See https://bugs.python.org/issue43760 discussion for recent CPython changes and discussions about adding new C API functions to abstract accesses to PyThreadState and PyFrameObject.

I'm the author and maintainer of pythoncapi_compat. Using it, it becomes to remove the following lines from src/greenlet/greenlet_cpython_compat.hpp:

#if PY_VERSION_HEX < 0x03090000
// The official version only became available in 3.9
#    define PyObject_GC_IsTracked(o) _PyObject_GC_IS_TRACKED(o)
#endif

But I suggest to do it in a separated change, to keep this PR as small as possible.

I wrote pythoncapi_compat to help C extension maintainers to keep up with recent Python C API changes.

Thanks for the heads up!

Rather than vendor an entire header file, I'll probably just add only the definitions greenlet needs to the existing header as it needs them.

Oh, pythoncapi_compat.h would also allow to remove another compatibility function:

#ifndef Py_SET_REFCNT
/* Py_REFCNT and Py_SIZE macros are converted to functions
https://bugs.python.org/issue39573 */
#    define Py_SET_REFCNT(obj, refcnt) Py_REFCNT(obj) = (refcnt)
#endif

Rather than vendor an entire header file, I'll probably just add only the definitions greenlet needs to the existing header as it needs them.

My long term plan is to ship pythoncapi_compat.h as part of Python itself and/or install it on the system, as the six module is now preinstalled or easy to install on any system. For now, I prefer to copy it in each project.

If you don't want to use pythoncapi_compat.h for now, here is a different PR without it: #268

Fixed by #268