_Py_IsFinalizing has been removed, meaning C/C++ threads cannot avoid termination during shutdown

[dgrisby]

Checklist

• I am confident this is a bug in CPython, not a bug in a third-party project
• I have searched the CPython issue tracker,
  and am confident this bug has not been reported before

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Output from running 'python -VV' on the command line:

Python 3.13.0a0 (main, Jul 29 2023, 00:25:05) [GCC 13.1.1 20230614 (Red Hat 13.1.1-4)]

A clear and concise description of the bug:

Threads that call PyEval_RestoreThread while the runtime is finalizing are terminated. The documented way to avoid this is to call _Py_IsFinalizing() to check. https://docs.python.org/3.13/c-api/init.html#c.PyEval_RestoreThread

_Py_IsFinalizing() has been removed from Python 3.13. Please provide a public API function that has the equivalent behavior, or some other way for a non-Python thread to safely attempt to call into Python code when the interpreter may be finalizing.

Rather than the calling code checking for finalization, an alternative solution would be to provide a variant of PyEval_RestoreThread that returns success or failure, so the calling code can know that it was unable to acquire the GIL, and react accordingly.

Linked PRs

• gh-108014: Add Py_IsFinalizing() function #108032

[vstinner]

Since sys.is_finalizing() was added to Python 3.5, IMO it's fine to add a public C API function for that: https://docs.python.org/dev/library/sys.html#sys.is_finalizing

cc @ericsnowcurrently

[ericsnowcurrently]

FYI, there is some work to hang remaining threads rather than killing them: gh-105805. I expect that would land in 3.13.
Presumably a Py_IsFinalizing() would still be helpful though.

[erlend-aasland]

I think it would make sense to also make _Py_IsInterpreterFinalizing public:

cpython/Python/pylifecycle.c

Lines 2181 to 2191 in 1344cfa

	int
	_Py_IsInterpreterFinalizing(PyInterpreterState *interp)
	{
	/* We check the runtime first since, in a daemon thread,
	interp might be dangling pointer. */
	PyThreadState *finalizing = _PyRuntimeState_GetFinalizing(&_PyRuntime);
	if (finalizing == NULL) {
	finalizing = _PyInterpreterState_GetFinalizing(interp);
	}
	return finalizing != NULL;
	}

[vstinner]

Function added by commit 3ff5ef2.

[vstinner]

Function added to pythoncapi-compat by commit python/pythoncapi-compat@1bb4e16.

[vstinner]

I proposed adding PythonFinalizationError exception in PR gh-109809.

[vstinner]

Follow-up: @wjakob asks to add this new function to the limited C API version 3.13.