kevinveenbirkenbach/docker-volume-backup
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fix(backup): mount the zfs snapshot itself so a container can read it

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The zfs snapshot was read through <subject>/.zfs/snapshot/<name>. The
kernel automounts that directory through a usermode helper that runs in
the host's mount namespace, so a containerised baudolo never sees the
mount and finds no snapshot. The snapshot is now mounted explicitly at
<subject>/.baudolo-<tag>, beside where btrfs carves its own, and removed
in order: umount, rmdir, zfs destroy. A failed mount destroys the
snapshot at once instead of leaking one per run.

The e2e suite could not reach this code at all: zpool create on the /img
file failed with "no such pool or dataset", because the kernel opens
vdevs from a taskq thread that resolves paths in the host's root, where
the container's /img does not exist. The pool now sits on a loop device,
whose /dev/loopN node the kernel creates on the host as well, and it is
destroyed on exit once this script created it, since a pool is
kernel-wide.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Kevin Veen-Birkenbach
2026-10-18 00:22:42 +02:00
parent 389b418f42
commit 0735e5fa7b
3 changed files with 90 additions and 15 deletions
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55
tests/unit/backup/test_snapshot.py
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@@ -21,6 +21,14 @@ class Runner:
return []
class Unmountable(Runner):
def __call__(self, command: list[str]) -> list[str]:
result = super().__call__(command)
if command[0] == "mount":
raise BackupError("permission denied")
return result
class TestBtrfs(unittest.TestCase):
def test_it_creates_a_read_only_snapshot_inside_the_subject(self) -> None:
run = Runner()
@@ -95,14 +103,57 @@ class TestZfs(unittest.TestCase):
run.calls[-1], ["zfs", "destroy", "tank/docker@baudolo-20260731"]
)
def test_it_maps_a_volume_path_through_the_dot_zfs_directory(self) -> None:
def test_it_mounts_the_snapshot_inside_the_subject(self) -> None:
run = self._run()
with volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run):
pass
self.assertIn(
[
"mount",
"-t",
"zfs",
"tank/docker@baudolo-20260731",
"/var/lib/docker/.baudolo-20260731",
],
run.calls,
)
def test_it_maps_a_volume_path_into_the_mounted_snapshot(self) -> None:
run = self._run()
with volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run) as resolve:
self.assertEqual(
resolve("/var/lib/docker/volumes/postgres_data/_data"),
"/var/lib/docker/.zfs/snapshot/baudolo-20260731/volumes/postgres_data/_data",
"/var/lib/docker/.baudolo-20260731/volumes/postgres_data/_data",
)
def test_it_unmounts_before_it_destroys(self) -> None:
run = self._run()
with volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run):
pass
self.assertEqual(
run.calls[-3:],
[
["umount", "/var/lib/docker/.baudolo-20260731"],
["rmdir", "/var/lib/docker/.baudolo-20260731"],
["zfs", "destroy", "tank/docker@baudolo-20260731"],
],
)
def test_a_failed_mount_leaves_no_snapshot_behind(self) -> None:
run = Unmountable({"zfs list": ["tank/docker"]})
with (
self.assertRaises(BackupError),
volume_snapshot("zfs", "/var/lib/docker", "20260731", run=run),
):
pass
self.assertEqual(
run.calls[-2:],
[
["rmdir", "/var/lib/docker/.baudolo-20260731"],
["zfs", "destroy", "tank/docker@baudolo-20260731"],
],
)
def test_an_unmounted_dataset_is_an_error(self) -> None:
run = Runner({"zfs list": [""]})
with (