Running a FastAPI app with nginx unit and getting a Starlette error

[bunny-therapist]

am running a FastAPI app using nginx unit, and using a python script to send multiple sequential (wait for response before sending next) requests to it with the "requests" library. When I reuse the same requests.Session object for all the requests, so that requests.Session.close is not called, then I get intermittent crashes in the application and thus status 500 back.

Traceback (most recent call last):
  File "<path>/venv310/lib/python3.10/site-packages/fastapi/applications.py", line 1054, in __call__
    await super().__call__(scope, receive, send)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/applications.py", line 123, in __call__
    await self.middleware_stack(scope, receive, send)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/errors.py", line 186, in __call__
    raise exc
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/errors.py", line 164, in __call__
    await self.app(scope, receive, _send)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 189, in __call__
    with collapse_excgroups():
  File "/usr/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/_utils.py", line 91, in collapse_excgroups
    raise exc
  File "<path>/venv310/lib/python3.10/site-packages/starlette/responses.py", line 257, in wrap
    await func()
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 217, in stream_response
    return await super().stream_response(send)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/responses.py", line 246, in stream_response
    async for chunk in self.body_iterator:
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 181, in body_stream
    raise app_exc
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 151, in coro
    await self.app(scope, receive_or_disconnect, send_no_error)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 189, in __call__
    with collapse_excgroups():
  File "/usr/lib/python3.10/contextlib.py", line 153, in __exit__
    self.gen.throw(typ, value, traceback)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/_utils.py", line 91, in collapse_excgroups
    raise exc
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 128, in receive_or_disconnect
    message = await wrap(wrapped_receive)
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 123, in wrap
    result = await func()
  File "<path>/venv310/lib/python3.10/site-packages/starlette/middleware/base.py", line 56, in wrapped_receive
    raise RuntimeError(f"Unexpected message received: {msg['type']}")
RuntimeError: Unexpected message received: http.request

The error is coming from this line: https://github.com/encode/starlette/blob/master/starlette/middleware/base.py#L56
which appears to be starlette waiting for a client disconnect but instead getting the next request. (No idea why starlette wants the request to be closed instead of allowing it to be reused for the next request, but maybe there is something I am not aware of.)

I did not encounter this issue when running my FastAPI app with uvicorn, it only happens with nginx unit, but the error is still coming from inside starlette and I cannot understand why this is happening. I have already posted the same question to nginx unit: nginx/unit#1100

[bunny-therapist]

This error goes away if I go back to starlette 0.27.0, where the code where the error is coming from does not exist: 554b9e2

[bunny-therapist]

An issue has been created on nginx unit and there I have commented exactly which commit(s) introduced this problem: nginx/unit#1104

I have no idea if it is nginx unit or starlette which is at fault here. I do not see these errors with uvicorn, which suggests it is unit. However, it is hard for me to judge.