gh-126595: fix a crash when calling itertools.count(sys.maxsize)

Lib/test/test_itertools.py

@@ -494,6 +494,14 @@ def test_count(self):
         self.assertEqual(take(2, zip('abc',count(-3))), [('a', -3), ('b', -2)])
         self.assertRaises(TypeError, count, 2, 3, 4)
         self.assertRaises(TypeError, count, 'a')
+        self.assertEqual(take(3, count(maxsize)),
+                        [maxsize, maxsize + 1, maxsize + 2])
+        self.assertEqual(take(3, count(maxsize, 2)),
+                         [maxsize, maxsize + 2, maxsize + 4])
+        self.assertEqual(take(3, count(maxsize, maxsize)),
+                         [maxsize, 2 * maxsize, 3 * maxsize])
+        self.assertEqual(take(3, count(-maxsize, maxsize)),
+                        [-maxsize, 0, maxsize])
         self.assertEqual(take(10, count(maxsize-5)),
                          list(range(maxsize-5, maxsize+5)))
         self.assertEqual(take(10, count(-maxsize-5)),

Misc/NEWS.d/next/Library/2024-11-09-10-31-10.gh-issue-126595.A-7MyC.rst

@@ -0,0 +1,2 @@
+Fix a crash when instantiating :class:`itertools.count` with an initial
+count of :data:`sys.maxsize` on debug builds. Patch by Bénédikt Tran.

Modules/itertoolsmodule.c

@@ -3287,7 +3287,10 @@ itertools_count_impl(PyTypeObject *type, PyObject *long_cnt,
         if (fast_mode) {
             assert(PyLong_Check(long_cnt));
             cnt = PyLong_AsSsize_t(long_cnt);
-            if (cnt == -1 && PyErr_Occurred()) {
+            if (cnt == PY_SSIZE_T_MAX) {
+                fast_mode = 0;
+            }
+            else if (cnt == -1 && PyErr_Occurred()) {
                 PyErr_Clear();
                 fast_mode = 0;
             }