Spawn worker threads with daemon=True on asyncio

[davidbrochart]

This was added in 635e25a:

anyio/anyio/_backends/_asyncio.py

Line 413 in 635e25a

thread = Thread(target=thread_worker, daemon=True)

And removed in #227:

anyio/src/anyio/_backends/_asyncio.py

Line 717 in 77759c8

thread = Thread(target=_thread_pool_worker, args=args, name='AnyIO worker thread')

Fixes jupyterlab/jupyterlab#10442

[graingert]

daemon=True isn't well supported with subinterpreters and skips cleanup. Perhaps this is something that trio should support too: trio.to_thread.run_sync(fn, daemon=True|False)

[agronholm]

I removed daemon=True once I learned the above. Why does JupyterLab hang on server shutdown? Does it not clean up the tasks it creates?

[agronholm]

There is also the matter of issue #323 which I just fixed (and which will require a prompt bugfix release). Could it be related?

[graingert]

python-trio/trio#2046

[davidbrochart]

There is also the matter of issue #323 which I just fixed (and which will require a prompt bugfix release). Could it be related?

It doesn't seem to be related, 49f666e doesn't fix the JupyterLab hang.

Why does JupyterLab hang on server shutdown? Does it not clean up the tasks it creates?

The issue is actually in Jupyter Server, which uses Tornado. Shutting down simply stops Tornado's event loop, and waits for each task to finish. But I think it is because of a task in a worker thread which is not thread-safe. I opened jupyter-server/jupyter_server#545 which should fix the issue.

I thought daemon=True was removed by mistake, but anyway this doesn't seem to be the root cause of our issue, so it is fine to close this PR. Thanks for your help!