Some multiprocessing APIs should be possible to import or use even when multiprocessing itself isn't available

[mhsmith]

Bug report

Bug description:

Android doesn't support multiprocessing because it doesn't support the System V IPC API, and iOS doesn't support subprocesses at all. However, there are a couple of things that should still work on these platforms, but currently don't:

• multiprocessing.dummy.Pool (aka multiprocessing.pool.ThreadPool) currently depends on the multiprocessing synchronization primitives, when it could use the threading equivalents instead.

• multiprocessing.synchronize throws an exception on import if the synchronization primitives are unavailable. But this breaks some packages like joblib, which imports the primitives but doesn't always use them (joblib/joblib#825).

Chaquopy has some monkey patches to work around these issues, but it would be better if they were fixed upstream.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

[terryjreedy]

(EDIT) I have no idea is these changed would be acceptible. In any case, we cannot/will-not grab other people's code. Someone with 'ownership' of chaquopy would have to sign our Contributor Licensing Agreement and submit a PR with their code. Or someone else (you?) would have to write new code that does not violate their copyright.

[freakboy3742]

@terryjreedy FTR: @mhsmith is the founder and principal author of Chaquopy.

[mhsmith]

Yes, I've already signed the CLA, and, if the fact that it's MIT licensed isn't already sufficient, I hereby give permission for anyone to contribute Chaquopy code to CPython.

However, a proper upstream solution will probably look a bit different to the monkey patches. I only linked to them to demonstrate that this could be fixed with a relatively small change.