docs: def GetMountPoint(self, path=None):
      """Walk back from the path to find the mount point.
  
      Args:
        path: a Unicode string containing the path or None. If path is None the
          value in self.path is used.
  
      Returns:
        path string of the mount point
      """
      path = os.path.abspath(
          client_utils.CanonicalPathToLocalPath(path or self.path))
  
      while not os.path.ismount(path):
        path = os.path.dirname(path)
  
      return path