ctypes._CData.from_buffer does not inherit _b_base_

Continuing work from #135669, I have noticed another inconsistency.

from_buffer docs say:

This method returns a ctypes instance that shares the buffer of the source object.

_b_base_ docs say:

The _b_base_ read-only member is the root ctypes object that owns the memory block.

So, I assumed that, when passing a ctypes object as source to from_buffer, the _b_base_ from the resulting ctypes object would be inherited from source, however that is not the case.

To better visualize the issue, lets consider this code:

import ctypes

bindata = bytearray([
    0, 0, # [0].offset
    2, 0, # [0].variant
    8, 0, # [1].offset
    1, 0, # [1].variant

    # dynamic_part
    1, 2, 3, 4,
    5, 6,

    0, 0,

    9, 8, 7, 6,
])

class A(ctypes.Structure):
    _fields_ = [
        ("foo", ctypes.c_int),
    ]
class B(ctypes.Structure):
    _fields_ = [
        ("bar", ctypes.c_byte * 6),
    ]

class Message(ctypes.Structure):
    _fields_ = [
        ("offset", ctypes.c_uint16),
        ("variant", ctypes.c_uint16),
    ]

    variants = {
        1: A,
        2: B,
    }

    def get(self):
        root = self._b_base_._b_base_
        _type = self.variants[self.variant]

        try:
            return _type.from_buffer(root.dynamic_part, self.offset)
        except ValueError as e:
            e.add_note(f"Invalid data offset={self.offset} variant={self.variant}")
            raise

class FinalData(ctypes.Structure):
    _fields_ = [
        ("messages", Message * 2),
        ("dynamic_part", ctypes.c_ubyte * 12),
    ]

x = FinalData.from_buffer(bindata)

print(x)
print(x.dynamic_part._b_base_)

a = x.messages[1].get()
b = x.messages[0].get()
print(a)
print(a._b_base_)
print(b)
print(b._b_base_)

I expected the a._b_base_ and b._b_base_ would be <__main__.FinalData object at 0x7fbee293d3d0>, like x.dynamic_part._b_base_, instead it is None.

Linked PRs

• gh-152706

Start by reviewing linked PR gh-152706 and running the supplied ctypes reproducer. Then inspect the implementation behind _CData.from_buffer and its existing tests; done means the resulting object’s b_base ownership chain matches the documented buffer ownership behavior.