gh-152235: Defer GC tracking during the lock-free fill in set.__init__

[corona10]

• Issue: Need to defer GC tracking of frozenset/set construction #152235

[corona10]

@vstinner @methane

Final PR of the set: list already has TSan-safe initialization, and this brings set.init up to the same level of protection.

cpython/Objects/listobject.c

Lines 3523 to 3544 in fe9c721

	list___init___impl(PyListObject *self, PyObject *iterable)
	/*[clinic end generated code: output=0f3c21379d01de48 input=b3f3fe7206af8f6b]*/
	{
	/* Verify list invariants established by PyType_GenericAlloc() */
	assert(0 <= Py_SIZE(self));
	assert(Py_SIZE(self) <= self->allocated || self->allocated == -1);
	assert(self->ob_item != NULL ||
	self->allocated == 0 || self->allocated == -1);
	/* Empty previous contents */
	if (self->ob_item != NULL) {
	Py_BEGIN_CRITICAL_SECTION(self);
	list_clear(self);
	Py_END_CRITICAL_SECTION();
	}
	if (iterable != NULL) {
	if (_list_extend(self, iterable) < 0) {
	return -1;
	}
	}
	return 0;
	}

cpython/Objects/listobject.c

Lines 1457 to 1511 in fe9c721

	_list_extend(PyListObject *self, PyObject *iterable)
	{
	// Special case:
	// lists and tuples which can use PySequence_Fast ops
	int res = -1;
	if ((PyObject *)self == iterable) {
	Py_BEGIN_CRITICAL_SECTION(self);
	res = list_inplace_repeat_lock_held(self, 2);
	Py_END_CRITICAL_SECTION();
	}
	else if (PyList_CheckExact(iterable)) {
	Py_BEGIN_CRITICAL_SECTION2(self, iterable);
	res = list_extend_lock_held(self, iterable);
	Py_END_CRITICAL_SECTION2();
	}
	else if (PyTuple_CheckExact(iterable)) {
	Py_BEGIN_CRITICAL_SECTION(self);
	res = list_extend_lock_held(self, iterable);
	Py_END_CRITICAL_SECTION();
	}
	else if (PyAnySet_CheckExact(iterable)) {
	Py_BEGIN_CRITICAL_SECTION2(self, iterable);
	res = list_extend_set(self, (PySetObject *)iterable);
	Py_END_CRITICAL_SECTION2();
	}
	else if (PyDict_CheckExact(iterable)) {
	Py_BEGIN_CRITICAL_SECTION2(self, iterable);
	res = list_extend_dict(self, (PyDictObject *)iterable, 0 /*keys*/);
	Py_END_CRITICAL_SECTION2();
	}
	else if (Py_IS_TYPE(iterable, &PyDictKeys_Type)) {
	PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
	Py_BEGIN_CRITICAL_SECTION2(self, dict);
	res = list_extend_dict(self, dict, 0 /*keys*/);
	Py_END_CRITICAL_SECTION2();
	}
	else if (Py_IS_TYPE(iterable, &PyDictValues_Type)) {
	PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
	Py_BEGIN_CRITICAL_SECTION2(self, dict);
	res = list_extend_dict(self, dict, 1 /*values*/);
	Py_END_CRITICAL_SECTION2();
	}
	else if (Py_IS_TYPE(iterable, &PyDictItems_Type)) {
	PyDictObject *dict = ((_PyDictViewObject *)iterable)->dv_dict;
	Py_BEGIN_CRITICAL_SECTION2(self, dict);
	res = list_extend_dictitems(self, dict);
	Py_END_CRITICAL_SECTION2();
	}
	else {
	Py_BEGIN_CRITICAL_SECTION(self);
	res = list_extend_iter_lock_held(self, iterable);
	Py_END_CRITICAL_SECTION();
	}
	return res;
	}

[vstinner]

The issue title is "Need to defer GC tracking of frozenset/set construction", but in fact, the problem only arises for frozenset.

PyFrozenSet_Type defines tp_new (frozenset_new) but has no tp_init function.

PySet_Type defines tp_new (set_new) and tp_init (set_init).

So changing set_init() only impacts the mutable set. I don't see which problem you are trying to solve. It's fine to mutate a set object.