Document that bf_getbuffer must prevent GC (only under freethreading?) to write to its `view` argument

https://docs.python.org/3/c-api/typeobj.html#c.PyBufferProcs.bf_getbuffer

does not mention any requirement for the callee to hold the GIL or otherwise prevent Python GC to write to the view argument. One might think this is safe, because one has no particular reason to think that the view object is shared between threads and that the bf_getbuffer implementation has exclusive access to it.

However, the view object can be allocated from GC memory, e.g., in https://github.com/python/cpython/blob/4f07fd59cbd2656ae31e164098b8d10c5b7e51f0/Objects/memoryobject.c#L72
and this means that we may see concurrent GC traversals if we do not prevent GC (e.g., by holding the GIL).

Under free-threading mode, I saw the following race in the JAX test suite which I believe stems from this problem:

WARNING: ThreadSanitizer: data race (pid=268192)
  Read of size 8 at 0x7fffc2221a38 by thread T69 (mutexes: read M0):
    #0 mbuf_traverse /__w/jax/jax/cpython/Objects/memoryobject.c:134:5 (python3.13+0x27f190) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #1 update_refs /__w/jax/jax/cpython/Python/gc_free_threading.c:441:5 (python3.13+0x44854a) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #2 _mi_heap_area_visit_blocks /__w/jax/jax/cpython/Objects/mimalloc/heap.c:630:14 (python3.13+0x2a9fe4) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #3 mi_heap_area_visitor /__w/jax/jax/cpython/Objects/mimalloc/heap.c:681:12 (python3.13+0x2aa547) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #4 mi_heap_visit_areas_page /__w/jax/jax/cpython/Objects/mimalloc/heap.c:661:10 (python3.13+0x2aa547)
    #5 mi_heap_visit_pages /__w/jax/jax/cpython/Objects/mimalloc/heap.c:46:12 (python3.13+0x2aa547)
    #6 mi_heap_visit_areas /__w/jax/jax/cpython/Objects/mimalloc/heap.c:667:10 (python3.13+0x2aa547)
    #7 mi_heap_visit_blocks /__w/jax/jax/cpython/Objects/mimalloc/heap.c:692:10 (python3.13+0x2aa547)
    #8 gc_visit_heaps_lock_held /__w/jax/jax/cpython/Python/gc_free_threading.c:267:14 (python3.13+0x4446ff) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #9 gc_visit_heaps /__w/jax/jax/cpython/Python/gc_free_threading.c:306:11 (python3.13+0x4446ff)
    #10 deduce_unreachable_heap /__w/jax/jax/cpython/Python/gc_free_threading.c:614:5 (python3.13+0x4458db) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #11 gc_collect_internal /__w/jax/jax/cpython/Python/gc_free_threading.c:1126:15 (python3.13+0x4458db)
    #12 gc_collect_main /__w/jax/jax/cpython/Python/gc_free_threading.c:1239:5 (python3.13+0x4458db)
    #13 _Py_RunGC /__w/jax/jax/cpython/Python/gc_free_threading.c:1684:5 (python3.13+0x44730e) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)

 Previous write of size 8 at 0x7fffc2221a38 by thread T68 (mutexes: read M0):
    #0 __tsan_memset  (python3.13+0xda21d) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #1 memset /usr/include/x86_64-linux-gnu/bits/string_fortified.h:59:10 (xla_extension.so+0xa7a60ec) (BuildId: 0cb922163d5a5c8d99583339143c71c4c2e776b1)
    #2 xla::(anonymous namespace)::PyArray_bf_getbuffer(_object*, Py_buffer*, int)::$_0::operator()() const /proc/self/cwd/external/xla/xla/python/py_array.cc:1601:5 (xla_extension.so+0xa7a60ec)
    #3 xla::(anonymous namespace)::PyArray_bf_getbuffer(_object*, Py_buffer*, int) /proc/self/cwd/external/xla/xla/python/py_array.cc:1518:25 (xla_extension.so+0xa7a60ec)
    #4 PyObject_GetBuffer /__w/jax/jax/cpython/Objects/abstract.c:442:15 (python3.13+0x1ba90d) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)
    #5 _PyManagedBuffer_FromObject /__w/jax/jax/cpython/Objects/memoryobject.c:97:9 (python3.13+0x27fdf5) (BuildId: 5001c71441642d7e6efdfc82265e7ee63d55228e)

Inizia con il riferimento alla C API all’indirizzo docs.python.org/3/c-api/typeobj.html, concentrandoti su PyBufferProcs.bf_getbuffer, e per contesto esamina il sito di allocazione collegato in Objects/memoryobject.c. Chiarisci se il requisito si applica solo al free-threading mode e documenta come le implementazioni devono proteggere le scritture nell’argomento view; il lavoro sarà considerato completato quando la documentazione dell’API dichiarerà esplicitamente il requisito di sincronizzazione applicabile.