gh-121617: Include <string.h> for Py_CLEAR() macro

[vstinner]

If the _Py_TYPEOF macro is not available, include <string.h> to get memcpy() for the Py_CLEAR() macro. The limited C API version 3.11 and newer doesn't include <string.h> anymore.

Add a Py_CLEAR() test in test_cext.

Modify also _Py_TYPEOF to use C23 typeof() if available.

• Issue: memcpy() usage without #include <string.h> in pwdmodule.c #121617

[vstinner]

If this change is merged in the main branch, I will write a backport to the 3.14 branch without the C23 change using typeof().

[vstinner]

Note: Py_SETREF() and Py_XSETREF() macros also use memcpy(), but these macros are only defined if the limited C API is not used. In this case, Python.h includes <string.h> and so we are already good (no change needed). Only Py_CLEAR() of the limited C API version 3.11 and newer is affected by the issue.

[encukou]

The documentation also needs an update. (3.14 has the list here.)

It's somewhat hard to explain, unfortunately. Is it enough to add something vague like “<string.h> (on certain compilers)”?

[vstinner]

I added the following sentence to the doc:

The <string.h> header is also included when using Limited API 3.11 or newer if Python is unable to get a typeof() implementation.

I'm not sure if it's worth it to document this corner case.

[encukou]

It looks hard to explain. Is it a good idea?

Looking more into this, I see <string.h> also defines NULL and size_t (as used in vectorcall).
These are also defined in these other headers that Python.h currently includes:

• <stdio.h> & <stdlib.h> -- not included in limited API 3.11 and below
• <wchar.h> -- which seems much less “fundamental” that string.h

Plus, I think we want to be able to use memcpy in macros/static inline functions -- as this bug shows, people tend to do that without thinking :)

I think we should go ahead and always include <string.h>.

[vstinner]

I think we should go ahead and always include <string.h>.

Ok, let's do that.

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @encukou for commit b6082ac 🤖

Results will be shown at:

https://buildbot.python.org/all/#/grid?branch=refs%2Fpull%2F144666%2Fmerge

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[encukou]

Thanks!

Let's wait for buildbots; they've been grumpy about test_c[pp]ext lately.

[vstinner]

Merged. Thanks for the review @encukou.

[vstinner]

Let's wait for buildbots; they've been grumpy about test_c[pp]ext lately.

The PEP 810 implementation introduced a minor incompatibility. It has been fixed quickly by commit 7854597.

 typedef enum {
     PyImport_LAZY_NORMAL,
     PyImport_LAZY_ALL,
-    PyImport_LAZY_NONE,
+    PyImport_LAZY_NONE
 } PyImport_LazyImportsMode;

The buildbot tests passed successfully on this PR ;-)