gh-112087: Update list_get_item_ref to optimistically avoid locking

[corona10]

• Issue: Make list objects thread-safe in --disable-gil builds #112087

[corona10]

!buildbot nogil

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit eda29bc 🤖

The command will test the builders whose names match following regular expression: nogil

The builders matched are:

• AMD64 Ubuntu NoGIL Refleaks PR
• x86-64 MacOS Intel ASAN NoGIL PR
• ARM64 MacOS M1 Refleaks NoGIL PR
• x86-64 MacOS Intel NoGIL PR
• AMD64 Windows Server 2022 NoGIL PR
• AMD64 Ubuntu NoGIL PR
• ARM64 MacOS M1 NoGIL PR

[corona10]

!buildbot nogil

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @corona10 for commit 9071dd4 🤖

The command will test the builders whose names match following regular expression: nogil

The builders matched are:

• AMD64 Ubuntu NoGIL Refleaks PR
• x86-64 MacOS Intel ASAN NoGIL PR
• ARM64 MacOS M1 Refleaks NoGIL PR
• x86-64 MacOS Intel NoGIL PR
• AMD64 Windows Server 2022 NoGIL PR
• AMD64 Ubuntu NoGIL PR
• ARM64 MacOS M1 NoGIL PR

[corona10]

@colesbury

IIUC, This PR should be the last PR for #112087
Please let me know if I missed some parts.

[colesbury]

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

[corona10]

We may need to create a issue for tracking this.

[colesbury]

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

[corona10]

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

[corona10]

• prealloc:

  cpython/Objects/listobject.c

  Lines 207 to 208 in 72714c0

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

• preallocate_exact:

  cpython/Objects/listobject.c

  Line 119 in 72714c0

  self->allocated=size;

• resize:

  cpython/Objects/listobject.c

  Line 97 in 72714c0

  self->allocated=new_allocated;

[colesbury]

We currently store it in the PyListObject. We also need to store it in the same memory allocation as the ob_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.

[corona10]

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

[colesbury]

This looks good to me. I think it's best to address the capacity/allocated changes in a separate PR.