bpo-39573: Use Py_SIZE() in s_set() of cfield.c

[shihai1991]

https://bugs.python.org/issue39573

[vstinner]

This change is unrelated to https://bugs.python.org/issue39573 If you consider that s_set() behavior is a bug, please open a separated issue.

Your change is backward incompatible. Example:

import ctypes

class MyStruct(ctypes.Structure):
    _fields_ = (
        ('char', ctypes.c_char * 5),
    )

s = MyStruct()
print(ascii(s.char), bytes(s))
s.char = b'a\0b\0'
print(ascii(s.char), bytes(s))

Current behavior:

b'' b'\x00\x00\x00\x00\x00'
b'a' b'a\x00\x00\x00\x00'

=> only b'a\0' (2 bytes) are copied

With your PR:

b'' b'\x00\x00\x00\x00\x00'
b'a' b'a\x00b\x00\x00'

=> only b'a\0b\0' (4 bytes) are copied

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[vstinner]

cc @corona10

[shihai1991]

This change is unrelated to https://bugs.python.org/issue39573 If you consider that s_set() behavior is a bug, please open a separated issue.

Your change is backward incompatible. Example:

import ctypes

class MyStruct(ctypes.Structure):
    _fields_ = (
        ('char', ctypes.c_char * 5),
    )

s = MyStruct()
print(ascii(s.char), bytes(s))
s.char = b'a\0b\0'
print(ascii(s.char), bytes(s))

Current behavior:

b'' b'\x00\x00\x00\x00\x00'
b'a' b'a\x00\x00\x00\x00'

=> only b'a\0' (2 bytes) are copied

With your PR:

b'' b'\x00\x00\x00\x00\x00'
b'a' b'a\x00b\x00\x00'

=> only b'a\0b\0' (4 bytes) are copied

Thanks for your quick feedback, victor.

This change is unrelated to https://bugs.python.org/issue39573

I found you do some macro porting operations, so I thought this macro porting operation relate to 39573 is fine(not the cause of a bug).

Your change is backward incompatible.

Great example, current tests not help me found this backward incompatible and your example have answer this question(/* XXX Why not Py_SIZE(value)? */).

I have debug it in my vm, the reason is:
value = b'a\0b\0'
data = PyBytes_AS_STRING(value) ==> "a"
strlen(data) ==> 1
Py_SIZE(value) ==> 4

Sorry for this noisy PR, guys!

[shihai1991]

To prevent other developers break this backward incompatible again, i add victor's example in #18424