When an endpoint raises an exception that is handled with the exception_handlers feature the endpoint will return the correct response to the client but a few seconds later a RuntimeError: No response returned. exception will be raised.
I've notice this only happens when Starlette is initialized with more than one middlewares that are a subclass of BaseHTTPMiddleware. If you remove one of the DummyMiddleware's it'll stop happening.
This started happening with version 0.15.0. It doesn't happen in version 0.14.2. I'm wondering if it could be related to the major change related to AnyIO.
I've been using CPython 3.9.5 but I've also seen the issue in Python 3.10.
I also think that the issue might be related to uvicorn. I've been able to reproduce it with gunicorn -k uvicorn.workers.UvicornWorker app:app but unable to reproduce it with hypecorn: hypercorn app:app.
Just install the requirements.txt.
pip install -r requirements.txt
uvicorn app:app --reload
Just query the timeout endpoint with curl or whatever you prefer:
curl http://localhost:8000/timeout
INFO: Will watch for changes in these directories: ['/home/ramon/tmp/starlette-issue']
INFO: Uvicorn running on http://127.0.0.1:8000 (Press CTRL+C to quit)
INFO: Started reloader process [170302] using statreload
INFO: Started server process [170342]
INFO: Waiting for application startup.
INFO: Application startup complete.
INFO: 127.0.0.1:42846 - "GET /timeout HTTP/1.1" 408 Request Timeout
ERROR: Exception in ASGI application
Traceback (most recent call last):
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/anyio/streams/memory.py", line 81, in receive
return self.receive_nowait()
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/anyio/streams/memory.py", line 76, in receive_nowait
raise WouldBlock
anyio.WouldBlock
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/middleware/base.py", line 41, in call_next
message = await recv_stream.receive()
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/anyio/streams/memory.py", line 101, in receive
raise EndOfStream
anyio.EndOfStream
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/uvicorn/protocols/http/h11_impl.py", line 373, in run_asgi
result = await app(self.scope, self.receive, self.send)
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/uvicorn/middleware/proxy_headers.py", line 75, in __call__
return await self.app(scope, receive, send)
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/applications.py", line 112, in __call__
await self.middleware_stack(scope, receive, send)
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/middleware/errors.py", line 181, in __call__
raise exc
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/middleware/errors.py", line 159, in __call__
await self.app(scope, receive, _send)
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/middleware/base.py", line 63, in __call__
response = await self.dispatch_func(request, call_next)
File "/home/ramon/tmp/starlette-issue/./app.py", line 26, in dispatch
return await call_next(request)
File "/home/ramon/.pyenv/versions/3.9.5/envs/starlette-issue/lib/python3.9/site-packages/starlette/middleware/base.py", line 45, in call_next
raise RuntimeError("No response returned.")
RuntimeError: No response returned.