run_task assert fails when other thread appends Handle to _ready

[bh-tneukom]

Hi!

Thanks for the great library! We might have found a small issue that causes the assert

qtinter/src/qtinter/_tasks.py

Lines 24 to 31 in b30b06b

	ntodo = len(loop._ready)
	task = asyncio.create_task(coro, **kwargs)
	# if task._source_traceback:
	# del task._source_traceback[-1]
	assert len(loop._ready) == ntodo + 1
	handle = loop._ready.pop()

to trigger when running functions in a thread using run_in_executor. With some bad luck the thread finishes exactly when the task is created and a new handle is added causing len(loop._ready) == ntodo + 2.

Uncommenting the monkey patch of the _ready deque shows what's happening:

import asyncio
import sys
import threading

import qtinter
from PySide6 import QtWidgets


class CrashWidget(QtWidgets.QWidget):
    def __init__(self):
        super().__init__()
        self.setLayout(QtWidgets.QVBoxLayout())

        btn = QtWidgets.QPushButton('Crash!')
        btn.clicked.connect(qtinter.asyncslot(self._run_in_executor))
        btn.clicked.connect(qtinter.asyncslot(self._set_event))
        btn.clicked.connect(qtinter.asyncslot(self._print_hello_world))
        self.layout().addWidget(btn)

    def _wait_until_event(self):
        self._event.wait()
        print('Joining')

    async def _run_in_executor(self):
        self._event = threading.Event()
        self._executor_task = asyncio.get_event_loop().run_in_executor(None, self._wait_until_event)
        await self._executor_task

    async def _set_event(self):
        self._event.set()

    async def _print_hello_world(self):
        print('Hello World')


if __name__ == '__main__':
    print(f'Main thread id: {threading.get_ident()}')
    app = QtWidgets.QApplication(sys.argv)

    with qtinter.using_asyncio_from_qt():
        # Useful for debugging
        # import collections
        # class MonkeyDeque(collections.deque):
        #     def append(self, __x):
        #         print(f'Adding {__x} to deque in thread {threading.get_ident()}')
        #         super().append(__x)
        # loop = asyncio.get_event_loop()
        # loop._ready = MonkeyDeque(loop._ready)

        widget = CrashWidget()
        widget.show()

        app.exec()

Log

Main thread id: 26280
...
Adding <Handle _chain_future.<locals>._set_state(<Future pendi...ask_wakeup()]>, <Future at 0x...rned NoneType>) at C:\dev\envs\py311\Lib\asyncio\futures.py:381> to deque in thread 17728
...
Traceback (most recent call last):
  File "C:\dev\projects\the_project\venv\Lib\site-packages\qtinter\_helpers.py", line 120, in handle
    transform(method, args, *extra)
  File "C:\dev\projects\the_project\venv\Lib\site-packages\qtinter\_slots.py", line 29, in _run_coroutine_function
    task = task_runner(coro)  # TODO: set name and context
           ^^^^^^^^^^^^^^^^^
  File "C:\dev\projects\the_project\venv\Lib\site-packages\qtinter\_tasks.py", line 30, in run_task
    assert len(loop._ready) == ntodo + 1
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

The _chain_future Handle is added not from the main thread. The assert fails immediately after that.

It takes a couple of tries to trigger the assert.

Best
Tobias