def METHOD_NAME(self):
    from relstorage.adapters.replica import ReplicaSelector
    with open(self.fn, 'w', encoding='utf-8') as f:
        f.write('localhost\n')
    rs = ReplicaSelector(self.fn, 600.0)
    self.assertEqual(rs.current(), 'localhost')
    self.assertEqual(rs.next(), None)