fix(backup): decide snapshot capture per volume

A volume with a backing store of its own is not in a snapshot of the docker data root: it appears there as an existing empty directory, so the copy succeeds, the generation is stamped complete, and the volume is empty in it. The existing check only asked whether the path was inside the snapshot, which that empty directory answers with yes.

The driver, its options and the filesystem the mountpoint sits on now decide, per volume. An uncaptured volume is copied live - correct data without the point in time - while every other volume of the same run keeps its snapshot. One NFS volume no longer costs the whole host its consistent backup.

A volume resolving outside the subject degrades the same way instead of aborting the run.

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

tests/e2e/test_e2e_snapshot.py

@@ -17,6 +17,7 @@ from .helpers import require_docker, run, unique
 
 REPO_SRC = Path(__file__).resolve().parents[2] / "src"
 DRIVER = Path(__file__).resolve().parent / "snapshot_driver.py"
+FAITHFUL_DRIVER = Path(__file__).resolve().parent / "faithful_driver.py"
 IMAGE = "alpine:3.20"
 PACKAGES = "apk add -q btrfs-progs e2fsprogs zfs python3 util-linux"
 ATTACH = (
@@ -76,20 +77,17 @@ def required(fstype: str) -> bool:
     return fstype in demanded.replace(",", " ").split()
 
 
-def stage() -> Path:
+def stage(driver: Path) -> Path:
     """Copy source and driver under /tmp, the only path the DinD daemon shares."""
     staged = Path("/tmp") / unique("baudolo-e2e-snapshot")
     shutil.copytree(REPO_SRC, staged / "src")
-    shutil.copy(DRIVER, staged / "driver.py")
+    shutil.copy(driver, staged / "driver.py")
     return staged
 
 
-def drive(fstype: str, kind: str, expect: str) -> str:
-    staged = stage()
-    script = (
-        f"set -e; {mount_script(fstype)}; "
-        f"python3 /driver.py {kind} /subject/docker {expect}"
-    )
+def drive(fstype: str, arguments: str, *, driver: Path = DRIVER) -> str:
+    staged = stage(driver)
+    script = f"set -e; {mount_script(fstype)}; python3 /driver.py {arguments}"
     try:
         proc = run(
             [
@@ -115,7 +113,7 @@ def drive(fstype: str, kind: str, expect: str) -> str:
         shutil.rmtree(staged, ignore_errors=True)
     if proc.returncode != 0:
         raise AssertionError(
-            f"{fstype}/{kind} driver failed:\n{proc.stdout}\n{proc.stderr}"
+            f"{fstype} driver failed on {arguments}:\n{proc.stdout}\n{proc.stderr}"
         )
     return proc.stdout
 
@@ -126,7 +124,7 @@ class TestE2ESnapshot(unittest.TestCase):
         require_docker()
 
     def assert_freezes(self, fstype: str) -> None:
-        output = drive(fstype, fstype, "supported")
+        output = drive(fstype, f"{fstype} /subject/docker supported")
         self.assertIn("PASS the snapshot exposes the volume", output)
         self.assertIn("PASS a later write does not reach the snapshot", output)
         self.assertIn("PASS the snapshot is removed afterwards", output)
@@ -148,13 +146,24 @@ class TestE2ESnapshot(unittest.TestCase):
         self.assert_freezes("zfs")
 
     def test_ext4_has_no_snapshot_and_says_so(self) -> None:
-        output = drive("ext4", "btrfs", "unsupported")
+        output = drive("ext4", "btrfs /subject/docker unsupported")
         self.assertIn("PASS refused loudly", output)
 
     def test_an_unknown_kind_is_refused_before_touching_the_filesystem(self) -> None:
-        output = drive("ext4", "lvm", "unsupported")
+        output = drive("ext4", "lvm /subject/docker unsupported")
         self.assertIn("PASS refused loudly", output)
 
+    def test_a_volume_with_its_own_storage_is_copied_live_not_from_the_snapshot(
+        self,
+    ) -> None:
+        output = drive("btrfs", "/subject/docker", driver=FAITHFUL_DRIVER)
+        self.assertIn(
+            "PASS the snapshot shows the other volume as an empty directory", output
+        )
+        self.assertIn("PASS the other volume degrades to live", output)
+        self.assertIn("PASS the plain volume is read from the snapshot", output)
+        self.assertIn("ALL OK", output)
+
 
 if __name__ == "__main__":
     unittest.main()