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/e2e/test_e2e_files_live_writer_precopy.py

@@ -0,0 +1,110 @@
+"""A writer that shrinks files mid-read fails the live pre-copy, not the run."""
+
+import unittest
+
+from .helpers import (
+    backup_path,
+    backup_run,
+    cleanup_docker,
+    create_minimal_compose_dir,
+    ensure_empty_dir,
+    latest_version_dir,
+    require_docker,
+    run,
+    unique,
+    wait_for_log,
+    write_databases_csv,
+)
+
+CHURN_FILES = 8
+
+WRITER = f"""
+trap 'exit 0' TERM
+echo hello > /data/hello.txt
+for i in $(seq 1 {CHURN_FILES}); do
+  (while :; do truncate -s 0 /data/churn$i; truncate -s 16M /data/churn$i; done) &
+done
+echo ready
+wait
+"""
+
+
+class TestE2EFilesLiveWriterPreCopy(unittest.TestCase):
+    @classmethod
+    def setUpClass(cls) -> None:
+        require_docker()
+        cls.prefix = unique("baudolo-e2e-live-writer")
+        cls.backups_dir = f"/tmp/{cls.prefix}/Backups"
+        ensure_empty_dir(cls.backups_dir)
+
+        cls.compose_dir = create_minimal_compose_dir(f"/tmp/{cls.prefix}")
+        cls.repo_name = cls.prefix
+
+        cls.volume = f"{cls.prefix}-vol"
+        cls.writer = f"{cls.prefix}-writer"
+        cls.containers = [cls.writer]
+        cls.volumes = [cls.volume]
+
+        run(["docker", "volume", "create", cls.volume])
+        run(
+            [
+                "docker",
+                "run",
+                "-d",
+                "--name",
+                cls.writer,
+                "-v",
+                f"{cls.volume}:/data",
+                "alpine:3.20",
+                "sh",
+                "-c",
+                WRITER,
+            ]
+        )
+        wait_for_log(cls.writer, "ready")
+
+        cls.databases_csv = f"/tmp/{cls.prefix}/databases.csv"
+        write_databases_csv(cls.databases_csv, [])
+
+        cls.result = backup_run(
+            backups_dir=cls.backups_dir,
+            repo_name=cls.repo_name,
+            compose_dir=cls.compose_dir,
+            databases_csv=cls.databases_csv,
+            database_containers=["dummy-db"],
+            images_no_stop_required=["dummy-image"],
+        )
+
+        cls.hash, cls.version = latest_version_dir(cls.backups_dir, cls.repo_name)
+
+    @classmethod
+    def tearDownClass(cls) -> None:
+        cleanup_docker(containers=cls.containers, volumes=cls.volumes)
+
+    def test_the_live_pre_copy_hit_the_writer(self) -> None:
+        self.assertIn(
+            f"WARNING: live pre-copy of volume '{self.volume}' failed",
+            self.result.stdout,
+            "the writer never shrank a file under rsync, so this run proves nothing",
+        )
+
+    def test_the_stopped_copy_captured_the_volume(self) -> None:
+        files = (
+            backup_path(self.backups_dir, self.repo_name, self.version, self.volume)
+            / "files"
+        )
+        self.assertEqual((files / "hello.txt").read_text().strip(), "hello")
+        self.assertEqual(
+            sorted(p.name for p in files.glob("churn*")),
+            sorted(f"churn{i}" for i in range(1, CHURN_FILES + 1)),
+        )
+
+    def test_the_writer_runs_again_after_the_backup(self) -> None:
+        state = run(
+            ["docker", "inspect", "-f", "{{.State.Running}}", self.writer]
+        ).stdout.strip()
+        self.assertEqual(state, "true")
+
+
+if __name__ == "__main__":
+    unittest.main()