Add is_subclass to PyWeakReference and do not clear those weakrefs in GC

Author: sergey-miryanov

Include/cpython/weakrefobject.h

@@ -38,6 +38,7 @@ struct _PyWeakReference {
      */
     PyMutex *weakrefs_lock;
 #endif
+    uint8_t is_subclass;
 };
 
 PyAPI_FUNC(void) _PyWeakref_ClearRef(PyWeakReference *self);

Objects/typeobject.c

@@ -9265,6 +9265,9 @@ add_subclass(PyTypeObject *base, PyTypeObject *type)
         return -1;
     }
 
+    ((PyWeakReference*)ref)->is_subclass = 1;
+
+
     // Only get tp_subclasses after creating the key and value.
     // PyWeakref_NewRef() can trigger a garbage collection which can execute
     // arbitrary Python code and so modify base->tp_subclasses.

Objects/weakrefobject.c

@@ -72,6 +72,7 @@ init_weakref(PyWeakReference *self, PyObject *ob, PyObject *callback)
     _PyObject_SetMaybeWeakref(ob);
     _PyObject_SetMaybeWeakref((PyObject *)self);
 #endif
+    self->is_subclass = 0;
 }
 
 // Clear the weakref and steal its callback into `callback`, if provided.