This is okay for now, but we need to store the capacity at the start of theob_item allocation in the free-threaded build.

We may need to create a issue for tracking this.

It's necessary for thread-safety so I think it's covered by#112087

By the way, we already store theop->allocated at theob_item allocation.
So adding assertion will be enough?
assert(cap != -1 && cap >= size);

• prealloc:

  cpython/Objects/listobject.c

  Lines 207 to 208 in72714c0

  	op->allocated=size;
  	return (PyObject*)op;

• preallocate_exact:

  cpython/Objects/listobject.c

  Line 119 in72714c0

  self->allocated=size;

• resize:

  cpython/Objects/listobject.c

  Line 97 in72714c0

  self->allocated=new_allocated;

We currently store it in thePyListObject. We also need to store it in the same memory allocation as theob_item array, like a pre-header.

Here are some pointers to the relevant code in nogil-3.12:

• https://github.com/colesbury/nogil-3.12/blob/cedde4f5ec3759ad723c89d44738776f362df564/Include/cpython/listobject.h#L24-L27
• https://github.com/colesbury/nogil-3.12/blob/cedde4f5ec3759ad723c89d44738776f362df564/Objects/listobject.c#L65-L70

The problem with the current code is that the list may be resized concurrently with the access. The bounds check may be stale. Putting the value of "allocated" as an immutable field avoids this problem.

Ah, I got it. I understood what you want to say. Let's handle it at a separate PR.