def ListDirectory(self, pathspec, depth=0):
    """A recursive generator of files."""
    # Limit recursion depth
    if depth >= self.request.max_depth:
      return

    try:
      fd = vfs.VFSOpen(pathspec, progress_callback=self.Progress)
      files = fd.ListFiles(ext_attrs=self.request.collect_ext_attrs)
    except (IOError, OSError) as e:
      if depth == 0:
        # We failed to open the directory the server asked for because dir
        # doesn't exist or some other reason. So we set status and return
        # back to the caller ending the Iterator.
        self.SetStatus(rdf_flows.GrrStatus.ReturnedStatus.IOERROR, e)
      else:
        # Can't open the directory we're searching, ignore the directory.
        logging.info("Find failed to ListDirectory for %s. Err: %s", pathspec,
                     e)
      return

    # If we are not supposed to cross devices, and don't know yet
    # which device we are on, we need to find out.
    if not self.request.cross_devs and self.filesystem_id is None:
      dir_stat = fd.Stat()
      self.filesystem_id = dir_stat.st_dev

    for file_stat in files:

      if stat.S_ISDIR(int(file_stat.st_mode)):
        # Do not traverse directories in a different filesystem.
        is_same_fs = self.filesystem_id == file_stat.st_dev
        if is_same_fs or self.request.cross_devs:
          for child_stat in self.ListDirectory(file_stat.pathspec, depth + 1):
            yield child_stat

      yield file_stat
A recursive generator of files.