[3.12] extern "C" not closed in tracemalloc.h when using Py_LIMITED_API

[mitya57]

Bug report

Bug description:

When building PyQt6 with the latest 3.12 branch, I get the following error:

g++ -c -pipe -g -O2 -ffile-prefix-map=/pyqt6=. -fstack-protector-strong -fstack-clash-protection -Wformat -Werror=format-security -fcf-protection -Wdate-time -D_FORTIFY_SOURCE=2 -O2 -fno-exceptions -Wall -Wextra -D_REENTRANT -fPIC -DPy_LIMITED_API=0x03090000 -DQT_NO_EXCEPTIONS -DQT_NO_DEBUG -DQT_PLUGIN -DQT_CORE_LIB -I. -I/usr/include/dbus-1.0 -I/usr/lib/x86_64-linux-gnu/dbus-1.0/include -I/usr/include/python3.12 -I/usr/include/x86_64-linux-gnu/qt6 -I/usr/include/x86_64-linux-gnu/qt6/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt6/mkspecs/linux-g++ -o dbus.o ../../../../dbus/dbus.cpp
In file included from /usr/include/c++/14/utility:68,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/qglobal.h:15,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/qcoreapplication.h:7,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/QCoreApplication:1,
                 from ../../../../dbus/dbus.cpp:29:
/usr/include/c++/14/bits/stl_relops.h:86:5: error: template with C linkage
   86 |     template <class _Tp>
      |     ^~~~~~~~
In file included from /usr/include/python3.12/Python.h:107,
                 from /usr/include/dbus-1.0/dbus/dbus-python.h:34,
                 from ../../../../dbus/dbus.cpp:27:
/usr/include/python3.12/tracemalloc.h:4:1: note: ‘extern "C"’ linkage started here
    4 | extern "C" {
      | ^~~~~~~~~~

This happened because of the commit 65bca19, which changed the structure of tracemalloc.h to the following:

#ifndef Py_TRACEMALLOC_H
#define Py_TRACEMALLOC_H
#ifdef __cplusplus
extern "C" {
#endif

#ifndef Py_LIMITED_API
/* ... */

#ifdef __cplusplus
}
#endif
#endif /* !Py_LIMITED_API */
#endif /* !Py_TRACEMALLOC_H */

Here, if Py_LIMITED_API is defined, the extern "C" block is opened but never closed.

This problem does not affect 3.13 and newer versions, because the order is different there. I am going to submit a trivial pull request to fix this.

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• [3.12] gh-127902: Make sure extern "C" is closed when using Py_LIMITED_API #127904
• [3.12] gh-127906: Backport test_cext from the main branch #127912

[eendebakpt]

@mitya57 Thanks for reporting. Making a PR would nice!

@vstinner Is there a reason why this was not noticed by the CI? I checked the CI on your PR #127817 and it was all green

[vstinner]

Oops, thanks for catching this regression!

Is there a reason why this was not noticed by the CI?

The limited C API is not well tested in Python 3.12.

[vstinner]

Fixed by change b184f48.

[vstinner]

The limited C API is not well tested in Python 3.12.

I created #127906 to add more tests to 3.12.