fix(backup): carve the btrfs snapshot inside its subject, not beside it

The kernel refuses a snapshot whose destination sits on another
filesystem. Placing it at <parent>/.baudolo-<tag> hit that on the most
sensible layout there is: a dedicated disk mounted straight onto
/var/lib/docker, where the parent directory belongs to a different
filesystem and every run failed with EXDEV.

Placing it at <subject>/.baudolo-<tag> makes source and destination the
same filesystem by construction, so the failure cannot occur on any
layout. It also aligns the two backends: the zfs path already resolves
its snapshot inside the subject, at <subject>/.zfs/snapshot/<tag>.

btrfs does not include nested subvolumes in a snapshot, so a leftover
from an interrupted run appears in the next snapshot as an empty
directory rather than recursing, and the copy only ever reads the volume
tree below it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/baudolo/backup/snapshot.py

@@ -41,7 +41,10 @@ def _resolver(subject: str, root: str) -> Callable[[str], str]:
 
 
 def _btrfs(subject: str, name: str, run: Callable[[str], list[str]]) -> tuple[str, str]:
-    target = os.path.join(os.path.dirname(os.path.abspath(subject)), f".{name}")
+    # The snapshot goes inside the subject, never beside it: the kernel rejects
+    # a snapshot whose destination is on another filesystem, which is exactly
+    # what the parent directory is when the subject is a mountpoint of its own.
+    target = os.path.join(os.path.abspath(subject), f".{name}")
     run(f"btrfs subvolume snapshot -r {subject} {target}")
     return target, f"btrfs subvolume delete {target}"
 

tests/unit/backup/test_app_snapshot.py

@@ -64,7 +64,7 @@ class TestSnapshotBranch(unittest.TestCase):
 
     def test_it_reads_from_the_snapshot_and_not_from_the_live_tree(self) -> None:
         source = drive(present=True)[0]["source"]
-        self.assertTrue(source.startswith("/var/lib/.baudolo-"), source)
+        self.assertTrue(source.startswith("/var/lib/docker/.baudolo-"), source)
 
     def test_it_compares_by_content_against_the_previous_generation(self) -> None:
         self.assertTrue(drive(present=True)[0]["authoritative"])

tests/unit/backup/test_snapshot.py

@@ -22,27 +22,27 @@ class Runner:
 
 
 class TestBtrfs(unittest.TestCase):
-    def test_it_creates_a_read_only_snapshot_beside_the_subject(self) -> None:
+    def test_it_creates_a_read_only_snapshot_inside_the_subject(self) -> None:
         run = Runner()
         with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run):
             pass
         self.assertEqual(
             run.calls[0],
-            "btrfs subvolume snapshot -r /var/lib/docker /var/lib/.baudolo-20260731",
+            "btrfs subvolume snapshot -r /var/lib/docker /var/lib/docker/.baudolo-20260731",
         )
 
     def test_it_removes_the_snapshot_afterwards(self) -> None:
         run = Runner()
         with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run):
             pass
-        self.assertEqual(run.calls[-1], "btrfs subvolume delete /var/lib/.baudolo-20260731")
+        self.assertEqual(run.calls[-1], "btrfs subvolume delete /var/lib/docker/.baudolo-20260731")
 
     def test_it_maps_a_volume_path_into_the_snapshot(self) -> None:
         run = Runner()
         with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
             self.assertEqual(
                 resolve("/var/lib/docker/volumes/postgres_data/_data"),
-                "/var/lib/.baudolo-20260731/volumes/postgres_data/_data",
+                "/var/lib/docker/.baudolo-20260731/volumes/postgres_data/_data",
             )
 
     def test_it_keeps_the_trailing_slash_rsync_reads_as_contents(self) -> None:
@@ -50,7 +50,7 @@ class TestBtrfs(unittest.TestCase):
         with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
             self.assertEqual(
                 resolve("/var/lib/docker/volumes/postgres_data/_data/"),
-                "/var/lib/.baudolo-20260731/volumes/postgres_data/_data/",
+                "/var/lib/docker/.baudolo-20260731/volumes/postgres_data/_data/",
             )
 
     def test_it_removes_the_snapshot_even_when_the_body_raises(self) -> None:
@@ -109,7 +109,7 @@ class TestRejections(unittest.TestCase):
     def test_the_subject_itself_resolves_to_the_snapshot_root(self) -> None:
         run = Runner()
         with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
-            self.assertEqual(resolve("/var/lib/docker"), "/var/lib/.baudolo-20260731")
+            self.assertEqual(resolve("/var/lib/docker"), "/var/lib/docker/.baudolo-20260731")
 
 
 class Busy(Runner):