fix(restore): refuse to write files into an unmounted backing store

Writing into a volume's mountpoint only restores it when the mountpoint is the storage. A volume with driver options - NFS, a bind device, tmpfs - keeps the same /var/lib/docker/volumes/<name>/_data path, but docker mounts the real backing store over it on demand and unmounts it again when the last consumer stops. Restoring while nothing holds it lands in the empty directory underneath, is hidden by the next mount, and rsync reports success.

The declaration decides, not the mount table: the driver and its options are true at every moment, where the mount table is only true while a container happens to hold the volume.

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

tests/unit/restore/test_files_backing_store.py

@@ -0,0 +1,62 @@
+import tempfile
+import unittest
+from unittest.mock import MagicMock, patch
+
+from baudolo.restore import files as files_mod
+
+
+class TestBackingStoreGuard(unittest.TestCase):
+    def restore(self, inspect: str, mounted: bool) -> tuple[int, list]:
+        calls = []
+
+        def _run(cmd, **kwargs):
+            calls.append(cmd)
+            return MagicMock(stdout=inspect.encode())
+
+        with (
+            patch.object(files_mod, "docker_volume_exists", return_value=True),
+            patch.object(files_mod.os.path, "ismount", return_value=mounted),
+            patch.object(files_mod, "run", side_effect=_run),
+        ):
+            code = files_mod.restore_volume_files("app_data", tempfile.mkdtemp())
+        return code, calls
+
+    def rsynced(self, calls: list) -> bool:
+        return any(cmd[0] == "rsync" for cmd in calls)
+
+    def test_plain_local_volume_is_restored_unmounted(self) -> None:
+        code, calls = self.restore("/var/lib/docker/volumes/a/_data|local|plain", False)
+        self.assertEqual(code, 0)
+        self.assertTrue(self.rsynced(calls))
+
+    def test_volume_with_driver_options_is_refused_while_unmounted(self) -> None:
+        code, calls = self.restore("/var/lib/docker/volumes/a/_data|local|opts", False)
+        self.assertEqual(code, 2)
+        self.assertFalse(
+            self.rsynced(calls),
+            "an NFS or bind volume writes under the mount and reports success",
+        )
+
+    def test_volume_with_driver_options_is_restored_once_mounted(self) -> None:
+        code, calls = self.restore("/var/lib/docker/volumes/a/_data|local|opts", True)
+        self.assertEqual(code, 0)
+        self.assertTrue(self.rsynced(calls))
+
+    def test_foreign_driver_is_refused_while_unmounted(self) -> None:
+        code, calls = self.restore("/mnt/gluster/a|glusterfs|plain", False)
+        self.assertEqual(code, 2)
+        self.assertFalse(self.rsynced(calls))
+
+    def test_an_unresolvable_mountpoint_still_fails_first(self) -> None:
+        code, calls = self.restore("|local|plain", False)
+        self.assertEqual(code, 2)
+        self.assertFalse(self.rsynced(calls))
+
+    def test_a_format_without_the_new_fields_is_treated_as_plain(self) -> None:
+        code, calls = self.restore("/var/lib/docker/volumes/a/_data", False)
+        self.assertEqual(code, 0)
+        self.assertTrue(self.rsynced(calls))
+
+
+if __name__ == "__main__":
+    unittest.main()