[BUG]: nullptr dereference in create_pp_content_once (internals.h)

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

What version (or hash if on master) of pybind11 are you using?

3.0.4

Problem description

The crash occurs on Windows when importing my pybind11 lib after pyarrow 18.1.0 is already imported. The conditions for it to crash are weirdly specific:

• It disappears if my library is compiled in Debug
• It disappears if i upgrade pyarrow to 19.0.0
• It disappears if I upgrade to python 3.14
  The nullptr dererefence happens in the function create_pp_content_once when locking pp_set_mutex_ even though the mutex seemed initialized (I cannot be sure however because I was looking at an optimized build).

I reproduced the crash with an empty module compiled with pybind11.

I am building with Visual Studio 2022.

Reproducible example code

#include <pybind11/pybind11.h>

PYBIND11_MODULE(foo, m){}

import pyarrow
import foo

Is this a regression? Put the last known working version here if it is.

3.0.1

Start with the Windows release-build reproduction using pyarrow 18.1.0, Python 3.14 or earlier, and the minimal foo module. Inspect create_pp_content_once and pp_set_mutex_ in internals.h, then compare behavior across pybind11 3.0.1 and 3.0.4. Done means importing pyarrow followed by the extension no longer dereferences nullptr.