def METHOD_NAME(self):
    """
    Something that is already btrfs but is subvolid=5, indicating that it's
    a loopback. When not in use, this will cleanly unmount.
    """
    artifacts_dir = Path(
        tempfile.mkdtemp(prefix=self.id(), dir=self._scratch_subvolume())
    )
    volume_dir = artifacts_dir / vfr.VOLUME_DIR
    os.mkdir(volume_dir)
    with tempfile.NamedTemporaryFile() as loopback:
        loopback.truncate(500 * 1024 * 1024)
        subprocess.run(["mkfs.btrfs", loopback.name], check=True)
        subprocess.run(["sudo", "mount", loopback.name, volume_dir], check=True)
        self.assertEqual(btrfsutil.subvolume_id(volume_dir), 5)
        self.assertEqual(
            vfr.get_volume_for_current_repo(
                artifacts_dir=artifacts_dir,
            ),
            volume_dir,
        )
        self.assertTrue(btrfsutil.is_subvolume(volume_dir))
        self.assertNotEqual(btrfsutil.subvolume_id(volume_dir), 5)