Race condition leads to AttributeError from `BaseEventLoop.__del__`

[bmerry]

Bug report

The following program will rarely (maybe 0.1% of the time) print a stack trace:

#!/usr/bin/env python3

import asyncio
import os
import signal
import sys
import threading

async def main():
    await asyncio.sleep(1)

sys.setswitchinterval(1e-6)
try:
    t = threading.Thread(target=os.kill, args=(0, signal.SIGINT))
    t.start()
    asyncio.run(main())
except KeyboardInterrupt:
    pass
finally:
    t.join()

Output:

Exception ignored while calling deallocator <function BaseEventLoop.__del__ at 0x721e011f2140>:
Traceback (most recent call last):
  File "/usr/lib/python3.15/asyncio/base_events.py", line 765, in __del__
    self.close()
    ~~~~~~~~~~^^
  File "/usr/lib/python3.15/asyncio/unix_events.py", line 71, in close
    super().close()
    ~~~~~~~~~~~~~^^
  File "/usr/lib/python3.15/asyncio/selector_events.py", line 105, in close
    self._close_self_pipe()
    ~~~~~~~~~~~~~~~~~~~~~^^
  File "/usr/lib/python3.15/asyncio/selector_events.py", line 112, in _close_self_pipe
    self._remove_reader(self._ssock.fileno())
                        ^^^^^^^^^^^
AttributeError: '_UnixSelectorEventLoop' object has no attribute '_ssock'
/home/bmerry/work/presentations/2026-10-pycon-asyncio-patterns/tests/./pause.py:19: RuntimeWarning: coroutine 'main' was never awaited
  pass

Tested on Ubuntu 24.04 with CPython 3.15.0rc3 from deadsnakes. I've also observed this with 3.14.6.

CPython versions tested on:

3.15

Operating systems tested on:

Linux