Store tp_weaklist on the interpreter state for static builtin types.

Author: ericsnowcurrently

Include/cpython/object.h

@@ -219,7 +219,7 @@ struct _typeobject {
     PyObject *tp_mro; /* method resolution order */
     PyObject *tp_cache;
     PyObject *tp_subclasses;
-    PyObject *tp_weaklist;
+    PyObject *tp_weaklist; /* not used for static builtin types */
     destructor tp_del;
 
     /* Type attribute cache version tag. Added in version 2.6 */

Include/internal/pycore_object.h

@@ -220,6 +220,12 @@ extern void _Py_PrintReferenceAddresses(FILE *);
 static inline PyObject **
 _PyObject_GET_WEAKREFS_LISTPTR(PyObject *op)
 {
+    if (PyType_Check(op) &&
+            ((PyTypeObject *)op)->tp_flags & _Py_TPFLAGS_STATIC_BUILTIN) {
+        static_builtin_state *state = _PyStaticType_GetState(
+                                                        (PyTypeObject *)op);
+        return &state->tp_weaklist;
+    }
     Py_ssize_t offset = Py_TYPE(op)->tp_weaklistoffset;
     return (PyObject **)((char *)op + offset);
 }

Include/internal/pycore_typeobject.h

@@ -45,6 +45,11 @@ struct type_cache {
 
 typedef struct {
     PyTypeObject *type;
+    /* We never clean up weakrefs for static builtin types since
+       they will effectively never get triggered.  However, there
+       are also some diagnostic uses for the list of weakrefs,
+       so we still keep it. */
+    PyObject *tp_weaklist;
 } static_builtin_state;
 
 struct types_state {

Objects/typeobject.c

@@ -127,6 +127,8 @@ static_builtin_state_init(PyTypeObject *self)
 
     static_builtin_state *state = static_builtin_state_get(interp, self);
     state->type = self;
+    /* state->tp_weaklist is left NULL until insert_head() or insert_after()
+       (in weakrefobject.c) sets it. */
 }
 
 static void
@@ -138,6 +140,7 @@ static_builtin_state_clear(PyTypeObject *self)
 
     static_builtin_state *state = static_builtin_state_get(interp, self);
     state->type = NULL;
+    /* state->tp_weaklist should already have been cleared out. */
     static_builtin_index_clear(self);
 
     assert(interp->types.num_builtins_initialized > 0);
@@ -502,6 +505,8 @@ static PyMemberDef type_members[] = {
     {"__basicsize__", T_PYSSIZET, offsetof(PyTypeObject,tp_basicsize),READONLY},
     {"__itemsize__", T_PYSSIZET, offsetof(PyTypeObject, tp_itemsize), READONLY},
     {"__flags__", T_ULONG, offsetof(PyTypeObject, tp_flags), READONLY},
+    /* Note that this value is misleading for static builtin types,
+       since the memory at this offset will always be NULL. */
     {"__weakrefoffset__", T_PYSSIZET,
      offsetof(PyTypeObject, tp_weaklistoffset), READONLY},
     {"__base__", T_OBJECT, offsetof(PyTypeObject, tp_base), READONLY},