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.

Inizia tracciando SET_ADD e LIST_APPEND attraverso _PySet_AddTakeRef() e set_add_entry_takeref(), usando il reproducer con thread fornito come verifica del comportamento. Eseguilo su un free-threaded build, incluso il thread-sanitized build, e determina se omettere il critical section è sicuro. Il lavoro è completato quando è stata stabilita la motivazione della sicurezza e sono state coperte tutte le spiegazioni o correzioni necessarie.