Context

In PYTHON-4451 we switch from using setuptools as the PEP 517 build backend to using Hatch, but still use setuptools to build the C extensions. There is an experimental Hatch plugin for scikit-build-core, that will eventually move to its own plugin. At that point we can switch to using CMake and scikit-build-core to build the C extensions, and migrate off of setuptools entirely.

This will enable us to use a widely supported build system, and even let us build C extensions for PyPy.

I did some experimentation and the following CMakelists.txt will build our C extension:

cmake_minimum_required(VERSION 3.18...3.22)

project(pymongo LANGUAGES C)

find_package(Python COMPONENTS Interpreter Development.Module REQUIRED)

Python_add_library(_cbson MODULE bson/_cbsonmodule.c bson/buffer.c bson/time64.c WITH_SOABI)
include_directories(bson)
Python_add_library(_cmessage MODULE pymongo/_cmessagemodule.c bson/_cbsonmodule.c bson/buffer.c bson/time64.c WITH_SOABI)

install(TARGETS _cbson LIBRARY DESTINATION bson)
install(TARGETS _cmessage LIBRARY DESTINATION pymongo)

Definition of done

Replace the custom build step introduced in PYTHON-4451 with the CMake plugin.

Pitfalls

We may want to do some manual testing or add new test cases to ensure there are no regressions from switching the C extension build system.