_ctypes fails to extend the return value per libffi contract

Bug report

Bug description:

The current implementation is relying on restype->size to write the data into a buffer, however, per libffi contract

integral types narrower than the system register size are widened

Which is not happening here. This has caused a failure on s390x when using libclang. cindex.py calls into libclang through ctypes, which builds the callback closures using libffi. With a c_int return, ctypes writes only the low 32 bits and skips the extension that both the ABI and libffi's closure contract require.

We have also opened a PR to fix this temporarily under llvm: https://github.com/llvm/llvm-project/pull/221024

CPython versions tested on:

3.11

Operating systems tested on:

Linux

Linked PRs

• gh-157045

从 Modules/_ctypes/callbacks.c 中链接的返回值处理部分开始，并阅读所引用的 libffi closure contract。复现或检查 s390x 上的 c_int callback 行为，然后验证窄整数返回值遵循所需的 ABI 扩展，而不是只复制 restype->size 个字节。检查链接的 CPython PR，了解已经在进行的工作。