[BUG] Traceback reference leak in CPython 3.12.0a6 and above

[yut23]

Describe the bug

In CPython 3.12.0a6 and newer with CYTHON_FAST_THREAD_STATE=1, traceback objects for Python code called from Cython have their refcounts incremented but never decremented. This is triggered when Cython code calls into a Python function that then raises an exception. The source of the leak is __Pyx_ErrRestoreInState(), which never calls Py_XDECREF on tb:

cython/Cython/Utility/Exceptions.c

Lines 178 to 193 in 9827c60

	static CYTHON_INLINE void __Pyx_ErrRestoreInState(PyThreadState *tstate, PyObject *type, PyObject *value, PyObject *tb) {
	#if PY_VERSION_HEX >= 0x030C00A6
	PyObject *tmp_value;
	assert(type == NULL || (value != NULL && type == (PyObject*) Py_TYPE(value)));
	CYTHON_UNUSED_VAR(type);
	if (value) {
	#if CYTHON_COMPILING_IN_CPYTHON
	if (unlikely(((PyBaseExceptionObject*) value)->traceback != tb))
	#endif
	// If this fails, we may lose the traceback but still set the expected exception below.
	PyException_SetTraceback(value, tb);
	}
	tmp_value = tstate->current_exception;
	tstate->current_exception = value;
	Py_XDECREF(tmp_value);
	#else

Code to reproduce the behaviour:

lib.pyx:

def get_spam(data):
    # this will raise a KeyError in the Python code for UserDict.__getitem__()
    return data["spam"]

repro.py:

from collections import UserDict
from lib import get_spam

def main():
    d = UserDict()
    d["lots of data"] = bytearray(1024**2)
    try:
        get_spam(d)
    except KeyError:
        pass

for i in range(10):
    main()

I've uploaded a full reproducer to https://github.com/yut23/cython_3.12_traceback_leak, along with some code I used to track down the leaks.

Expected behaviour

No response

OS

Linux, macOS

Python version

3.12.0a6+

Cython version

3.0.0+

Additional context

We first encountered this when running yt's test suite on Python 3.12, and reported it to CPython in python/cpython#109602. Thanks to @neutrinoceros, @Xarthisius, and @mdboom for their work in simplifying the reproducer!

I'll make a PR with a proposed fix shortly.

[ThomasWaldmann]

@scoder Does this issue also apply to Cython 0.29.x and if so, was a fix already released (could not find it in changelog)?

[scoder]

The issue milestone says 3.0.3, so the fix was (probably) not backported. Looks more like a 3.0.x issue to me. There were a lot of changes in the exception handling code since 0.29.x. It doesn't look like 0.29.x has any special casing code for Py3.12 here.