gh-85299: Add note warning about entry point guard for asyncio example

[slateny]

#85299

https://docs.python.org/3/library/asyncio-eventloop.html#asyncio.loop.run_in_executor

[bedevere-bot]

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[gvanrossum]

Okay, I am beginning to understand the issue. It's due to the process pool example. If I comment out this block of code from the example:

    # 3. Run in a custom process pool:
    with concurrent.futures.ProcessPoolExecutor() as pool:
        result = await loop.run_in_executor(
            pool, cpu_bound)
        print('custom process pool', result)

everything is fine. If I leave that in the run (on macOS) fails spectacularly with a RuntimeError that has many lines of traceback, then a multi-line error message, and then more traceback lines. Adding the __main__ guard makes this go away.

I presume that's one of the many reasons not to use ProcessPoolExecutor. :-(

I propose to rephrase the note to mention the platform and just link to https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods, not to the change note.

[miss-islington]

Thanks @slateny for the PR, and @gvanrossum for merging it 🌮🎉.. I'm working now to backport this PR to: 3.10. 🐍🍒⛏🤖

[miss-islington]

Thanks @slateny for the PR, and @gvanrossum for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11. 🐍🍒⛏🤖

[bedevere-bot]

GH-98334 is a backport of this pull request to the 3.11 branch.

[gvanrossum]

Let’s see if we can backport this.

[bedevere-bot]

GH-98335 is a backport of this pull request to the 3.10 branch.