Data race: ctypes `PyCData_NewGetBuffer` reads `b_ptr` without the critical section `_ctypes_resize` holds

Bug report

Bug description:

#131336 / #128182 made ctypes.resize and addressof/byref thread-safe by taking a critical section in _ctypes_resize, which reallocates obj->b_ptr:

https://github.com/python/cpython/blob/a2a846678e54b1b5defdccd451c573679094ff02/Modules/_ctypes/callproc.c#L1875-L1935

But the buffer-protocol getbuffer, PyCData_NewGetBuffer (reached via memoryview(cdata)), reads self->b_ptr without that critical section:

https://github.com/python/cpython/blob/a2a846678e54b1b5defdccd451c573679094ff02/Modules/_ctypes/_ctypes.c#L3103-L3130

So memoryview(obj) racing ctypes.resize(obj) reads a b_ptr that resize concurrently reallocates.

Reproducer:

import ctypes
from threading import Thread

buf = (ctypes.c_char * 64)()

def viewer():
    for _ in range(20000):
        try:
            memoryview(buf)
        except Exception:
            pass

def resizer():
    for i in range(20000):
        try:
            ctypes.resize(buf, 128 if (i & 1) else 256)
        except Exception:
            pass

threads  = [Thread(target=viewer)  for _ in range(6)]
threads += [Thread(target=resizer) for _ in range(2)]
for t in threads: t.start()
for t in threads: t.join()

TSAN Report:

==================
WARNING: ThreadSanitizer: data race (pid=1524546)
  Read of size 8 at 0x7fffb6b50208 by thread T1:
    #0 PyCData_NewGetBuffer /cpython/./Modules/_ctypes/_ctypes.c:3129:23 
    #1 PyObject_GetBuffer /cpython/Objects/abstract.c:455:15
    #2 _PyManagedBuffer_FromObject /cpython/Objects/memoryobject.c:97:9 
    #3 PyMemoryView_FromObjectAndFlags /cpython/Objects/memoryobject.c:813:42 
    #4 PyMemoryView_FromObject /cpython/Objects/memoryobject.c:856:12
    #5 memoryview_impl /cpython/Objects/memoryobject.c:1017:12 
    #6 memoryview /cpython/Objects/clinic/memoryobject.c.h:63:20 
    #7 type_call /cpython/Objects/typeobject.c:2472:11 
    #8 _PyObject_MakeTpCall /cpython/Objects/call.c:242:18
    #9 _PyObject_VectorcallTstate /cpython/./Include/internal/pycore_call.h:142:16)
    #10 PyObject_Vectorcall /cpython/Objects/call.c:327:12
    #11 _Py_VectorCall_StackRefSteal /cpython/Python/ceval.c:726:11 
    #12 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:4559:35 

  Previous write of size 8 at 0x7fffb6b50208 by thread T7:
    #0 _ctypes_resize_impl /cpython/./Modules/_ctypes/callproc.c
    #1 _ctypes_resize /cpython/./Modules/_ctypes/clinic/callproc.c.h:139:20
    #2 _Py_BuiltinCallFast_StackRef /cpython/Python/ceval.c:817:11
    #3 _PyEval_EvalFrameDefault /cpython/Python/generated_cases.c.h:2510:35
    #4 _PyEval_EvalFrame /cpython/./Include/internal/pycore_ceval.h:122:16

SUMMARY: ThreadSanitizer: data race /cpython/./Modules/_ctypes/_ctypes.c:3129:23 in PyCData_NewGetBuffer
==================

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-157759

Bắt đầu với Modules/_ctypes/_ctypes.c tại PyCData_NewGetBuffer và so sánh quyền truy cập b_ptr của nó với critical section trong Modules/_ctypes/callproc.c tại _ctypes_resize_impl. Chạy reproducer đa luồng được cung cấp bằng ThreadSanitizer. Hoàn tất khi race giữa memoryview và ctypes.resize không còn được báo cáo, trong khi hành vi hiện có vẫn được giữ nguyên.