GH-95245: Move weakreflist into the pre-header. (GH-95996)

1 files changed, 28 insertions, 15 deletions

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 67dfc6f..c881aeb 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -2497,10 +2497,11 @@ subtype_getweakref(PyObject *obj, void *context)
         return NULL;
     }
     _PyObject_ASSERT((PyObject *)type,
-                     type->tp_weaklistoffset > 0);
+                     type->tp_weaklistoffset > 0 ||
+                     type->tp_weaklistoffset == MANAGED_WEAKREF_OFFSET);
     _PyObject_ASSERT((PyObject *)type,
-                     ((type->tp_weaklistoffset + sizeof(PyObject *))
-                      <= (size_t)(type->tp_basicsize)));
+                     ((type->tp_weaklistoffset + (Py_ssize_t)sizeof(PyObject *))
+                      <= type->tp_basicsize));
     weaklistptr = (PyObject **)((char *)obj + type->tp_weaklistoffset);
     if (*weaklistptr == NULL)
         result = Py_None;
@@ -3093,9 +3094,9 @@ type_new_descriptors(const type_new_ctx *ctx, PyTypeObject *type)
     }
 
     if (ctx->add_weak) {
-        assert(!ctx->base->tp_itemsize);
-        type->tp_weaklistoffset = slotoffset;
-        slotoffset += sizeof(PyObject *);
+        assert((type->tp_flags & Py_TPFLAGS_MANAGED_WEAKREF) == 0);
+        type->tp_flags |= Py_TPFLAGS_MANAGED_WEAKREF;
+        type->tp_weaklistoffset = MANAGED_WEAKREF_OFFSET;
     }
     if (ctx->add_dict) {
         assert((type->tp_flags & Py_TPFLAGS_MANAGED_DICT) == 0);
@@ -5116,9 +5117,9 @@ compatible_for_assignment(PyTypeObject* oldto, PyTypeObject* newto, const char*
          !same_slots_added(newbase, oldbase))) {
         goto differs;
     }
-    /* The above does not check for managed __dicts__ */
-    if ((oldto->tp_flags & Py_TPFLAGS_MANAGED_DICT) ==
-        ((newto->tp_flags & Py_TPFLAGS_MANAGED_DICT)))
+    /* The above does not check for the preheader */
+    if ((oldto->tp_flags & Py_TPFLAGS_PREHEADER) ==
+        ((newto->tp_flags & Py_TPFLAGS_PREHEADER)))
     {
         return 1;
     }
@@ -5217,7 +5218,7 @@ object_set_class(PyObject *self, PyObject *value, void *closure)
     if (compatible_for_assignment(oldto, newto, "__class__")) {
         /* Changing the class will change the implicit dict keys,
          * so we must materialize the dictionary first. */
-        assert((oldto->tp_flags & Py_TPFLAGS_MANAGED_DICT) == (newto->tp_flags & Py_TPFLAGS_MANAGED_DICT));
+        assert((oldto->tp_flags & Py_TPFLAGS_PREHEADER) == (newto->tp_flags & Py_TPFLAGS_PREHEADER));
         _PyObject_GetDictPtr(self);
         if (oldto->tp_flags & Py_TPFLAGS_MANAGED_DICT &&
             _PyDictOrValues_IsValues(*_PyObject_DictOrValuesPointer(self)))
@@ -5360,7 +5361,7 @@ object_getstate_default(PyObject *obj, int required)
         {
             basicsize += sizeof(PyObject *);
         }
-        if (Py_TYPE(obj)->tp_weaklistoffset) {
+        if (Py_TYPE(obj)->tp_weaklistoffset > 0) {
             basicsize += sizeof(PyObject *);
         }
         if (slotnames != Py_None) {
@@ -6150,7 +6151,7 @@ inherit_special(PyTypeObject *type, PyTypeObject *base)
         if (type->tp_clear == NULL)
             type->tp_clear = base->tp_clear;
     }
-    type->tp_flags |= (base->tp_flags & Py_TPFLAGS_MANAGED_DICT);
+    type->tp_flags |= (base->tp_flags & Py_TPFLAGS_PREHEADER);
 
     if (type->tp_basicsize == 0)
         type->tp_basicsize = base->tp_basicsize;
@@ -6571,7 +6572,7 @@ type_ready_fill_dict(PyTypeObject *type)
 }
 
 static int
-type_ready_dict_offset(PyTypeObject *type)
+type_ready_preheader(PyTypeObject *type)
 {
     if (type->tp_flags & Py_TPFLAGS_MANAGED_DICT) {
         if (type->tp_dictoffset > 0 || type->tp_dictoffset < -1) {
@@ -6583,6 +6584,18 @@ type_ready_dict_offset(PyTypeObject *type)
         }
         type->tp_dictoffset = -1;
     }
+    if (type->tp_flags & Py_TPFLAGS_MANAGED_WEAKREF) {
+        if (type->tp_weaklistoffset != 0 &&
+            type->tp_weaklistoffset != MANAGED_WEAKREF_OFFSET)
+        {
+            PyErr_Format(PyExc_TypeError,
+                        "type %s has the Py_TPFLAGS_MANAGED_WEAKREF flag "
+                        "but tp_weaklistoffset is set",
+                        type->tp_name);
+            return -1;
+        }
+        type->tp_weaklistoffset = MANAGED_WEAKREF_OFFSET;
+    }
     return 0;
 }
 
@@ -6802,7 +6815,7 @@ type_ready_post_checks(PyTypeObject *type)
             return -1;
         }
     }
-    else if (type->tp_dictoffset < sizeof(PyObject)) {
+    else if (type->tp_dictoffset < (Py_ssize_t)sizeof(PyObject)) {
         if (type->tp_dictoffset + type->tp_basicsize <= 0) {
             PyErr_Format(PyExc_SystemError,
                          "type %s has a tp_dictoffset that is too small");
@@ -6847,7 +6860,7 @@ type_ready(PyTypeObject *type)
     if (type_ready_inherit(type) < 0) {
         return -1;
     }
-    if (type_ready_dict_offset(type) < 0) {
+    if (type_ready_preheader(type) < 0) {
         return -1;
     }
     if (type_ready_set_hash(type) < 0) {