Torn reads of GC statistics from external process via get_gc_stats

Bug report

Bug description:

If an external tool sampling GC statistics does not pause the target process (and we designed get_gc_stats not to pause), it can read torn data under certain circumstances.

There are two distinct problems:

1. Weakly ordered platforms: The stores of ts_start and ts_stop may be reordered by the CPU/optimizer. We need to add memory barriers to mitigate this.

2. Large memcpy window: We are copying a sufficiently large memory region via memcpy. If two or more GC cycles occur during this copy, we can end up with inconsistent data spanning multiple GC states.

@maurycy has an idea to use a sequence counter for consistency checks — I think he should definitely give it a try.

cc @pablogsal

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-155828

まず get_gc_stats と ts_start および ts_stop を格納している箇所を追跡し、次に外部プロセスが GC データをどのように観測するかについて memcpy パスを確認してください。最初にリンクされた PR gh-155828 と関連するテストをすべて読んでください。弱いメモリ順序付けのプラットフォームで、外部サンプリングが GC サイクルをまたいで分断されたスナップショットを返さなくなれば完了です。