nested tasks causing deadlock?

[Androidown]

question

The following code will lead to a infinite polling in both windows and linux. I'm quite new to nested asyncio so I want to know if it is a bug or not.

code

import asyncio
import nest_asyncio

nest_asyncio.apply()

async def nested(t):
    asyncio.run(t)
    print('finished')

async def main():
    t = asyncio.create_task(nested(asyncio.sleep(1)))
    asyncio.run(nested(t))   # deadlock?
    # await nested(t)   # finished normally

asyncio.run(main())

enviroment

cpython 3.8.10 nest_asyncio==1.5.4

stack

(gdb) py-bt-full
#6 Frame 0x1987b90, for file /usr/lib/python3.8/selectors.py, line 468, in select (self=<EpollSelector(_fd_to_key={7: <SelectorKey at remote 0x7f635d6b7a40>}, _map=<_SelectorMapping(_selector=<...>) at remote 0x7f635d6a8df0>, _selector=<select.epoll at remote 0x7f635d6b4150>) at remote 0x7f635d6a8d90>, timeout=-1, max_ev=1, ready=[])
    fd_event_list = self._selector.poll(timeout, max_ev)
#12 Frame 0x7f635d6c4040, for file /usr/local/lib/python3.8/dist-packages/nest_asyncio.py, line 1122, in _run_once (self=<_UnixSelectorEventLoop(_timer_cancelled_count=1, _closed=False, _stopping=False, _ready=<collections.deque at remote 0x7f635d6aabe0>, _scheduled=[], _default_executor=None, _internal_fds=1, _thread_id=140064757851968, _clock_resolution=<float at remote 0x7f635d7ddd10>, _exception_handler=None, _debug=False, slow_callback_duration=<float at remote 0x7f635ddafed0>, _current_handle=None, _task_factory=None, _coroutine_origin_tracking_enabled=False, _coroutine_origin_tracking_saved_depth=None, _asyncgens=<WeakSet(data=set(), _remove=<function at remote 0x7f635d6b9a60>, _pending_removals=[], _iterating=set()) at remote 0x7f635d6a8d30>, _asyncgens_shutdown_called=False, _selector=<EpollSelector(_fd_to_key={7: <SelectorKey at remote 0x7f635d6b7a40>}, _map=<_SelectorMapping(_selector=<...>) at remote 0x7f635d6a8df0>, _selector=<select.epoll at remote 0x7f635d6b4150>) at remote 0x7f635d6a8d90>, _ssock=...(truncated)
#18 Frame 0x7f635d6c1be0, for file /usr/local/lib/python3.8/dist-packages/nest_asyncio.py, line 75, in run_until_complete (self=<_UnixSelectorEventLoop(_timer_cancelled_count=1, _closed=False, _stopping=False, _ready=<collections.deque at remote 0x7f635d6aabe0>, _scheduled=[], _default_executor=None, _internal_fds=1, _thread_id=140064757851968, _clock_resolution=<float at remote 0x7f635d7ddd10>, _exception_handler=None, _debug=False, slow_callback_duration=<float at remote 0x7f635ddafed0>, _current_handle=None, _task_factory=None, _coroutine_origin_tracking_enabled=False, _coroutine_origin_tracking_saved_depth=None, _asyncgens=<WeakSet(data=set(), _remove=<function at remote 0x7f635d6b9a60>, _pending_removals=[], _iterating=set()) at remote 0x7f635d6a8d30>, _asyncgens_shutdown_called=False, _selector=<EpollSelector(_fd_to_key={7: <SelectorKey at remote 0x7f635d6b7a40>}, _map=<_SelectorMapping(_selector=<...>) at remote 0x7f635d6a8df0>, _selector=<select.epoll at remote 0x7f635d6b4150>) at remote 0x7f635d6a8d90>, ...(truncated)
    self._run_once()
#24 Frame 0x19c9e50, for file /usr/local/lib/python3.8/dist-packages/nest_asyncio.py, line 38, in run (main=<Task(_loop=<_UnixSelectorEventLoop(_timer_cancelled_count=1, _closed=False, _stopping=False, _ready=<collections.deque at remote 0x7f635d6aabe0>, _scheduled=[], _default_executor=None, _internal_fds=1, _thread_id=140064757851968, _clock_resolution=<float at remote 0x7f635d7ddd10>, _exception_handler=None, _debug=False, slow_callback_duration=<float at remote 0x7f635ddafed0>, _current_handle=None, _task_factory=None, _coroutine_origin_tracking_enabled=False, _coroutine_origin_tracking_saved_depth=None, _asyncgens=<WeakSet(data=set(), _remove=<function at remote 0x7f635d6b9a60>, _pending_removals=[], _iterating=set()) at remote 0x7f635d6a8d30>, _asyncgens_shutdown_called=False, _selector=<EpollSelector(_fd_to_key={7: <SelectorKey at remote 0x7f635d6b7a40>}, _map=<_SelectorMapping(_selector=<...>) at remote 0x7f635d6a8df0>, _selector=<select.epoll at remote 0x7f635d6b4150>) at remote 0x7f635d6a8d90>, _ssock=<...(truncated)
    return loop.run_until_complete(task)

[erdewit]

To call async code from async code you can just use await, this project is for calling async code from sync code.

Btw it looks like in the snippet there's a circular dependency, with two tasks waiting on the other to finish.