Pickle `load_build` function checks if `slotstate` is False, not a dict

Bug report

Bug description:

This issue is similar to Issue https://github.com/python/cpython/issues/128965. Inside of the load_build() function, Python pickle assumes that slotstate is a dictionary while C _pickle explicitly checks to ensure it's a dictionary. The behavior of these two implementations diverge when slotstate is falsey but not a dictionary.

payload:      b'NN\x8f\x86b.'

pickle:       None
_pickle.c:    FAILURE slot state is not a dictionary
pickletools:
    0: N    NONE
    1: N    NONE
    2: \x8f EMPTY_SET
    3: \x86 TUPLE2
    4: b    BUILD
    5: .    STOP
highest protocol among opcodes = 4

In this case, opcode 2 (EMPTY_SET) is slotstate. When running in pickle.py, this is falsey and thus the code inside setting attributes will not be run, causing the pickle to continue deserializing without any errors.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Lib/pickle.py#L1868

When running in _pickle.c, this value is not a dictionary, causing an error to be thrown.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Modules/_pickle.c#L6903

I guess also in general I'm not understanding what the if slotstate in pickle.py is meant to do anyway. If it's supposed to check if slotstate == None, then the C _pickle module doesn't do that correctly. It checks if slotstate is NULL, which is not the same as PyNone.

https://github.com/python/cpython/blob/29acc08c8dad664cd5713cb392e5beba65724c10/Modules/_pickle.c#L6899

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-144582

So sánh triển khai của load_build trong Lib/pickle.py với Modules/_pickle.c, sử dụng payload b'NN\x8f\x86b.' để tái hiện hành vi khác biệt. Xem xét PR được liên kết gh-144582 và xác nhận rằng cả hai triển khai đều nhất quán từ chối một slotstate falsy không phải là dictionary, với hành vi pickle liên quan được bao phủ bởi các bài kiểm thử nếu PR xác định chúng.