gh-145272: Fix data race when accessing func_code in PyFunctionObject

[Krishna-web-hub]

• Fix a data race when accessing PyFunctionObject.func_code in free-threaded builds.

• This patch replaces the direct read with _Py_atomic_load_ptr() and updates the setter to use _Py_atomic_exchange_ptr() so that updates to func_code are performed atomically.

• The race was reproduced using ThreadSanitizer and verified to be fixed after this change.

• Issue: Data race in func_set_code #145272

[vstinner]

Is this issue specific to func_code? What about other PyFunction methods such as PyFunction_GET_GLOBALS() or PyFunction_GET_MODULE()?

[Krishna-web-hub]

@vstinner actually this issue was mentioned only to fixation of the func_code. But the func_code have high chance of being reassigned at runtime. But others like func_globals and func_module are set at the time of the function creation. But there are few objects like func_annotations, func_defaults they can cause the race situation but i think applying the atomic pointer fix can slow down the cpython. Based on this i want to know your opinion on what i should be doing next ?