Callable iterator can SystemError if call exhausts the iterator

[ionite34]

Bug report

If the callable of a callable iterator (with sentinel) exhausts the iterator itself during the call, a SystemError occurs.

Example discovered by @chilaxan

def bug():
    if bug.clear:
        return 1
    else:
        bug.clear = True
        list(bug.iterator)
        return 0

bug.iterator = iter(bug, 1)
bug.clear = False
next(bug.iterator)

SystemError: Objects\object.c:722: bad argument to internal function

Likely cause

iterobject.calliter_iternext does not check whether it->it_sentinel is NULL after the _PyObject_CallNoArgs call
https://github.com/python/cpython/blob/main/Objects/iterobject.c#L207-L237

static PyObject *
calliter_iternext(calliterobject *it)
{
    PyObject *result;

    if (it->it_callable == NULL) {
        return NULL;
    }

+   result = _PyObject_CallNoArgs(it->it_callable);
    if (result != NULL) {
        int ok;

+       ok = PyObject_RichCompareBool(it->it_sentinel, result, Py_EQ);
        if (ok == 0) {
            return result; /* Common case, fast path */
        }
(...)

Your environment

Appears to affect 3.7-3.12

Linked PRs

• gh-101892: Fix SystemError when a callable iterator call exhausts the iterator #101896
• [3.11] gh-101892: Fix SystemError when a callable iterator call exhausts the iterator (GH-101896) #102418
• [3.10] gh-101892: Fix SystemError when a callable iterator call exhausts the iterator (GH-101896) #102422