Semi-stable C API tier

[encukou]

Hi,
I'd like to introduce a subset of the C API that can change in every minor release, without deprecation warnings, for use cases like debuggers and JIT compilers (e.g. PEP-523). We already have API like this, but it's marked as such in inconsistent ways (leading underscores, notes in docs or in a PEP).
To prevent using it accidentally, it would be opt-in (exposed only by defining Py_USING_SEMI_STABLE_API). API that's now public or documented would have a deprecation period of at least one release rather than the usual two until this opt-in is mandatory.
Details are discussed in python/cpython#91744, background is on python-dev

The bad news is that this API is still changing (e.g. the PyCode_New signature changed last week). It will probably be necessary to adjust the semi-stable API in early betas. (I see it as part of stabilization, but views might vary.) It should be as stable as the ABI -- formally in the first RC, but hopefully sooner.

Is the SC OK with this? If yes, how much oversight over details does it want? Does this need a PEP?

(I added this to the agenda)

[gvanrossum]

I'm confused. You state "can change in every minor release" but also "as stable as the ABI". Those seem wildly different. Which is it? What am I misunderstanding?

[encukou]

I'm talking about the regular, version-specific ABI, like cp310-cp310-manylinux_2_17_x86_64 in wheels. Not the Stable ABI (abi3).

[gvanrossum]

Thanks; your use of "stable" and "ABI" in the same phrase confused me. ;-)

[encukou]

It needs a PEP. I'll write one.