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

@colesbury

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

pythongh-126688: Reinit import lock after fork

Merge branch 'main' into pythongh-126688-import-lock

gh-126688: Reinit import lock after fork (#126692)

pythongh-126688: Reinit import lock after fork (pythonGH-126692)

[3.13] gh-126688: Reinit import lock after fork (GH-126692) (GH-126765)

I think this can be closed now.

There may be other code that makes the same assumption about the thread ID. For example, in PyOS_AfterFork_Child():

cpython/Modules/posixmodule.c

Lines 655 to 658 in 09c240f

    
               assert(tstate->thread_id == PyThread_get_thread_ident()); 
        
           #ifdef PY_HAVE_THREAD_NATIVE_ID 
        
               tstate->native_thread_id = PyThread_get_thread_native_id(); 
        
           #endif

This would break only on debug builds (or at least !defined(NDEBUG)).

I'm guessing there are other places too.

pythongh-126688: Reinit import lock after fork (python#126692)

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CPython should not assume that pthread_self returns the same value in fork parent and child #126688

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	assert(tstate->thread_id == PyThread_get_thread_ident());
	#ifdef PY_HAVE_THREAD_NATIVE_ID
	tstate->native_thread_id = PyThread_get_thread_native_id();
	#endif

CPython should not assume that pthread_self returns the same value in fork parent and child #126688

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oskar-skog commented on Nov 12, 2024

ericsnowcurrently commented on Nov 21, 2024

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