'Test diff restore not allowed when no restore point found.
Detail conditions:
1. backup base is diff-format
2. restore point does not exist'

@common_mocks
def test_diff_restore_allowed_with_no_restore_point(self):

    not_allowed = (False, None)
    backup_base = 'backup.base'
    rbd_io = self._get_wrapped_rbd_io(self.service.rbd.Image())
    args_vols_different = [backup_base, self.backup, self.alt_volume, rbd_io, self.mock_rados]
    with mock.patch.object(self.service, '_rbd_image_exists') as mock_rbd_image_exists:
        mock_rbd_image_exists.return_value = (True, backup_base)
        with mock.patch.object(self.service, '_get_restore_point') as mock_get_restore_point:
            mock_get_restore_point.return_value = None
            args = args_vols_different
            resp = self.service._diff_restore_allowed(*args)
            self.assertEqual(not_allowed, resp)
            self.assertTrue(mock_rbd_image_exists.called)
            mock_get_restore_point.assert_called_once_with(backup_base, self.backup['id'])
