Add some comments

Author: sergey-miryanov

Include/internal/pycore_weakref.h

@@ -128,8 +128,13 @@ extern void _PyWeakref_ClearWeakRefsNoCallbacks(PyObject *obj);
 
 PyAPI_FUNC(int) _PyWeakref_IsDead(PyObject *weakref);
 
+// Create sentinel callback object for subclasses weakrefs
 int _PyWeakref_InitSubclassSentinel(PyInterpreterState *interp);
+
+// Create new PyWeakReference object with sentinel callback
 PyObject * _PyWeakref_NewSubclassRef(PyObject *ob);
+
+// Check if weakref has sentinel callback
 int _PyWeakref_IsSubclassRef(PyWeakReference *weakref);
 
 #ifdef __cplusplus

Python/gc.c

@@ -906,6 +906,8 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
              * reference cycle.  If we don't clear the weakref, the callback
              * will run and potentially cause a crash.  See bpo-38006 for
              * one example.
+             *
+             * If this is a subclass weakref we can safely ignore it's cleanup.
              */
             if (!_PyWeakref_IsSubclassRef((PyWeakReference *)op)) {
                 _PyWeakref_ClearRef((PyWeakReference *)op);
@@ -935,6 +937,11 @@ handle_weakrefs(PyGC_Head *unreachable, PyGC_Head *old)
             PyGC_Head *wrasgc;                  /* AS_GC(wr) */
 
             wr_next = wr->wr_next;
+
+            /* If this is a subclass weakref we can safely ignore it's cleanup.
+             * It has only sentinel callback (no-op) and we also can safely
+             * not invoke them.
+             */
             if (_PyWeakref_IsSubclassRef(wr) == 1) {
                 continue;
             }