gh-103987: fix crash in mmap module

Lib/test/test_mmap.py

@@ -264,6 +264,26 @@ def test_bad_file_desc(self):
         # Try opening a bad file descriptor...
         self.assertRaises(OSError, mmap.mmap, -2, 4096)
 
+    def test_if_crash(self): # test for issue gh-103987
+        with open("TESTFN", "wb+") as f:
+            data = b'aabaac\x00deef\x00\x00aa\x00'
+            n = len(data)
+            f.write(data)
+            f.flush()
+
+            class X:
+                def __index__(self):
+                    m.close()
+                    return 1
+
+            m = mmap.mmap(f.fileno(), n, access=mmap.ACCESS_READ)
+
+        self.assertEqual(m[1], 97)
+        with self.assertRaises(ValueError):
+            m[X()] # should not crash
+
+        m.close()
+
     def test_tougher_find(self):
         # Do a tougher .find() test.  SF bug 515943 pointed out that, in 2.2,
         # searching for data with embedded \0 bytes didn't work.

Modules/mmapmodule.c

@@ -933,9 +933,9 @@ mmap_item(mmap_object *self, Py_ssize_t i)
 static PyObject *
 mmap_subscript(mmap_object *self, PyObject *item)
 {
-    CHECK_VALID(NULL);
     if (PyIndex_Check(item)) {
         Py_ssize_t i = PyNumber_AsSsize_t(item, PyExc_IndexError);
+        CHECK_VALID(NULL);
         if (i == -1 && PyErr_Occurred())
             return NULL;
         if (i < 0)