CancelledError in uasyncio/stream.py in _serve when calling server.wait_closed()

[Beormund]

I migrated to MicroPython v1.20.0 on 2023-04-26; Raspberry Pi Pico W with RP2040.

I'm getting an unhandled CancelledError when calling server.close followed by await server.wait_closed on the server object returned from await asyncio.start_server.

File "uasyncio/stream.py", line 1, in wait_closed
File "uasyncio/core.py", line 1, in run_until_complete
File "uasyncio/stream.py", line 1, in _serve

I've set an exception handler on the loop which doesn't catch this CancelledError. Once this error is thrown all sockets created outside of start_server also become unresponsive, even if I wrap wait_closed in try/except.

[Josverl]

Did the same code work on an earlier build?
Can you share (a link to) the relevant section of the code, without that it's really hard to help

[Beormund]

I will put a small test demo together to demonstrate.

[peterhinch]

Looking at the uasyncio code this is expected behaviour.

    def close(self):
        self.task.cancel()

    async def wait_closed(self):
        await self.task

I don't think there's any need to issue wait_closed(). Task cancellation occurs the next time the task is scheduled, so in the following sequence shoud suffice:

async def foo():
    server.close()
    await asyncio.sleep(0)
    # It's closed now

[Beormund]

Not sure why you get a CancelledError being thrown in stream.py _serve when the CancelledError is supposed to be caught:

import uasyncio as asyncio
  
async def handle(r,w):
    pass
    
async def main():
    loop = asyncio.new_event_loop()
    server = await asyncio.start_server(handle, '0.0.0.0', 80)
    server.close()
    await server.wait_closed()
    loop.run_forever()
    
asyncio.run(main())

'''
Traceback (most recent call last):
  File "<stdin>", line 13, in <module>
  File "uasyncio/core.py", line 1, in run
  File "uasyncio/core.py", line 1, in run_until_complete
  File "uasyncio/core.py", line 1, in run_until_complete
  File "<stdin>", line 10, in main
  File "uasyncio/stream.py", line 1, in wait_closed
  File "uasyncio/core.py", line 1, in run_until_complete
  File "uasyncio/stream.py", line 1, in _serve
CancelledError: '''

micropython/extmod/uasyncio/stream.py

Lines 135 to 151 in 786013d

	async def _serve(self, s, cb):
	# Accept incoming connections
	while True:
	try:
	yield core._io_queue.queue_read(s)
	except core.CancelledError:
	# Shutdown server
	s.close()
	return
	try:
	s2, addr = s.accept()
	except:
	# Ignore a failed accept
	continue
	s2.setblocking(False)
	s2s = Stream(s2, {"peername": addr})
	core.create_task(cb(s2s, s2s))

[peterhinch]

This code looks wrong to me. The commented lines should be deleted.

import uasyncio as asyncio
  
async def handle(r,w):
    pass
    
async def main():
    # loop = asyncio.new_event_loop()  # Delete
    server = await asyncio.start_server(handle, '0.0.0.0', 80)
    server.close()
    await server.wait_closed()
    # loop.run_forever()  # Delete
    
asyncio.run(main())

The event loop stuff largely dates from old versions of Python's asyncio and I'm not sure of the implications of using them here