[Free-Threaded] Allow Free-Threaded Python Build Fail Gracefully

[FlickerSoul]

As discussed in here, it would be nice to allow PyO3 to fail when building against GIL-less Python. I'm planning to implement this feature so I opened this issue to track discussion regarding this issue.

I was advised to look at the pyo3-build-config crate and the following is my plan of implementation of this feature:

• Add an additional boolean field (possibly named free_threaded) in InterpreterConfig to indicate whether the interpreter is free-threaded or not.
• In the build.rs located at pyo3-ffi crate or the one in top-level workspace, add an additional check in configure_pyo3 to validate the interpreter is not free-threaded (from the obtained InterpreterConfig instance), and panic otherwise.
• Add an additional environment variable (possibly named PYO3_ALLOW_FREE_THREADED) to override the second bullet, to facilitate building during development.

Any advice would be much appreciated! Thanks! :)

Related to: #4265

[davidhewitt]

This sounds great, yes please!

• Add an additional boolean field (possibly named free_threaded) in InterpreterConfig to indicate whether the interpreter is free-threaded or not.

Depending on the CPython configuration, it might be that an additional field isn't needed and this can go into build_flags.

• In the build.rs located at pyo3-ffi crate or the one in top-level workspace, add an additional check in configure_pyo3 to validate the interpreter is not free-threaded (from the obtained InterpreterConfig instance), and panic otherwise.

Yes (though configure_pyo3 returns a result rather than panicking).

• Add an additional environment variable (possibly named PYO3_ALLOW_FREE_THREADED) to override the second bullet, to facilitate building during development.

Yes please, I think maybe UNSAFE_PYO3_BUILD_FREE_THREADED perhaps? We expect builds to be broken until the moment we've finalised support, so I think hinting at this in the env var might be a good idea.

[davidhewitt]

Fixed in #4327, thanks @FlickerSoul