Strange occasional `RuntimeError: Event loop is closed` out of nowhere

从 ipykernel/iostream.py 开始，重点查看 _watch_pipe_fd、write、_schedule_flush，以及 traceback 中显示的 pub-thread 调度路径。将该生命周期与 Kernel.do_execute 以及外部解释器对 stdout 和 stderr 的处理进行比较，然后调查 asyncio 循环何时被关闭。当识别出一个可复现的关闭竞态，或确认负责该问题的用户代码，并且在问题位于 ipykernel 时添加回归测试后，即视为完成。

I'm implementing a kernel by inheriting from ipykernel.kernelbase.Kernel, as described at Making simple Python wrapper kernels.

That generally works well, but I have a strange problem: Occasionally I get an exception RuntimeError: Event loop is closed:

Traceback (most recent call last):
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/ipykernel/iostream.py", line 390, in _watch_pipe_fd
    self.write(bts.decode(errors='replace'))
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/ipykernel/iostream.py", line 664, in write
    self._schedule_flush()
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/ipykernel/iostream.py", line 561, in _schedule_flush
    self.pub_thread.schedule(_schedule_in_thread)
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/ipykernel/iostream.py", line 269, in schedule
    f()
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/ipykernel/iostream.py", line 559, in _schedule_in_thread
    self._io_loop.call_later(self.flush_interval, self._flush)
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/tornado/ioloop.py", line 597, in call_later
    return self.call_at(self.time() + delay, callback, *args, **kwargs)
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/site-packages/tornado/platform/asyncio.py", line 206, in call_at
    return self.asyncio_loop.call_later(
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/asyncio/base_events.py", line 721, in call_later
    timer = self.call_at(self.time() + delay, callback, *args,
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/asyncio/base_events.py", line 732, in call_at
    self._check_closed()
  File "/home/ca/Store/micromamba/envs/std/lib/python3.10/asyncio/base_events.py", line 515, in _check_closed
    raise RuntimeError('Event loop is closed')
RuntimeError: Event loop is closed

The traceback does not go back to my code, so I'm not certain whether I did something wrong, or there is a bug in ipykernel.kernelbase? I am unable to reproduce the error, in the exact same situation it sometimes happens and sometimes doesn't.

I found two other issues which mention this exception, but they are about UI events (#825) or parallelization (#534), both of which I do not use. I inherited from ipykernel.kernelbase.Kernel and implemented do_execute, which sends code to an external interpreter and sends its stdout and stderr via self.send_response.