Ctypes unions with bitfield members that do not share memory

[OhSoDank]

BPO	39030
Nosy	@abalkin, @LewisGaul, @OhSoDank
Files	bitfield_union.py

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-12-12.11:15:24.265>
labels = ['ctypes', 'type-bug', '3.7']
title = 'Ctypes unions with bitfield members that do not share memory'
updated_at = <Date 2019-12-12.15:41:56.428>
user = 'https://github.com/OhSoDank'

bugs.python.org fields:

activity = <Date 2019-12-12.15:41:56.428>
actor = 'LewisGaul'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['ctypes']
creation = <Date 2019-12-12.11:15:24.265>
creator = 'dankreso'
dependencies = []
files = ['48772']
hgrepos = []
issue_num = 39030
keywords = []
message_count = 1.0
messages = ['358300']
nosy_count = 3.0
nosy_names = ['belopolsky', 'LewisGaul', 'dankreso']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39030'
versions = ['Python 3.7']

[OhSoDank]

I've found what looks like a corner case bug. Specifically, the behaviour that looks suspicious is when a ctypes union has bit field members, where the members have bit widths that are smaller than the size types:

class BitFieldUnion(Union):
 _fields_ = [("a", c_uint32, 16), ("b", c_uint32, 16)]

buff = bytearray(4)

bitfield_union = BitFieldUnion.from_buffer(buff)
bitfield_union.a = 1
bitfield_union.b = 2

print("a is {}".format(bitfield_union.a)) # Prints "a is 1"
print("b is {}".format(bitfield_union.b)) # Prints "b is 2"
print("Buffer: {}".format(buff)) # Prints "Buffer: b'\x01\x00\x00\x00'".

(Example of this script can be found at https://rextester.com/XJFGAK37131. I've also tried it on my system which is Ubuntu 16.04.2 LTS with Python 3.6.)

Here I would expect both 'a' and 'b' to be set to 2, and for the buffer to look like '\x02\x00\x00\x00'. Here's the equivalent code in C which behaves as expected: https://rextester.com/HWDUMB56821.

If at least one of the bitwidths in the above example are changed from 16 to 32, however, then 'a', 'b', and the buffer look as expected.

I've also attached some further examples of weird behaviour with unions with bitfield members - online version can be found at https://rextester.com/VZRB77320.

[encukou]

Fixed in #97702.