'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'

def GetMountPoint(self, path=None):

    path = os.path.abspath(client_utils.CanonicalPathToLocalPath((path or self.path)))
    while (not os.path.ismount(path)):
        path = os.path.dirname(path)
    return path
