru"
      ]
      // caArgs
    )
    // {
      meta = args.meta or { };
      passthru = args.passthru or { };
    };

  input1 = mkDerivation {
    shell = busybox;
    name = "build-remote-input-1";
    buildCommand = "echo hi-input1; echo FOO > $out";
    requiredSystemFeatures = [ "foo" ];
  };

  input2 = mkDerivation {
    shell = busybox;
    name = "build-remote-input-2";
    buildCommand = "echo hi; echo BAR > $out";
    requiredSystemFeatures = [ "bar" ];
  };

  input3 = mkDerivation {
    shell = busybox;
    name = "build-remote-input-3";
    buildCommand = ''
      echo hi-input3
      read x < ${input2}
      echo $x BAZ > $out
    '';
    requiredSystemFeatures = [ "baz" ];
  };

in

mkDerivation {
  shell = busybox;
  name = "build-remote";
  passthru = { inherit input1 input2 input3; };
  buildCommand = ''
    read x < ${input1}
    read y < ${input3}
    echo "$x $y" > $out
  '';
}