[3.14] gh-139653: Add PyUnstable_ThreadState_SetStackProtection() (GH-139668)

[encukou]

This backports recent changes to the stack protection mechanism, and the unstable API to control it:

• gh-139231: Fix estimation of available stack size for recursion limit on macOS #139232
• GH-139291: Fix C stack limits by factoring out finding hardware stack limits #139294
• gh-139653: Add PyUnstable_ThreadState_SetStackProtection() #139668
• gh-139653: Remove assertions in _Py_InitializeRecursionLimits() #141551

• Issue: Python 3.14 stack overflow detection is incompatible with C++ Boost make_fcontext() coroutines #139653

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📚 Documentation preview 📚: https://cpython-previews--141661.org.readthedocs.build/

[encukou]

@hugovk, this will change the internal ABI.

[vstinner]

I would prefer to wait to see how the discussion https://discuss.python.org/t/python-3-14-0-is-incompatible-with-stack-switching-systems-what-do-we-do/104880 goes and see if this API solves the issue, or if we need another solution.

[vstinner]

If this API cannot be used by most projects and a better fix can be designed, I would even suggest to remove this API from the main branch.

[vstinner]

#141404 has been merged causing conflicts.

Can you please update your PR to solve conflicts?

[vstinner]

I would prefer to wait to see how the discussion https://discuss.python.org/t/python-3-14-0-is-incompatible-with-stack-switching-systems-what-do-we-do/104880 goes and see if this API solves the issue, or if we need another solution.

If this API cannot be used by most projects and a better fix can be designed, I would even suggest to remove this API from the main branch.

#141711 has been merged: it should fix #139653 for most impacted projects. But according to @markshannon, PyUnstable_ThreadState_SetStackProtection() remains useful for these projects. So let's backport this function to the 3.14 branch.

[vstinner]

LGTM