Raise exception from background task on BaseHTTPMiddleware

starlette/middleware/base.py

@@ -103,6 +103,7 @@ async def __call__(self, scope: Scope, receive: Receive, send: Send) -> None:
         request = _CachedRequest(scope, receive)
         wrapped_receive = request.wrapped_receive
         response_sent = anyio.Event()
+        app_exc: Exception | None = None
 
         async def call_next(request: Request) -> Response:
             app_exc: Exception | None = None
@@ -165,9 +166,6 @@ async def body_stream() -> typing.AsyncGenerator[bytes, None]:
                     if not message.get("more_body", False):
                         break
 
-                if app_exc is not None:
-                    raise app_exc
-
             response = _StreamingResponse(status_code=message["status"], content=body_stream(), info=info)
             response.raw_headers = message["headers"]
             return response
@@ -180,6 +178,9 @@ async def body_stream() -> typing.AsyncGenerator[bytes, None]:
                 response_sent.set()
                 recv_stream.close()
 
+        if app_exc is not None:
+            raise app_exc
+
     async def dispatch(self, request: Request, call_next: RequestResponseEndpoint) -> Response:
         raise NotImplementedError()  # pragma: no cover
 

tests/middleware/test_base.py

@@ -297,6 +297,29 @@ async def send(message: Message) -> None:
     assert background_task_run.is_set()
 
 
+def test_run_background_tasks_raise_exceptions(test_client_factory: TestClientFactory) -> None:
+    # test for https://github.com/encode/starlette/issues/2625
+
+    async def sleep_and_set() -> None:
+        await anyio.sleep(0.1)
+        raise ValueError("TEST")
+
+    async def endpoint_with_background_task(_: Request) -> PlainTextResponse:
+        return PlainTextResponse(background=BackgroundTask(sleep_and_set))
+
+    async def passthrough(request: Request, call_next: RequestResponseEndpoint) -> Response:
+        return await call_next(request)
+
+    app = Starlette(
+        middleware=[Middleware(BaseHTTPMiddleware, dispatch=passthrough)],
+        routes=[Route("/", endpoint_with_background_task)],
+    )
+
+    client = test_client_factory(app)
+    with pytest.raises(ValueError, match="TEST"):
+        client.get("/")
+
+
 @pytest.mark.anyio
 async def test_do_not_block_on_background_tasks() -> None:
     response_complete = anyio.Event()