test: test backup preservation failures

Make sure write errors fail without killing the entire backup process.

test/test_object_store.py

@@ -31,3 +31,31 @@ def test_object_store_cancel_backup_preservation(tmp_path: Path) -> None:
     store.cancel_backup_preservation(request_name)
     requests = storage.list_path("site_name/preservation_request")
     assert len(requests) == 0
+
+
+def test_object_store_try_request_backup_preservation_returns_none_on_failure(tmp_path: Path) -> None:
+    storage_dir = tmp_path / "storage"
+    storage_dir.mkdir()
+    storage_dir.chmod(0o000)
+    try:
+        storage = LocalTransfer(directory=str(storage_dir))
+        store = ObjectStore(storage, prefix="site_name", site=None, pgdata=str(tmp_path / "pgdata"))
+        preserve_until = datetime.datetime(2022, 12, 18, 10, 20, 30, 123456, tzinfo=datetime.timezone.utc)
+        request_name = store.try_request_backup_preservation("2022_12_10", preserve_until=preserve_until)
+        assert request_name is None
+    finally:
+        storage_dir.chmod(0o700)
+
+
+def test_object_store_try_cancel_backup_preservation_silently_fails(tmp_path: Path) -> None:
+    storage_dir = tmp_path / "storage"
+    storage_dir.mkdir()
+    storage = LocalTransfer(directory=str(storage_dir))
+    store = ObjectStore(storage, prefix="site_name", site=None, pgdata=str(tmp_path / "pgdata"))
+    preserve_until = datetime.datetime(2022, 12, 18, 10, 20, 30, 123456, tzinfo=datetime.timezone.utc)
+    request_name = store.request_backup_preservation("2022_12_10", preserve_until=preserve_until)
+    storage_dir.chmod(0o000)
+    try:
+        store.try_cancel_backup_preservation(request_name)
+    finally:
+        storage_dir.chmod(0o700)