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"
    ''
)