trio_asyncio shielding cancellation causes errors to be silenced.

[elektito]

I was having a hard time figuring out why my program suddenly stopped working without any errors and without keyboard interrupts working. After a lot of digging, I found that there was a simple AttributeError but it wasn't being raised. And it only happened with trio-asyncio running. I managed to come up with this minimal example:

import trio
import trio_asyncio

async def foo():
    async with trio_asyncio.open_loop():
        await trio.sleep(0)

async def main():
    async with trio.open_nursery() as nursery:
        nursery.start_soon(foo)
        raise RuntimeError('error!')

if __name__ == '__main__':
    trio.run(main)

Digging further, I found this bit in the _base.py module is to blame:

with trio.CancelScope(shield=True):
    while not self._stopped.is_set():
        await self._main_loop_one()

I'm not familiar with trio_asyncio internals, so I don't know why the shield=True is necessary, but it can clearly cause issues like the one I was experiencing.