WindowsSelectorEventLoopPolicy doesn't raise KeyboardInterrupt immediately

[davetapley]

Bug report

Run the snippet below and Ctrl+C,
observe interrupted will only be printed after ten seconds.

Comment out set_event_loop_policy and repeat,
observe interrupted is printed immediately.

import asyncio
import sys

if sys.platform == 'win32':
   asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())

async def sleep():
    print('sleeping')
    await asyncio.sleep(10)
    print('done')

try:
    asyncio.run(sleep())
except KeyboardInterrupt:
    print('interrupted')

---

For context: I need WindowsSelectorEventLoopPolicy to mitigate this error with pyzmq:

RuntimeError: Proactor event loop does not implement add_reader family of methods required for zmq. zmq will work 
with proactor if tornado >= 6.1 can be found. Use `asyncio.set_event_loop_policy(WindowsSelectorEventLoopPolicy())` or install 'tornado>=6.1' to avoid this error.

Your environment

• CPython versions tested on: 3.9.10
• Operating system and architecture: Windows 10, 64-bit

[gvanrossum]

Can you repro this in 3.11? Or in main? And why can't you use the other mitigation (tornado >= 6.1)?

[eryksun]

The main thread is blocked in a Winsock select() call. Since the interpreter calls signal handlers on the main thread, the runner's SIGINT handler (i.e. asyncio.runners.Runner._on_sigint()) won't be called until the interpreter resumes executing on the main thread.

You can complete the select() call using the event loop's _write_to_self() method, which is allowed to be called from another thread. A native Windows console control handler is always called on a new thread. For example:

import asyncio
import sys

if sys.platform == 'win32':
    import ctypes
    import signal

    asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
    runner = None

    @ctypes.WINFUNCTYPE(ctypes.c_int, ctypes.c_ulong) 
    def ctrl_handler(event):
        if event == signal.CTRL_C_EVENT:
            if runner is not None:
                loop = runner.get_loop()
                if loop is not None: 
                    loop._write_to_self()
        # Chain to the next registered control handler, which is probably 
        # the C runtime library's SIGINT/SIGBREAK implementation.
        return False

    kernel32 = ctypes.WinDLL('kernel32', use_last_error=True)
    kernel32.SetConsoleCtrlHandler(ctrl_handler, True)

async def sleep():
    print('sleeping')
    await asyncio.sleep(10)
    print('done')

try:
    with asyncio.Runner() as runner:
        runner.run(sleep())
except KeyboardInterrupt:
    print('interrupted')

[kumaraditya303]

Looks like a duplicate of #77373 but I haven't verified it.