[ravi0800]

The Cython Packer checks for live buffer exports only on entry to pack(), pack_map_pairs() and pack_ext_type(). Python code that runs mid-pack (a default callback, the pairs iterable, a list or dict subclass) can call memoryview(packer), and the next msgpack_pack_write() that grows the buffer PyMem_Reallocs pk.buf under that view. With Packer(buf_size=16, autoreset=False, default=...) where default stores memoryview(packer) and returns b"x" * 65536, the stored view reads b' ' instead of the b'\x91' array header, i.e. freed heap memory.

__getbuffer__ now raises BufferError while one of those three methods is running, using a nesting counter so a reentrant pack() from a callback doesn't clear the outer guard. The pure Python Packer already raises BufferError here because BytesIO refuses to resize while exported, so the new test in test_buffer.py passes on both implementations and fails on the unpatched C extension.