Bug report

On Windows, the Py_GIL_DISABLED macro is not defined when building C API extensions with the free-threaded build of CPython 3.13 (i.e., --disable-gil). Now that biased reference counting (6dfb8fe) landed, this leads to incorrect/failed compilation of extensions on Windows ¹. The problem does not occur on Linux/macOS and other ./configure based builds because that build system defines the Py_NOGIL macro as part of pyconfig.h. On Windows, Py_GIL_DISABLED instead defined as a pre-processor definition in pyproject.props, but that's used by C-API extensions.

Among other problems, this leads to test_peg_generator failing on the nogil Windows buildbot (example failure).

I can think of two possible approaches to fixing this:

1. Modify setuptools (maybe setuptools.distutils?) to define Py_GIL_DISABLED when invoking the compiler if sysconfig.get_config_var("Py_NOGIL") is True.
2. In the CPython Windows build, generate a header file defining/undefing Py_GIL_DISABLED depending on if the build is invoked with --disable-gil or not.

The first option seems a little more straightforward to me, but there is a risk that we might encounter this same issue across multiple extension build systems. The second option seems like it would more robustly address this issue.

Linked PRs

• gh-111650: Generate pyconfig.h on Windows #112179
• gh-111650: Ensure pyconfig.h includes Py_GIL_DISABLED on Windows #112778