fix(backup): let the stopped copy replace a failed live pre-copy

A volume whose containers must stop was copied twice: live first, then
with the containers stopped and --checksum. A live writer that truncates
a file under rsync (read errors mapping ...: No data available (61),
exit 23) aborted the run in the first pass, before the containers were
stopped, although the second pass rewrites every differing file anyway.
Seen in infinito-nexus CI on kix_kix_shared/objecticons.

The live copy now only warns when a stopped copy follows. Without a stop
it stays the only copy and still fails the run, and a failing stopped
copy still fails it.

The e2e test drives a writer that shrinks files mid-read; without the
fix it reproduces the CI failure verbatim.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/unit/backup/test_app_live_precopy.py

@@ -0,0 +1,70 @@
+"""Contract of app.main's live copy when no snapshot is taken."""
+
+from __future__ import annotations
+
+import unittest
+from unittest import mock
+
+from baudolo.backup import app
+from baudolo.backup.shell import BackupError
+from baudolo.backup.volume import Backing
+
+from . import BASE_ARGV
+
+
+def drive(*, stop: bool, fail_live: bool) -> list[str]:
+    events: list[str] = []
+
+    def record(versions_dir, volume_name, volume_dir, *, authoritative, source):
+        events.append("authoritative" if authoritative else "live")
+        if fail_live and not authoritative:
+            raise BackupError("rsync exit code 23")
+
+    with (
+        mock.patch("sys.argv", BASE_ARGV),
+        mock.patch.object(app, "get_machine_id", return_value="machine"),
+        mock.patch.object(app, "create_version_directory", return_value="/gen"),
+        mock.patch.object(app, "create_volume_directory", return_value="/gen/vol"),
+        mock.patch.object(app, "load_databases_df", return_value=None),
+        mock.patch.object(app, "docker_volume_names", return_value=["vol"]),
+        mock.patch.object(app, "containers_using_volume", return_value=["c"]),
+        mock.patch.object(app, "volume_is_fully_ignored", return_value=False),
+        mock.patch.object(app, "backup_dumps_for_volume", return_value=(False, False)),
+        mock.patch.object(
+            app,
+            "inspect_backing",
+            return_value=Backing("/var/lib/docker/volumes/vol/_data"),
+        ),
+        mock.patch.object(app, "requires_stop", return_value=stop),
+        mock.patch.object(app, "filter_stoppable", return_value=["c"]),
+        mock.patch.object(
+            app,
+            "change_containers_status",
+            side_effect=lambda containers, status: events.append(status),
+        ),
+        mock.patch.object(app, "write_manifest"),
+        mock.patch.object(app, "stamp_directory"),
+        mock.patch.object(app, "handle_docker_compose_services"),
+        mock.patch.object(app, "backup_volume", side_effect=record),
+    ):
+        app.main()
+    return events
+
+
+class TestLivePreCopy(unittest.TestCase):
+    def test_a_failed_pre_copy_is_replaced_by_the_stopped_copy(self) -> None:
+        self.assertEqual(
+            drive(stop=True, fail_live=True),
+            ["live", "stop", "authoritative", "start"],
+        )
+
+    def test_a_failed_live_copy_without_a_stop_aborts_the_run(self) -> None:
+        with self.assertRaises(BackupError):
+            drive(stop=False, fail_live=True)
+
+    def test_a_volume_without_a_stop_is_copied_live_once(self) -> None:
+        self.assertEqual(drive(stop=False, fail_live=False), ["live"])
+
+
+if __name__ == "__main__":
+    unittest.main()