Test case for cffi ffi.gc() callback-upon-deletion subinterpreter issue

build_cffi.py

from cffi import FFI
ffibuilder = FFI()

ffibuilder.cdef(
"""
    typedef struct c_class c_class;
    typedef void destroy_callback_function();

    extern c_class* c_class_create(destroy_callback_function *callback);
    extern void c_class_destroy(c_class *obj);

    extern "Python" void _py_destroy_callback_function_impl();
""")

ffibuilder.set_source('_c_code',  # name of the output C extension
"""
    #include "c_code.h"
""",
    sources=['c_code.c'])

if __name__ == '__main__':
    ffibuilder.compile(verbose=True)

c_code.c

#include <stdlib.h>

#include "c_code.h"


struct c_class {
    destroy_callback_function *destroy_callback;
};


extern void c_class_destroy(c_class *obj)
{
    if (obj->destroy_callback) {
        obj->destroy_callback();
    }

    free(obj);
}

extern c_class* c_class_create(destroy_callback_function *callback)
{
    c_class *obj = malloc(sizeof (c_class));
    *obj = (c_class) {
        .destroy_callback = callback
    };
    return obj;
}

c_code.h

#ifndef C_CODE_H
#define C_CODE_H

typedef struct c_class c_class;
typedef void destroy_callback_function();

extern c_class* c_class_create(destroy_callback_function *callback);
extern void c_class_destroy(c_class *obj);

#endif // C_CODE_H

test.py

import wrapper

# Removing this function causes the callback to execute successfully, for some reason?
def unused_function():
    print('(Nothing calls or references this function, so this print call will never execute)')

obj = wrapper.make_class()

print('End of test.py')

# (obj goes out of scope here)

wrapper.py

from _c_code import lib, ffi


@ffi.def_extern()
def _py_destroy_callback_function_impl():
    print('Destroy callback')


def make_class():
    return ffi.gc(
        lib.c_class_create(lib._py_destroy_callback_function_impl),
        lib.c_class_destroy,
    )