depot/third_party/nixpkgs/nixos/lib/make-system-tarball.nix

{
  stdenv,
  closureInfo,
  pixz,

  # The file name of the resulting tarball
  fileName ? "nixos-system-${stdenv.hostPlatform.system}",

  # The files and directories to be placed in the tarball.
  # This is a list of attribute sets {source, target} where `source'
  # is the file system object (regular file or directory) to be
  # grafted in the file system at path `target'.
  contents,

  # In addition to `contents', the closure of the store paths listed
  # in `packages' are also placed in the Nix store of the tarball.  This is
  # a list of attribute sets {object, symlink} where `object' if a
  # store path whose closure will be copied, and `symlink' is a
  # symlink to `object' that will be added to the tarball.
  storeContents ? [ ],

  # Extra commands to be executed before archiving files
  extraCommands ? "",

  # Extra tar arguments
  extraArgs ? "",
  # Command used for compression
  compressCommand ? "pixz -t",
  # Extension for the compressed tarball
  compressionExtension ? ".xz",
  # extra inputs, like the compressor to use
  extraInputs ? [ pixz ],
}:

let
  symlinks = map (x: x.symlink) storeContents;
  objects = map (x: x.object) storeContents;
in

stdenv.mkDerivation {
  name = "tarball";
  builder = ./make-system-tarball.sh;
  nativeBuildInputs = extraInputs;

  inherit
    fileName
    extraArgs
    extraCommands
    compressCommand
    ;

  # !!! should use XML.
  sources = map (x: x.source) contents;
  targets = map (x: x.target) contents;

  # !!! should use XML.
  inherit symlinks objects;

  closureInfo = closureInfo {
    rootPaths = objects;
  };

  extension = compressionExtension;
}