Avoid boilerplate for traversing a mutable heap type

[ZeroIntensity]

Throughout the codebase, instances of mutable heap types must always implement the GC protocol to traverse their type. This leads to the following being repeated a lot:

static int
object_visit(PyObject *op, visitproc visit, void *arg)
{
    Py_VISIT(Py_TYPE(op));
    return 0;
}

To reduce code duplication, I think it would be beneficial to have a common helper for this case. I'm going to call this _PyObject_VisitType for now.

cc @picnixz

Linked PRs

• gh-138342: Use a common utility for visiting an object's type #138343
• gh-138342: Move _PyObject_VisitType() to the internal C API #139734

[serhiy-storchaka]

When this boilerplate was added at first place, I suggested to do this automatically for each heap type with the Py_TPFLAGS_HAVE_GC flag. It belongs to the garbage collector (which does type-specific things in any case), no need to pollute the source code with Py_VISIT(Py_TYPE(op)). For some reasons my suggestion was ignored.

[picnixz]

I think you were talking about #84398 right?

[serhiy-storchaka]

Exactly. Now, after adding all these Py_VISIT(Py_TYPE(op)), we cannot use that approach and are doomed to add Py_VISIT(Py_TYPE(op)) in each tp_traverse.