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

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

Linked PRs

• gh-148050

先檢視 multiprocessing.dummy.Pool 和 multiprocessing.pool.ThreadPool 進入點，接著檢查 multiprocessing.synchronize，以及它在沒有同步原語的平台上於匯入時失敗的情況。將所需行為與 threading 的對應功能進行比較，並檢視連結的 PR gh-148050；當受支援的 API 不需使用不可用的 multiprocessing 功能即可匯入或使用時，即表示完成。