os.fork() in a multi-threaded application is a likely source of deadlocks on many platforms. We should raise a warning when people call os.fork() from a process that we know has other threads running.

This came from discussion in https://discuss.python.org/t/switching-default-multiprocessing-context-to-spawn-on-posix-as-well/21868/, though I believe many of us have pondered doing it in the past.

Linked PRs

• gh-100228: Warn from os.fork() if other threads exist. #100229
• gh-100228: Document the os.fork threads DeprecationWarning. #109767
• [3.12] gh-100228: Document the os.fork threads DeprecationWarning. (GH-109767) #109773