TCP Transport RuntimeError with Python 3.13 + uvloop

[dylangroos]

TCP Transport RuntimeError with Python 3.13 + uvloop

Summary

uvloop is causing RuntimeError: unable to perform operation on <TCPTransport closed=True> errors when used with Python 3.13. Switching to --loop asyncio fixes the issue.

Environment

• Python: 3.13.3
• uvloop: (latest)
• Starlette: 0.46.2
• FastAPI: 0.115.14

Reproduction

1. Use Python 3.13
2. Run uvicorn with --loop uvloop
3. Make concurrent HTTP requests (e.g., browser loading static files)
4. Client disconnects/refreshes during response

Workaround

Use --loop asyncio instead of uvloop.

Expected

uvloop should handle closed connections gracefully like asyncio does.

Edit

I had a stray uvicorn process that had wracked up 9hrs of CPU time, this could potentially be the issue?

[Kludex]

Is this the same as #506 (comment) ?

[dylangroos]

Let me take a look... will provide more info if need be. Thank you

[aekasitt]

Experiencing the same issue

[aekasitt]

In case this reflects the original post's, I had fixed my problem throwing TCP Transport RuntimeError but my context is a little different. First of all, mine occurs in a pytest-asyncio context that recently got fixed here
Async fixtures may break current event loop
by setting the following lines in pyproject.toml

+ [tool.pytest.ini_options]
+ asyncio_mode = "auto"
+ asyncio_default_fixture_loop_scope = "session"
+ asyncio_default_test_loop_scope = "session"

I mentioned the above fix because I had my test running on uvloop.EventLoopPolicy but python closing the event loop might not be related to uvloop behaviors. In tests that I change the event loop, prior to pytest-asyncio configuration adjustment, I get similar errors to the OP as such:

  /opt/homebrew/Cellar/python@3.13/3.13.2/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/streams.py:343 in writelines

    340 │   │   self._transport.write(data)
    341 │
    342 │   def writelines(self, data):
  ❱ 343 │   │   self._transport.writelines(data)
    344 │
    345 │   def write_eof(self):
    346 │   │   return self._transport.write_eof()

  343: in writelines
  ❱  uvloop/handles/stream.pyx:691 in ???
  691: in uvloop.loop.UVStream.writelines
  ❱  uvloop/handles/handle.pyx:159 in ???
  159: in uvloop.loop.UVHandle._ensure_alive
  ❱  E RuntimeError: unable to perform operation on <TCPTransport closed=True reading=False 0x10783dad0>; the handler is closed

But running the tests without uvloop.EventLoopPolicy override, I still get

/opt/homebrew/Cellar/python@3.13/3.13.2/Frameworks/Python.framework/Versions/3.13/lib/python3.13/asyncio/base_events.py:556 in _check_closed

     553 │
     554 │   def _check_closed(self):
     555 │   │   if self._closed:
  ❱  556 │   │   │   raise RuntimeError('Event loop is closed')
     557 │
     558 │   def _check_default_executor(self):
     559 │   │   if self._executor_shutdown_called:

  556: in _check_closed
  ❱  E RuntimeError: Event loop is closed

So I don't suspect event loop being closed to be an implementation fault of uvloop.