h11._util.RemoteProtocolError: can't handle event type ConnectionClosed when role=SERVER and state=SEND_RESPONSE

I intermittently got this error when load testing uvicorn endpoint.

This error comes from a proxy endpoint where I am also using encode/http3 to perform HTTP client calls.

  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 365, in post
    timeout=timeout,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 497, in request
    timeout=timeout,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 112, in send
    allow_redirects=allow_redirects,
  File "/project/venv/lib/python3.7/site-packages/http3/client.py", line 145, in send_handling_redirects
    request, verify=verify, cert=cert, timeout=timeout
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection_pool.py", line 121, in send
    raise exc
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection_pool.py", line 116, in send
    request, verify=verify, cert=cert, timeout=timeout
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/connection.py", line 59, in send
    response = await self.h11_connection.send(request, timeout=timeout)
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/http11.py", line 65, in send
    event = await self._receive_event(timeout)
  File "/project/venv/lib/python3.7/site-packages/http3/dispatch/http11.py", line 109, in _receive_event
    event = self.h11_state.next_event()
  File "/project/venv/lib/python3.7/site-packages/h11/_connection.py", line 439, in next_event
    exc._reraise_as_remote_protocol_error()
  File "/project/venv/lib/python3.7/site-packages/h11/_util.py", line 72, in _reraise_as_remote_protocol_error
    raise self
  File "/project/venv/lib/python3.7/site-packages/h11/_connection.py", line 422, in next_event
    self._process_event(self.their_role, event)
  File "/project/venv/lib/python3.7/site-packages/h11/_connection.py", line 238, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "/project/venv/lib/python3.7/site-packages/h11/_state.py", line 238, in process_event
    self._fire_event_triggered_transitions(role, event_type)
  File "/project/venv/lib/python3.7/site-packages/h11/_state.py", line 253, in _fire_event_triggered_transitions
    .format(event_type.__name__, role, self.states[role]))
h11._util.RemoteProtocolError: can't handle event type ConnectionClosed when role=SERVER and state=SEND_RESPONSE

[lch277]

There are two web servers. testapp.py

from fastapi import FastAPI
from starlette.requests import Request
from starlette.responses import JSONResponse
import http3

app = FastAPI()
http_client = http3.AsyncClient()

@app.middleware('http')
async def sso_middleware(request: Request, call_next):
    r = await http_client.post('http://127.0.0.1:8001')
    if r.status_code != 200:
        return JSONResponse({'ok': 0, 'data': {'status_code': r.status_code}})
    ret = r.json()
    await r.close()
    print(ret)
    response = await call_next(request)
    return response

@app.get('/')
def index(request: Request):
    return {"ok": 1, "data": "welcome to test app!"}

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="127.0.0.1", port=8000)
    pass

testapp1.py

from fastapi import FastAPI
from starlette.requests import Request

app = FastAPI()

@app.get('/')
@app.post('/')
def index(request: Request):
    return {"ok": 1, "data": "welcome to test app 11111111111!"}

if __name__ == "__main__":
    import uvicorn
    uvicorn.run(app, host="127.0.0.1", port=8001)
    pass

The reproduce steps:

1. run the two servers
2. open the http://127.0.0.1:8000/ in browser
3. refresh the page. the response is 200 and ok.
4. then stop refreshing, do some other works for seconds
5. switch back and refresh again, the first refresh result is 500 most of the time

This happens, not every time. The logs again:

INFO: ('127.0.0.1', 14501) - "GET / HTTP/1.1" 200
{'ok': 1, 'data': 'welcome to test app 11111111111!'}
INFO: ('127.0.0.1', 14501) - "GET / HTTP/1.1" 200
INFO: ('127.0.0.1', 14553) - "GET / HTTP/1.1" 500
ERROR: Exception in ASGI application
Traceback (most recent call last):
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\uvicorn\protocols\http\h11_impl.py", line 370, in run_asgi
    result = await app(self.scope, self.receive, self.send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\applications.py", line 133, in __call__
    await self.error_middleware(scope, receive, send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 122, in __call__
    raise exc from None
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\errors.py", line 100, in __call__
    await self.app(scope, receive, _send)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\starlette\middleware\base.py", line 25, in __call__
    response = await self.dispatch_func(request, self.call_next)
  File "testapp.py", line 13, in sso_middleware
    r = await http_client.post('http://127.0.0.1:8001')
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 406, in post
    timeout=timeout,
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 548, in request
    timeout=timeout,
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 145, in send
    allow_redirects=allow_redirects,
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\client.py", line 178, in send_handling_redirects
    request, verify=verify, cert=cert, timeout=timeout
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\connection_pool.py", line 130, in send
    raise exc
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\connection_pool.py", line 121, in send
    request, verify=verify, cert=cert, timeout=timeout
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\connection.py", line 59, in send
    response = await self.h11_connection.send(request, timeout=timeout)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\http11.py", line 58, in send
    http_version, status_code, headers = await self._receive_response(timeout)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\http11.py", line 130, in _receive_response
    event = await self._receive_event(timeout)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\http3\dispatch\http11.py", line 161, in _receive_event
    event = self.h11_state.next_event()
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_connection.py", line 439, in next_event
    exc._reraise_as_remote_protocol_error()
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_util.py", line 72, in _reraise_as_remote_protocol_error
    raise self
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_connection.py", line 422, in next_event
    self._process_event(self.their_role, event)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_connection.py", line 238, in _process_event
    self._cstate.process_event(role, type(event), server_switch_event)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_state.py", line 238, in process_event
    self._fire_event_triggered_transitions(role, event_type)
  File "C:\Users\lch\.virtualenvs\authcenter-5r0y-06W\lib\site-packages\h11\_state.py", line 253, in _fire_event_triggered_transitions
    .format(event_type.__name__, role, self.states[role]))
h11._util.RemoteProtocolError: can't handle event type ConnectionClosed when role=SERVER and state=SEND_RESPONSE
{'ok': 1, 'data': 'welcome to test app 11111111111!'}
INFO: ('127.0.0.1', 14570) - "GET / HTTP/1.1" 200

Sorry for my poor English.

[njsmith]

I don’t think there are any actual behavioral bugs being discussed in this thread. It’s just about how to give a better error message. So I think either there’s something wrong with your server setup that’s causing it to close connections abruptly without sending responses, or else you’ve found a different bug in httpx and should probably open a new issue so your problem doesn’t get lost in the noise.