nix/tests/functional/hermetic.nix

{
  busybox,
  seed,
  # If we want the final derivation output to have references to its
  # dependencies. Some tests need/want this, other don't.
  withFinalRefs ? false,
}:

with import ./config.nix;

let
  contentAddressedByDefault = builtins.getEnv "NIX_TESTS_CA_BY_DEFAULT" == "1";
  caArgs =
    if contentAddressedByDefault then
      {
        __contentAddressed = true;
        outputHashMode = "recursive";
        outputHashAlgo = "sha256";
      }
    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 = "hermetic-input-1";
    buildCommand = "echo hi-input1 seed=${toString seed}; echo FOO > $out";
  };

  input2 = mkDerivation {
    shell = busybox;
    name = "hermetic-input-2";
    buildCommand = "echo hi; echo BAR > $out";
  };

  input3 = mkDerivation {
    shell = busybox;
    name = "hermetic-input-3";
    buildCommand = ''
      echo hi-input3
      read x < ${input2}
      echo ${input2} $x BAZ > $out
    '';
  };

in

mkDerivation {
  shell = busybox;
  name = "hermetic";
  passthru = { inherit input1 input2 input3; };
  buildCommand = ''
    read x < ${input1}
    read y < ${input3}
    echo ${
      if (builtins.trace withFinalRefs withFinalRefs) then "${input1} ${input3}" else ""
    } "$x $y" > $out
  '';
}