LIST_APPEND and SET_ADD opcodes safety in Free Threading

Hi,

While reviewing PR gh-152273, I moved _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(so); check to set_add_entry_takeref() to check for bugs. I discovered that SET_ADD opcode calls _PySet_AddTakeRef() without getting the set critical section. That's surprising and IMO it would deserve adding a comment to explain why it's safe to omit locking here.

LIST_APPEND has a similar design (also omit locking).

The following code uses the evil gc.get_objects() function to call set.add() in a different thread while the main thread is building a set:

import dis
import gc
import threading

GLOBAL_SET = None

def worker():
    EVENT.wait()
    obj = GLOBAL_SET
    for i in range(100):
        # Call set.add() which uses the critical section
        obj.add(i)


MARKER = b"MARKER".decode()
EVENT = threading.Event()

class EvilHash:
    def __init__(self, hash_value):
        self.hash_value = hash_value

    def __hash__(self):
        global GLOBAL_SET, EVENT
        if GLOBAL_SET is None:
            # Invoke the evil gc.get_objects()!
            for obj in gc.get_objects():
                if isinstance(obj, set) and MARKER in obj:
                    GLOBAL_SET = obj
                    EVENT.set()
        return self.hash_value

    def __repr__(self):
        return f"EvilHash({self.hash_value})"

def func():
    build_set = {
        # Use *list so following items are added by SET_ADD opcode
        *[MARKER],
        # Added by SET_ADD which calls _PySet_AddTakeRef()
        # without the critical section
        EvilHash(0), EvilHash(1), EvilHash(2), EvilHash(3), EvilHash(4),
        EvilHash(5)}

    print(build_set)
    print("Length:", len(build_set))
    if len(build_set) != 107:
        raise Exception("race condition!")

thread = threading.Thread(target=worker)
thread.start()
func()
thread.join()
#dis.dis(func)

I expected the code to fail randomly, but so far I failed to trigger a race condition on Free Threading. Running the code on Python built with --with-thread-sanitizer doesn't show any warning.

Commencez par suivre SET_ADD et LIST_APPEND à travers _PySet_AddTakeRef() et set_add_entry_takeref(), en utilisant le reproducer avec threads fourni comme vérification du comportement. Exécutez-le sur un free-threaded build, y compris le thread-sanitized build, et déterminez si l’omission du critical section est sûre. Le travail est terminé lorsque la justification de la sécurité est établie et que toute explication ou correction nécessaire est couverte.