I just might be wrong because if this indeed a race condition it should be breaking more things. Anyway, I got this exception (line numbers might be wrong due to debug statements):

Traceback (most recent call last):
  File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
    self.run()
  File "/.../lib/python3.8/site-packages/aqt/mediasrv.py", line 89, in run
    self.server.run()
  File "/.../lib/python3.8/site-packages/waitress/server.py", line 323, in run
    self.asyncore.loop(
  File "/.../lib/python3.8/site-packages/waitress/wasyncore.py", line 285, in loop
    poll_fun(timeout, map)
  File "/.../lib/python3.8/site-packages/waitress/wasyncore.py", line 211, in poll
    r, w, e = select.select(r, w, e, timeout)
OSError: [Errno 9] Bad file descriptor

This error was extremely rare but since I was getting it while running tests I could just run a lot of them until one failed, which I did, and I think the problem is a follows.

1. First, thread Thread-1 that the app I'm testing is launching, one that runs waitress server, assembles the descriptor lists for select:

   waitress/src/waitress/wasyncore.py

   Lines 154 to 166 in 603d2c1

   	r = []
   	w = []
   	e = []
   	for fd, obj in list(map.items()): # list() call FBO py3
   	is_r = obj.readable()
   	is_w = obj.writable()
   	if is_r:
   	r.append(fd)
   	# accepting sockets should not be writable
   	if is_w and not obj.accepting:
   	w.append(fd)
   	if is_r or is_w:
   	e.append(fd)

2. Then, thread waitress-0 deletes one of the channels, in my case it was <waitress.channel.HTTPChannel 127.0.0.1:54044 at 0x7f10ec052400>, and immediately closes the socket:

   waitress/src/waitress/wasyncore.py

   Lines 460 to 470 in 603d2c1

   	def close(self):
   	self.connected = False
   	self.accepting = False
   	self.connecting = False
   	self.del_channel()
   	if self.socket is not None:
   	try:
   	self.socket.close()
   	except OSError as why:
   	if why.args[0] not in (ENOTCONN, EBADF):
   	raise

   Stack of waitiress-0 at the moment:

     File "/usr/lib/python3.8/threading.py", line 890, in _bootstrap
       self._bootstrap_inner()
     File "/usr/lib/python3.8/threading.py", line 932, in _bootstrap_inner
       self.run()
     File "/usr/lib/python3.8/threading.py", line 870, in run
       self._target(*self._args, **self._kwargs)
     File "/.../lib/python3.8/site-packages/waitress/task.py", line 84, in handler_thread
       task.service()
     File "/.../lib/python3.8/site-packages/waitress/channel.py", line 426, in service
       task.service()
     File "/.../lib/python3.8/site-packages/waitress/task.py", line 168, in service
       self.execute()
     File "/.../lib/python3.8/site-packages/waitress/task.py", line 451, in execute
       self.channel.write_soon(app_iter)
     File "/.../lib/python3.8/site-packages/waitress/channel.py", line 377, in write_soon
       (flushed, exception) = self._flush_exception(self._flush_some)
     File "/.../lib/python3.8/site-packages/waitress/channel.py", line 132, in _flush_exception
       return (flush(), False)
     File "/.../lib/python3.8/site-packages/waitress/channel.py", line 270, in _flush_some
       num_sent = self.send(chunk)
     File "/.../lib/python3.8/site-packages/waitress/wasyncore.py", line 479, in send
       self.handle_close()
     File "/.../lib/python3.8/site-packages/waitress/channel.py", line 317, in handle_close
       wasyncore.dispatcher.close(self)
     File "/.../lib/python3.8/site-packages/waitress/wasyncore.py", line 519, in close
       traceback.print_stack()

3. Then, thread Thread-1 is trying to see if the file descriptor of the socked closed above is writable, which leads to the the exception above:

   waitress/src/waitress/wasyncore.py

   Lines 171 to 177 in 603d2c1

   	try:
   	r, w, e = select.select(r, w, e, timeout)
   	except OSError as err:
   	if err.args[0] != EINTR:
   	raise
   	else:
   	return

Python 3.8.10, waitress 2.1.1, Ubuntu 20.04 LTS focal @ WSL2