Asyncio.Event.wait doesn't resume execution when the event is set if no other tasks are active.

[agustinhuman]

Bug report

Bug description:

This is a toy program that sets an asyncio.Event when a signal is received, and runs an asynchronous function (passive_wait) that waits for that signal to be set.

When the following code is executed and a signal is sent externally, it is detected by the program, but the function waiting function does not resume execution.

It does finish correctly if another async function is executed alongside, like when uncommenting the line active_wait(),.

import asyncio
import signal

event = asyncio.Event()

def _signal_handler(*_) -> None:
    event.set()
    print("Event triggered")

signal.signal(signal.SIGINT, _signal_handler)
signal.signal(signal.SIGTERM, _signal_handler)

async def passive_wait():
    print("Pre event")
    await event.wait()
    print("Post event")

async def active_wait():
    while not event.is_set():
        await asyncio.sleep(1)

async def main():
    await asyncio.gather(
        passive_wait(),
        # Uncommenting the following line makes it stop correctly
        #active_wait(),
    )

asyncio.run(main())
# Send the signal externally

If that's the intended behavior I would love some pointers.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

[gvanrossum]

Try wrapping the event.set() call in call_threadsafe()

[agustinhuman]

Using loop.call_soon_threadsafe to set the event does indeed make it work.

Checking the current thread within the coro and the handler shows both on _MainThread. I'm a bit confuse on why was that the problem.

[graingert]

Yes this is intentional behavior. I wonder if python could set a flag when executing from a reentrant context like a signal handler or finalizer so that Event.set could crash in this case

[willingc]

Removing the bug label based on @graingert's explanation.