fix(backup): mount the zfs snapshot itself so a container can read it

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>

tests/e2e/test_e2e_snapshot.py

@@ -36,9 +36,14 @@ ZFS_READY = "{ [ -c /dev/zfs ] || modprobe zfs 2>/dev/null; }; [ -c /dev/zfs ]"
 def mount_script(fstype: str) -> str:
     """Build a filesystem on a loop device and carve out the snapshot subject."""
     if fstype == "zfs":
+        # Exception: a loop device, not the /img file, because the kernel opens
+        # vdevs from a taskq thread that resolves paths in the host's root.
+        # The pool is kernel-wide, so it is destroyed on exit, but only once
+        # this script created it.
         return (
-            f"{PACKAGES} && {ZFS_READY} && truncate -s 400M /img "
-            "&& zpool create -m none baudolo /img "
+            f"{PACKAGES} && {ZFS_READY} && truncate -s 400M /img && {ATTACH} "
+            '&& zpool create -m none baudolo "$LOOP" '
+            "&& trap 'zpool destroy -f baudolo; losetup -d \"$LOOP\"' EXIT "
             "&& zfs create -o mountpoint=/subject/docker baudolo/docker"
         )
     return (

tests/unit/backup/test_snapshot.py

@@ -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 (