Deprecate and Then Remove PyGILState_GetThisThreadState()

Feature or enhancement

Proposal:

(See https://docs.python.org/3/c-api/init.html#c.PyGILState_GetThisThreadState.)

PyGILState_GetThisThreadState() was added as part of the PEP 311 implementation. However, it wasn't part of that proposal. It isn't clear that it was even meant to be public API.

If a user wants to know what the current thread state is then they should use PyThreadState_Get(). PyGILState_GetThisThreadState() is subtly different and surfaces some of the tricky PyGILState corner cases that PyThreadState_Get() blissfully ignores. A real issue here is that users are reasonably likely to stumble across PyGILState_GetThisThreadState() and, due to the name, use it instead of the proper API.

I can think of one thing that PyGILState_GetThisThreadState() does exclusively: it will tell you what existing thread state PyGILState_Ensure() will use, if any. It isn't clear to me if that's a meaningful use case.

Ultimately, I expect we would be fine to get rid of PyGILState_GetThisThreadState(). If still useful for core development, it could be moved to the internal C-API.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Start with the C-API documentation entry for PyGILState_GetThisThreadState() and compare its stated behavior with PyThreadState_Get(). Determine the agreed deprecation and removal scope, including whether any internal C-API use must remain; done requires a maintainer-approved plan and corresponding documentation and implementation changes.