GH-119462: Enforce invariants of type versioning (GH-120731)

* Remove uses of Py_TPFLAGS_VALID_VERSION_TAG

Author: markshannon

Diff for: Doc/c-api/typeobj.rst

@@ -1328,8 +1328,8 @@ and :c:data:`PyType_Type` effectively act as defaults.)
       To indicate that a class has changed call :c:func:`PyType_Modified`
 
       .. warning::
-         This flag is present in header files, but is an internal feature and should
-         not be used. It will be removed in a future version of CPython
+         This flag is present in header files, but is not be used.
+         It will be removed in a future version of CPython
 
 
 .. c:member:: const char* PyTypeObject.tp_doc

Diff for: Include/object.h

@@ -566,7 +566,7 @@ given type object has a specified feature.
 /* Objects behave like an unbound method */
 #define Py_TPFLAGS_METHOD_DESCRIPTOR (1UL << 17)
 
-/* Object has up-to-date type attribute cache */
+/* Unused. Legacy flag */
 #define Py_TPFLAGS_VALID_VERSION_TAG  (1UL << 19)
 
 /* Type is abstract and cannot be instantiated */

Diff for: Lib/test/test_type_cache.py

@@ -93,6 +93,21 @@ class C:
         new_version = type_get_version(C)
         self.assertEqual(new_version, 0)
 
+    def test_119462(self):
+
+        class Holder:
+            value = None
+
+            @classmethod
+            def set_value(cls):
+                cls.value = object()
+
+        class HolderSub(Holder):
+            pass
+
+        for _ in range(1050):
+            Holder.set_value()
+            HolderSub.value
 
 @support.cpython_only
 @requires_specialization
@@ -106,8 +121,10 @@ def _assign_valid_version_or_skip(self, type_):
         if type_get_version(type_) == 0:
             self.skipTest("Could not assign valid type version")
 
-    def _assign_and_check_version_0(self, user_type):
+    def _no_more_versions(self, user_type):
         type_modified(user_type)
+        for _ in range(1001):
+            type_assign_specific_version_unsafe(user_type, 1000_000_000)
         type_assign_specific_version_unsafe(user_type, 0)
         self.assertEqual(type_get_version(user_type), 0)
 
@@ -136,7 +153,7 @@ def load_foo_1(type_):
         self._check_specialization(load_foo_1, A, "LOAD_ATTR", should_specialize=True)
         del load_foo_1
 
-        self._assign_and_check_version_0(A)
+        self._no_more_versions(A)
 
         def load_foo_2(type_):
             return type_.foo
@@ -187,7 +204,7 @@ def load_x_1(instance):
         self._check_specialization(load_x_1, G(), "LOAD_ATTR", should_specialize=True)
         del load_x_1
 
-        self._assign_and_check_version_0(G)
+        self._no_more_versions(G)
 
         def load_x_2(instance):
             instance.x
@@ -206,7 +223,7 @@ def store_bar_1(type_):
         self._check_specialization(store_bar_1, B(), "STORE_ATTR", should_specialize=True)
         del store_bar_1
 
-        self._assign_and_check_version_0(B)
+        self._no_more_versions(B)
 
         def store_bar_2(type_):
             type_.bar = 10
@@ -226,7 +243,7 @@ def call_class_1(type_):
         self._check_specialization(call_class_1, F, "CALL", should_specialize=True)
         del call_class_1
 
-        self._assign_and_check_version_0(F)
+        self._no_more_versions(F)
 
         def call_class_2(type_):
             type_()
@@ -245,7 +262,7 @@ def to_bool_1(instance):
         self._check_specialization(to_bool_1, H(), "TO_BOOL", should_specialize=True)
         del to_bool_1
 
-        self._assign_and_check_version_0(H)
+        self._no_more_versions(H)
 
         def to_bool_2(instance):
             not instance

Diff for: Misc/NEWS.d/next/Core and Builtins/2024-06-19-11-10-50.gh-issue-119462.DpcqSe.rst

@@ -0,0 +1,4 @@
+Make sure that invariants of type versioning are maintained:
+* Superclasses always have their version number assigned before subclasses
+* The version tag is always zero if the tag is not valid.
+* The version tag is always non-if the tag is valid.

Diff for: Modules/_testinternalcapi.c

@@ -2014,7 +2014,6 @@ type_assign_specific_version_unsafe(PyObject *self, PyObject *args)
     }
     assert(!PyType_HasFeature(type, Py_TPFLAGS_IMMUTABLETYPE));
     _PyType_SetVersion(type, version);
-    type->tp_flags |= Py_TPFLAGS_VALID_VERSION_TAG;
     Py_RETURN_NONE;
 }
 

Diff for: Modules/pyexpat.c

@@ -1651,7 +1651,7 @@ PyDoc_STRVAR(pyexpat_module_documentation,
 static int init_handler_descrs(pyexpat_state *state)
 {
     int i;
-    assert(!PyType_HasFeature(state->xml_parse_type, Py_TPFLAGS_VALID_VERSION_TAG));
+    assert(state->xml_parse_type->tp_version_tag == 0);
     for (i = 0; handler_info[i].name != NULL; i++) {
         struct HandlerInfo *hi = &handler_info[i];
         hi->getset.name = hi->name;