fix(runtime): fail closed when sandbox teardown fails

strix/runtime/backends.py

@@ -119,11 +119,12 @@ async def start_session_with_retry(
             if session is not None:
                 try:
                     await client.delete(session)
-                except Exception:  # noqa: BLE001
+                except Exception as teardown_error:
                     logger.warning(
-                        "Failed to tear down sandbox after start failure",
+                        "Failed to tear down sandbox after start failure; aborting retry",
                         exc_info=True,
                     )
+                    raise exc from teardown_error
             transient = _is_transient_start_error(exc)
             if not transient or attempt == max_attempts:
                 raise

tests/test_backend_retries.py

@@ -33,9 +33,10 @@ class _FakeSession:
 
 
 class _FakeClient:
-    def __init__(self) -> None:
+    def __init__(self, *, delete_error: BaseException | None = None) -> None:
         self.created = 0
         self.deleted: list[_FakeSession] = []
+        self.delete_error = delete_error
 
     async def create(self) -> _FakeSession:
         self.created += 1
@@ -54,6 +55,8 @@ class _FakeClient:
 
     async def delete(self, session: _FakeSession) -> None:
         self.deleted.append(session)
+        if self.delete_error is not None:
+            raise self.delete_error
 
 
 async def test_transient_workspace_failure_retries_and_tears_down(
@@ -90,6 +93,28 @@ async def test_non_transient_workspace_failure_does_not_retry() -> None:
     assert client.deleted == [session]
 
 
+async def test_teardown_failure_raises_original_error_without_retry() -> None:
+    start_error = WorkspaceStartError(
+        path=Path("/workspace"),
+        cause=TimeoutError("transient transport timeout"),
+    )
+    teardown_error = RuntimeError("teardown failed")
+    client = _FakeClient(delete_error=teardown_error)
+    session = _FakeSession([start_error])
+
+    async def create_session() -> _FakeSession:
+        client.created += 1
+        return session
+
+    with pytest.raises(WorkspaceStartError) as caught:
+        await start_session_with_retry(client, create_session, attempts=3)
+
+    assert caught.value is start_error
+    assert caught.value.__cause__ is teardown_error
+    assert client.created == 1
+    assert client.deleted == [session]
+
+
 async def test_each_transient_attempt_is_torn_down(monkeypatch: pytest.MonkeyPatch) -> None:
     client = _FakeClient()
     client.created = 0