[BUG]: Data race when using static variables with free-threaded Python

[rostan-t]

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

v2.13.6

Problem description

With free-threaded Python, I get data races when using static class variables defined like this:

#include <pybind11/pybind11.h>

namespace py = pybind11;

class Foo {
};

Foo Bar;

PYBIND11_MODULE(foo, m, py::mod_gil_not_used()) {
    py::class_<Foo>(m, "Foo")
        .def_readonly_static("Bar", &Bar);
}

I get multiple kinds of errors when accessing Foo.Bar from multiple threads, from segmentation faults to "pybind11_object_dealloc(): Tried to deallocate unregistered instance!" exceptions.

Further investigation suggested that it might come from Foo.Bar being freed too many times. I'm unsure if it's a pybind11 or CPython issue so I'm posting it here.

Reproducible example code

import threading

from foo import Foo


def create_foo():
    for _ in range(1000):
        Foo.Bar


# keeping a reference to Foo.Bar prevents the issue from happening
# bar = Foo.Bar

nb_threads = 10
threads = [threading.Thread(target=create_foo) for _ in range(nb_threads)]

for thread in threads:
    thread.start()

for thread in threads:
    thread.join()

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

Not a regression