[C API] PyOS_ReadlineFunctionPointer API rely on private _PyOS_ReadlineTState variable removed in Python 3.13

[vstinner]

Hi,

I removed the _PyOS_ReadlineTState variable from the public C API (it was moved to the internal C API). The gdb project sets its readline callback and uses the _PyOS_ReadlineTState variable and so fails to build on Python 3.13.0 alpha2: https://bugzilla.redhat.com/show_bug.cgi?id=2250652

gdb: PyOS_ReadlineFunctionPointer = gdbpy_readline_wrapper;

The gdb function uses _PyOS_ReadlineTState to use the Python C API:

PyEval_RestoreThread (_PyOS_ReadlineTState);
gdbpy_convert_exception (except);
PyEval_SaveThread ();

source: py-gdb-readline.c

The _PyOS_ReadlineTState variable was added in 2004 by commit 30ea2f2.

The callback is called with the GIL released. The _PyOS_ReadlineTState variable is used to acquire again the GIL.

---

In 2020, I made fixes related to GIL but not directly to this issue:

• commit c353764
• commit fa7ab6a

[vstinner]

The documentation https://docs.python.org/dev/c-api/veryhigh.html#c.PyOS_ReadlineFunctionPointer should also mention the internal lock, at least explain that the callback cannot be called twice at the same time in two threads.

[vstinner]

Well, if a readline callback needs to use the Python C API, PyGILState_Ensure() and PyGILState_Release() to be used to temporarily acquire/release the GIL. I'm not sure of the advantage of using PyEval_RestoreThread (_PyOS_ReadlineTState) and PyEval_SaveThread() instead.