`gc.get_objects` can corrupt in-progress GC in free threading build

[colesbury]

Bug report

Background

The free threading GC uses two queue-like data structures to keep track of objects:

• struct worklist, which is a singly linked list that repurposes ob_tid for the linked list pointer

• _PyObjectStack, which is effectively a dynamically sized array of PyObject*. (Implemented using a linked list of fixed size array buffers).

The struct worklist data structure is convenient because enqueueing objects doesn't require a memory allocation and so can't fail. However, an object can only be part of one "worklist" at a time, because each object has only one ob_tid field.

Bug

Other threads can run while the GC is finalizing cyclic garbage and while it's calling tp_clear() and other clean-up.

During that time, some thread may call gc.get_objects(), which can return otherwise "unreachable" objects. The implementation of _PyGC_GetObjects temporarily pushes objects to a struct worklist, including objects that might already be part of some other worklist, overwriting the linked list pointer. This essentially corrupts the state of the in-progress GC and causes assertion failures.

Proposed fix

• We should probably exclude objects in the "unreachable" (i.e. _PyGC_BITS_UNREACHABLE) from being returned by gc.get_objects()
• We should limit the use of struct worklist to the actual GC and use _PyObjectStack (or some other data structure) in _PyGC_GetObjects(). This reduces the risk of bugs causing an object to be added to more than one worklist.

Linked PRs

• gh-125859: Fix crash when gc.get_objects is called during GC #125882
• [3.13] gh-125859: Fix crash when gc.get_objects is called during GC (GH-125882) #125921

[rruuaanng]

Hi @colesbury, if there is no objection, I would like to take over this task. I will submit a PR in the next few days. I will take plan We should probably exclude objects in the "unreachable" (i.e. _PyGC_BITS_UNREACHABLE) from being returned by gc.get_objects().
I think using plan
We should limit the use of struct worklist to the actual GC and use _PyObjectStack (or some other data structure) in _PyGC_GetObjects(). This reduces the risk of bugs causing an object to be added to more than one worklist.
may cause the memory usage to become higher. So I choose a simple conditional return

[colesbury]

@rruuaanng - thank you for the offer, but I've already started working on this