[3.13] gh-91153: prevent a crash in bytearray.__setitem__(ind, ...) when ind.__index__ has side-effects (GH-132379) (#136582)

(cherry picked from commit 5e1e21dee3)

Co-authored-by: Bast <52266665+bast0006@users.noreply.github.com>

Lib/test/test_bytes.py

@@ -1827,6 +1827,8 @@ def test_repeat_after_setslice(self):
         self.assertEqual(b3, b'xcxcxc')
 
     def test_mutating_index(self):
+        # bytearray slice assignment can call into python code
+        # that reallocates the internal buffer
         # See gh-91153
 
         class Boom:
@@ -1844,6 +1846,39 @@ def __index__(self):
             with self.assertRaises(IndexError):
                 self._testlimitedcapi.sequence_setitem(b, 0, Boom())
 
+    def test_mutating_index_inbounds(self):
+        # gh-91153 continued
+        # Ensure buffer is not broken even if length is correct
+
+        class MutatesOnIndex:
+            def __init__(self):
+                self.ba = bytearray(0x180)
+
+            def __index__(self):
+                self.ba.clear()
+                self.new_ba = bytearray(0x180)  # to catch out-of-bounds writes
+                self.ba.extend([0] * 0x180)     # to check bounds checks
+                return 0
+
+        with self.subTest("skip_bounds_safety"):
+            instance = MutatesOnIndex()
+            instance.ba[instance] = ord("?")
+            self.assertEqual(instance.ba[0], ord("?"), "Assigned bytearray not altered")
+            self.assertEqual(instance.new_ba, bytearray(0x180), "Wrong object altered")
+
+        with self.subTest("skip_bounds_safety_capi"):
+            instance = MutatesOnIndex()
+            instance.ba[instance] = ord("?")
+            self._testlimitedcapi.sequence_setitem(instance.ba, instance, ord("?"))
+            self.assertEqual(instance.ba[0], ord("?"), "Assigned bytearray not altered")
+            self.assertEqual(instance.new_ba, bytearray(0x180), "Wrong object altered")
+
+        with self.subTest("skip_bounds_safety_slice"):
+            instance = MutatesOnIndex()
+            instance.ba[instance:1] = [ord("?")]
+            self.assertEqual(instance.ba[0], ord("?"), "Assigned bytearray not altered")
+            self.assertEqual(instance.new_ba, bytearray(0x180), "Wrong object altered")
+
 
 class AssortedBytesTest(unittest.TestCase):
     #