The PyType_GenericAlloc() function tracks the newly created object in the garbage collector (GC) as soon as memory is initialized, but before all object members are initialized.

If a GC collection happens before the object is fully initialized, the traverse function of the newly created object can crash.

This case is hypothetical for built-in types since their constructor should not trigger a GC collection. It is more likely in third party extensions and subclasses.

Anyway, I propose to add a new _PyType_AllocNoTrack() function which allocates memory without tracking the newly allocated object directly in the GC.

This function can be used to only track explicitly the object in the GC once it is fully initialized.

In bpo-40142, I tried to modify _PyObject_GC_TRACK() to visit the object before tracking it, as done by PyObject_GC_Track().

Problem: PyType_GenericAlloc() cannot traverse the object since the object members are not initialized yet. For example, dict_traverse() can only be called at dict_new() exit.

Here I propose a different approach. First, fix built-in types to only track instances when they are fully initialized. Avoid PyType_GenericAlloc().

> Anyway, I propose to add a new _PyType_AllocNoTrack() function which allocates memory without tracking the newly allocated object directly in the GC.

How is that going to help third party extensions and subclasses?

New changeset 823460daa9fab3d0cf00ec553d1e35635ef73d40 by Victor Stinner in branch 'main':
bpo-44531: Fix type_repr() if tp_name is NULL (GH-26948)
https://github.com/python/cpython/commit/823460daa9fab3d0cf00ec553d1e35635ef73d40

> How is that going to help third party extensions and subclasses?

It doesn't solve bpo-40142 issue for third party extensions.

About subclasses, I'm not sure if it's safe to call the traverse function if a subclass overrides it and visits its own member which are not in the built-in type (like dict or tuple).

> About subclasses, I'm not sure if it's safe to call the traverse function if a subclass overrides it and visits its own member which are not in the built-in type (like dict or tuple).

Technically the traverse should only call visit on the members, but in practice they can do whatever. I agree is not clear is a safe operation

For the background of this issue, please see bpo-38392:
https://bugs.python.org/issue38392#msg354074

New changeset 818628c2da99ba0376313971816d472c65c9a9fc by Victor Stinner in branch 'main':
bpo-44531: Add _PyType_AllocNoTrack() function (GH-26947)
https://github.com/python/cpython/commit/818628c2da99ba0376313971816d472c65c9a9fc