Python 3.13 free-threading compatibility

[lysnikolaou]

Hey all! 👋

This aims to be a tracking issue for all work necessary to support the free-threaded build of Python 3.13. A very high-level list of steps is (more details to follow as I investigate more issues):

• [ ] Set up CI for free-threading (WIP by @hugovk) https://github.com/python-pillow/Pillow/pull/8200
• [ ] Move away from APIs that use borrowed references under the free-threaded build (also WIP by @hugovk)
• [ ] Audit C extension module for thread-safety issues
• [ ] Mark C extension modules as thread-safe with the Py_mod_gil slot (or PyUnstable_Module_SetGIL)
• [ ] Upload wheels for the free-threaded build. Other projects have started uploading nightlies for the free-threaded on the scientific-python-nightly-wheels index so that we can enable faster iteration, we can discuss whether we wanna do this for Pillow as well.

[hugovk]

Thanks for opening this issue!

As some background, the acceptance of PEP 703 lays out the three phases for disabling the GIL. Python 3.13 is the first in the experimental phase I, where a free-threaded build is available as an option in the Windows and macOS installers, and can be built for Linux (and the other platforms) using an extra flag.

So now is a good time to start testing on free-threaded builds, and getting the code ready. It will be good to produce wheels at some point, to help others test and integrate. These will be experimental and not for production.

I've made a start on the first two steps, I'll open a PR for the CI first.

Some handy references:

• https://dev.to/hugovk/help-us-test-free-threaded-python-without-the-gil-1hgf
• https://docs.python.org/3.13/howto/free-threading-extensions.html
• https://github.com/Quansight-Labs/free-threaded-compatibility

[hugovk]

Please see PR https://github.com/python-pillow/Pillow/pull/8200 to set up CI for free-threading.