core/pkgs/build-support/buildenv/default.nix

# buildEnv creates a tree of symlinks to the specified paths.  This is
# a fork of the hardcoded buildEnv in the Nix distribution.

{ buildPackages, runCommand, lib, substituteAll }:

let
  builder = substituteAll {
    src = ./builder.pl;
    inherit (builtins) storeDir;
  };

in lib.makeOverridable ({ name

  , # The manifest file (if any).  A symlink $out/manifest will be
  # created to it.
  manifest ? ""

  , # The paths to symlink.
  paths

  , # Whether to ignore collisions or abort.
  ignoreCollisions ? false

  , # If there is a collision, check whether the contents and permissions match
  # and only if not, throw a collision error.
  checkCollisionContents ? true

  , # The paths (relative to each element of `paths') that we want to
  # symlink (e.g., ["/bin"]).  Any file not inside any of the
  # directories in the list is not symlinked.
  pathsToLink ? [ "/" ]

  , # The package outputs to include. By default, only the default
  # output is included.
  extraOutputsToInstall ? [ ]

  , # Root the result in directory "$out${extraPrefix}", e.g. "/share".
  extraPrefix ? ""

  , # Shell commands to run after building the symlink tree.
  postBuild ? ""

    # Additional inputs
  , nativeBuildInputs ? [ ] # Handy e.g. if using makeWrapper in `postBuild`.
  , buildInputs ? [ ]

  , passthru ? { }, meta ? { } }:

  runCommand name rec {
    inherit manifest ignoreCollisions checkCollisionContents passthru meta
      pathsToLink extraPrefix postBuild nativeBuildInputs buildInputs;
    pkgs = builtins.toJSON (map (drv: {
      paths =
        # First add the usual output(s): respect if user has chosen explicitly,
        # and otherwise use `meta.outputsToInstall`. The attribute is guaranteed
        # to exist in mkDerivation-created cases. The other cases (e.g. runCommand)
        # aren't expected to have multiple outputs.
        (if (!drv ? outputSpecified || !drv.outputSpecified)
        && drv.meta.outputsToInstall or null != null then
          map (outName: drv.${outName}) drv.meta.outputsToInstall
        else
          [ drv ])
        # Add any extra outputs specified by the caller of `buildEnv`.
        ++ lib.filter (p: p != null)
        (builtins.map (outName: drv.${outName} or null) extraOutputsToInstall);
      priority = drv.meta.priority or 5;
    }) paths);
    preferLocalBuild = true;
    allowSubstitutes = false;
    # XXX: The size is somewhat arbitrary
    passAsFile =
      if builtins.stringLength pkgs >= 128 * 1024 then [ "pkgs" ] else [ ];
  } ''
    ${buildPackages.perl}/bin/perl -w ${builder}
    eval "$postBuild"
  '')