Suppose that program:

====================================

import asyncio
import socket
def receiver(loop):
    (a, b) = socket.socketpair()
    loop.call_later(1, lambda: print('Should be called inside the loop'))
    end = loop.time() + 3
    print('Starting busy receiver')
    while loop.time() < end:
        a.send(b'test')
        yield from loop.sock_recv(b, 65536)
        # yield from asyncio.sleep(0) # <=====================
    print('Busy receiver complete')
    # just not to stop ioloop immediatelly
    yield from asyncio.sleep(0.5)
def main():
    loop = asyncio.get_event_loop()
    loop.run_until_complete(receiver(loop))
    loop.close()
if __name__ == '__main__':
    main()

====================================

Without asyncio.sleep(0) it will not fire time-delayed calls! If I add asyncio.sleep(0), everything work right. As I think, It is because recv() syscall is always succeeded, and we never return to ioloop (to epoll() I mean).

In other words, it is classical reader starvation as mentioned in "man epoll".

It is not documented, that this function may block event loop, in spite of it returns coroutine! I thought that this function will setup EPOLLIN event for socket's FD + call recv() after that. I spent many time to debug program.

You should develop using asyncio debug mode:

https://docs.python.org/dev/library/asyncio-dev.html#asyncio-dev

haypo@selma$ PYTHONASYNCIODEBUG=1 ./python x.py 
Starting busy receiver
Traceback (most recent call last):
  File "x.py", line 21, in <module>
    main()
  File "x.py", line 18, in main
    loop.run_until_complete(receiver(loop))
  File "/home/haypo/prog/python/default/Lib/asyncio/base_events.py", line 341, in run_until_complete
    return future.result()
  File "/home/haypo/prog/python/default/Lib/asyncio/futures.py", line 276, in result
    raise self._exception
  File "/home/haypo/prog/python/default/Lib/asyncio/tasks.py", line 238, in _step
    result = coro.send(value)
  File "x.py", line 11, in receiver
    yield from loop.sock_recv(b, 65536)
  File "/home/haypo/prog/python/default/Lib/asyncio/selector_events.py", line 316, in sock_recv
    raise ValueError("the socket must be non-blocking")
ValueError: the socket must be non-blocking
/home/haypo/prog/python/default/Lib/asyncio/base_events.py:384: ResourceWarning: unclosed event loop <_UnixSelectorEventLoop running=False closed=False debug=True>
sys:1: ResourceWarning: unclosed <socket object at 0x7f0b03d7d688>
sys:1: ResourceWarning: unclosed <socket object at 0x7f0b03d7d5f8>

You should develop using asyncio debug mode:

Maybe we should promote this check to the production mode?

Adding of b.setblocking(0) after socketpair() does not help.

Le lundi 29 juin 2015, Yury Selivanov <report@bugs.python.org> a écrit :

> You should develop using asyncio debug mode:

Maybe we should promote this check to the production mode?

asyncio must be slow. The check has a cost, I prefer to keep it only in
debug mode. It's probably more a doc issue. We should maybe repeat the info
at the beginning of the asyncio doc.

The issue is in your code as it is trying to use blocking sockets in asyncio.