gh-161471: Fix crash when subscripting a ctypes pointer without _type_

Lib/test/test_ctypes/test_pointers.py

@@ -414,6 +414,20 @@ class BadType(ctypes._Pointer):
             with self.assertRaises(ctypes.ArgumentError):
                 func(object())
 
+    def test_incomplete_pointer_subscript(self):
+        class BadType(ctypes._Pointer):
+            pass
+
+        buf = bytearray(8)
+        buf[0] = 1
+        obj = BadType.from_buffer(buf)
+        with self.assertRaisesRegex(TypeError, "has no _type_"):
+            obj[0:1]
+        with self.assertRaisesRegex(TypeError, "has no _type_"):
+            obj[0]
+        with self.assertRaisesRegex(TypeError, "has no _type_"):
+            obj[0] = 0
+
 class PointerTypeCacheTestCase(unittest.TestCase):
     # dummy tests to check warnings and base behavior
     def tearDown(self):

Modules/_ctypes/_ctypes.c

@@ -5665,14 +5665,21 @@ Pointer_item_impl(PyObject *myself, Py_ssize_t index)
     assert(stginfo); /* Cannot be NULL for pointer object instances */
 
     proto = stginfo->proto;
-    assert(proto);
+    if (proto == NULL) {
+        PyErr_SetString(PyExc_TypeError,
+                        "Cannot access items: has no _type_");
+        return NULL;
+    }
 
     StgInfo *iteminfo;
     if (PyStgInfo_FromType(st, proto, &iteminfo) < 0) {
         return NULL;
     }
-    assert(iteminfo); /* proto is the item type of the pointer, a ctypes
-                         type, so this cannot be NULL */
+    if (iteminfo == NULL) {
+        PyErr_SetString(PyExc_SystemError,
+                        "pointer _type_ has no storage info");
+        return NULL;
+    }
 
     size = iteminfo->size;
     offset = index * iteminfo->size;
@@ -5722,14 +5729,21 @@ Pointer_ass_item_impl(PyObject *myself, Py_ssize_t index, PyObject *value)
     assert(stginfo); /* Cannot be NULL for pointer instances */
 
     proto = stginfo->proto;
-    assert(proto);
+    if (proto == NULL) {
+        PyErr_SetString(PyExc_TypeError,
+                        "Cannot access items: has no _type_");
+        return -1;
+    }
 
     StgInfo *iteminfo;
     if (PyStgInfo_FromType(st, proto, &iteminfo) < 0) {
         return -1;
     }
-    assert(iteminfo); /* Cannot be NULL because the itemtype of a pointer
-                         is always a ctypes type */
+    if (iteminfo == NULL) {
+        PyErr_SetString(PyExc_SystemError,
+                        "pointer _type_ has no storage info");
+        return -1;
+    }
 
     size = iteminfo->size;
     offset = index * iteminfo->size;
@@ -5957,12 +5971,20 @@ Pointer_subscript(PyObject *myself, PyObject *item)
         }
         assert(stginfo); /* Cannot be NULL for pointer instances */
         proto = stginfo->proto;
-        assert(proto);
+        if (proto == NULL) {
+            PyErr_SetString(PyExc_TypeError,
+                            "Cannot access items: has no _type_");
+            return NULL;
+        }
         StgInfo *iteminfo;
         if (PyStgInfo_FromType(st, proto, &iteminfo) < 0) {
             return NULL;
         }
-        assert(iteminfo);
+        if (iteminfo == NULL) {
+            PyErr_SetString(PyExc_SystemError,
+                            "pointer _type_ has no storage info");
+            return NULL;
+        }
         if (iteminfo->getfunc == _ctypes_get_fielddesc("c")->getfunc) {
             char *dest;