gh-117549: Match declaration order for _Py_BackoffCounter initializer

[mdboom]

This is to fix the build for Scipy, which includes this header from C++ code which seems to really care about such things.

To be clear, I didn't confirm that this fixes the Scipy build -- there are a bunch of other patches required to its build system to even compile with Python 3.13 and I'm not sure it's worth the effort there.

But this warning seems legitimate and the fix is harmless. It's possible there are other errors in Scipy behind it, but this is the only struct initializer added in #117144.

• Issue: New tier 2 counters break some C extensions due to order of field mismatch #117549

[gvanrossum]

LGTM. Let's wait for @tacaswell to give us a thumbs up here before we merge.

[tacaswell]

I can confirm that scipy compiles with cpython from this branch.

[gvanrossum]

Thanks for your help! Merging now...

[methane]

Cython includes this file and g++ 9 and 10 (available on Ubuntu 22.04 LTS) doesn't compile this.

msgpack/msgpack-python#593 (comment)

// foo.cpp
#define Py_BUILD_CORE 1
#include <Python.h>

#include "frameobject.h"
#include "internal/pycore_frame.h"

int main() {}
// g++-10 -std=c++17 -I include/python3.13 foo.cpp

I confirmed Using temporary variable fix it.

    _Py_BackoffCounter c;
    c.backoff = backoff;
    c.value = value;
    return c;

But I am not sure we should support g++ 9/10 in Python 3.13.

[gvanrossum]

I am despairing. We make all these efforts to indicate things are internal and they are still included in half the packages in the world. :-(

[methane]

FYI, this caused compile error on MSVC too. Should we add /std:c++20 in cflags on msvc for better interoperability with cython&c++?
msgpack/msgpack-python#597

[gvanrossum]

Wait— if it failed on MSVC how could the original code have passed CI?

[methane]

Because we don't test C++.
This is problem only when using C++ with Cython.

[methane]

/std:c++20 didn't help me. I need to stop using C++ or Cython.

  C:\hostedtoolcache\windows\Python\3.13.0-alpha.6\x64\include\internal\pycore_backoff.h(47): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax
  C:\hostedtoolcache\windows\Python\3.13.0-alpha.6\x64\include\internal\pycore_backoff.h(53): error C4576: a parenthesized type followed by an initializer list is a non-standard explicit type conversion syntax

[gvanrossum]

So let’s fix it. Do you have time for a PR?

[gvanrossum]

That should be fixed now.

[methane]

thank you.

[methane]

By the way, Cython uses this only for building custom traceback object.
We hided f_lineno in PyFrameObject so Cython includes pycore_frame.h.

Adding _PyFrame_SetLineno() in Include/cpython/frameobject.h makes it possible to Cython remove this include.

https://github.com/cython/cython/blob/45db483cbe128c29a1e9ffe9f5a4db12cf77cddd/Cython/Utility/Exceptions.c#L1007C1-L1007C52
https://github.com/cython/cython/blob/45db483cbe128c29a1e9ffe9f5a4db12cf77cddd/Cython/Utility/ModuleSetupCode.c#L839

[gvanrossum]

Adding _PyFrame_SetLineno() in Include/cpython/frameobject.h makes it possible to Cython remove this include.

So IIUC you are proposing to add a new public API to set the f_lineno attribute in a frame object? It's too bad that today is the 3.13 beta 1 feature freeze, so this will have to go into 3.14. Can you open a new issue and PR so we won't forget?