fix: prevent starting multiple responses

litestar/middleware/_internal/exceptions/middleware.py

@@ -29,6 +29,7 @@
         ExceptionHandler,
         ExceptionHandlersMap,
         Logger,
+        Message,
         Receive,
         Scope,
         Send,
@@ -146,9 +147,19 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         Returns:
             None
         """
+        scope_state = ScopeState.from_scope(scope)
+
+        async def capture_response_started(event: Message) -> None:
+            if event["type"] == "http.response.start":
+                scope_state.response_started = True
+            await send(event)
+
         try:
-            await self.app(scope, receive, send)
+            await self.app(scope, receive, capture_response_started)
         except Exception as e:  # noqa: BLE001
+            if scope_state.response_started:
+                raise LitestarException("Exception caught after response started") from e
+
             litestar_app = scope["app"]
 
             if litestar_app.logging_config and (logger := litestar_app.logger):

litestar/utils/scope/state.py

@@ -44,6 +44,7 @@ class ScopeState:
         "msgpack",
         "parsed_query",
         "response_compressed",
+        "response_started",
         "session_id",
         "url",
         "_compat_ns",
@@ -68,6 +69,7 @@ def __init__(self) -> None:
         self.msgpack = Empty
         self.parsed_query = Empty
         self.response_compressed = Empty
+        self.response_started = False
         self.session_id = Empty
         self.url = Empty
         self._compat_ns: dict[str, Any] = {}
@@ -90,6 +92,7 @@ def __init__(self) -> None:
     msgpack: Any | EmptyType
     parsed_query: tuple[tuple[str, str], ...] | EmptyType
     response_compressed: bool | EmptyType
+    response_started: bool
     session_id: str | None | EmptyType
     url: URL | EmptyType
     _compat_ns: dict[str, Any]

tests/unit/test_middleware/test_exception_handler_middleware.py

@@ -9,7 +9,7 @@
 from structlog.testing import capture_logs
 
 from litestar import Litestar, MediaType, Request, Response, get
-from litestar.exceptions import HTTPException, InternalServerException, ValidationException
+from litestar.exceptions import HTTPException, InternalServerException, LitestarException, ValidationException
 from litestar.exceptions.responses._debug_response import get_symbol_name
 from litestar.logging.config import LoggingConfig, StructLoggingConfig
 from litestar.middleware._internal.exceptions.middleware import (
@@ -20,7 +20,7 @@
 from litestar.status_codes import HTTP_400_BAD_REQUEST, HTTP_500_INTERNAL_SERVER_ERROR
 from litestar.testing import TestClient, create_test_client
 from litestar.types import ExceptionHandlersMap
-from litestar.types.asgi_types import HTTPScope
+from litestar.types.asgi_types import HTTPReceiveMessage, HTTPScope, Message, Receive, Scope, Send
 from litestar.utils.scope.state import ScopeState
 from tests.helpers import cleanup_logging_impl
 
@@ -400,3 +400,29 @@ def handler() -> None:
     with create_test_client([handler], type_encoders={Foo: lambda f: f.value}) as client:
         res = client.get("/")
         assert res.json()["extra"] == {"foo": "bar"}
+
+
+async def test_exception_handler_middleware_response_already_started(scope: HTTPScope) -> None:
+    assert not ScopeState.from_scope(scope).response_started
+
+    async def mock_receive() -> HTTPReceiveMessage:  # type: ignore[empty-body]
+        pass
+
+    mock = MagicMock()
+
+    async def mock_send(message: Message) -> None:
+        mock(message)
+
+    start_message: Message = {"type": "http.response.start", "status": 200, "headers": []}
+
+    async def asgi_app(scope: Scope, receive: Receive, send: Send) -> None:
+        await send(start_message)
+        raise RuntimeError("Test exception")
+
+    mw = ExceptionHandlerMiddleware(asgi_app, None)
+
+    with pytest.raises(LitestarException):
+        await mw(scope, mock_receive, mock_send)
+
+    mock.assert_called_once_with(start_message)
+    assert ScopeState.from_scope(scope).response_started