CPython should not assume that pthread_self returns the same value in fork parent and child

[nico]

Summarized from SerenityOS/serenity#25263 by request of @colesbury:

It seems that at least Python 3.13 relies on pthread_self returning the same value in both the parent and the child process.
PyThread_get_thread_ident_ex

POSIX doesn't seem to mention if the value returned by pthread_self should be inherited by the child process.
pthread_self
fork

(It is the case on Linux, Hurd, FreeBSD, OpenBSD, Cygwin. It isn't on SerenityOS, and also not on Solaris 9/HP-UX 11 per https://bugs.python.org/issue7242.

It looks like the code add broke things is from this commit e21057b added in #118523 as part of #117657

Linked PRs

• gh-126688: Reinit import lock after fork #126692
• [3.13] gh-126688: Reinit import lock after fork (GH-126692) #126765
• gh-126688: Support Fork Parent and Child With Different Thread ID #127121

[oskar-skog]

I think this can be closed now.