Thought question: suppose finalize_garbage() is called with a collectable list all of whose members are in fact going to be destroyed?
I don't see how the loop iteration logic could reliably work then. For concreteness, suppose there's only object - A - in the list. It's a circular list so gc starts as A. A is finalized, we see it's refcount is 1, `gc = gc->gc.gc_prev` sets gc to A again, and A is destroyed by the Py_DECREF. We go back to the top of the loop, and then `gc = gc->gc.gc_next` reads up trash (free'd) memory.
Or so it seems to me ;-)