Test hangs when using mp.Pool on Windows/macOS using CLI runner when --import-mode=importlib

This is a duplicate of #5429 (which does not appear to actually be resolved, or has regressed) but I come bearing more details, in particular a matrix that should help diagnose the source of the problem.

Here's a minimal example:

from multiprocessing import Pool

def times_two(x):
    return x * 2

def test_foo():
    with Pool(1) as pool:
        assert pool.map(times_two, [1, 2, 3, 4]) == [2, 4, 6, 8]

This is using pytest 7.3.1.

Tests were performed on Github Actions ubuntu-latest, windows-latest, and macos-latest, which correspond to Ubuntu 22.04, Windows Server 2022, and macOS 12 Monterey (not Ventura). I also validated the macOS and Linux results on macOS 13 Ventura and Ubuntu 20.04 on my own machines.

All of the below were tested with Python 3.11 on Github Actions runners. On my own machines, I tested with both 3.10 and 3.11, same results.

The last row in particular suggests that the issue is likely in the test runner. Running all tests via PyCharm.app/Contents/plugins/python/helpers/pycharm/_jb_pytest_runner.py does not hang.

– Dylan

Start by reproducing the minimal multiprocessing example with the CLI runner on Windows and macOS using Python 3.11 and --import-mode=importlib, then compare it with Linux and the PyCharm runner. Investigate the test-runner path involved in collection or process startup; done means the example completes consistently on the affected platforms without regressing the Linux behavior.