/// traversal couldn't start, `*undeleted_files` must be set to 0 and
  /// `*undeleted_dirs` must be set to 1.
  ///
  /// TODO(b/139060984): After all filesystems are converted, consider
  /// invariant about `*undeleted_files` and `*undeleted_dirs`.
  ///
  /// Plugins:
  ///   * Must set `status` to `TF_OK` if directory was deleted.
  ///   * Must set `status` to `TF_NOT_FOUND` if `path` doesn't exist.