Issue #23996: Avoid a crash when a delegated generator raises an unnormalized…

Issue #23996: Avoid a crash when a delegated generator raises an unnormalized StopIteration exception.  Patch by Stefan Behnel.

Misc/NEWS

@@ -10,6 +10,9 @@ Release date: tba
 Core and Builtins
 -----------------
 
+- Issue #23996: Avoid a crash when a delegated generator raises an
+  unnormalized StopIteration exception.  Patch by Stefan Behnel.
+
 - Issue #24022: Fix tokenizer crash when processing undecodable source code.
 
 - Issue #23309: Avoid a deadlock at shutdown if a daemon thread is aborted

Objects/genobject.c

@@ -396,13 +396,30 @@ _PyGen_FetchStopIterationValue(PyObject **pvalue) {
 
     if (PyErr_ExceptionMatches(PyExc_StopIteration)) {
         PyErr_Fetch(&et, &ev, &tb);
-        Py_XDECREF(et);
-        Py_XDECREF(tb);
         if (ev) {
-            value = ((PyStopIterationObject *)ev)->value;
-            Py_INCREF(value);
-            Py_DECREF(ev);
+            /* exception will usually be normalised already */
+            if (Py_TYPE(ev) == (PyTypeObject *) et
+                || PyObject_IsInstance(ev, PyExc_StopIteration)) {
+                value = ((PyStopIterationObject *)ev)->value;
+                Py_INCREF(value);
+                Py_DECREF(ev);
+            } else if (et == PyExc_StopIteration) {
+                /* avoid normalisation and take ev as value */
+                value = ev;
+            } else {
+                /* normalisation required */
+                PyErr_NormalizeException(&et, &ev, &tb);
+                if (!PyObject_IsInstance(ev, PyExc_StopIteration)) {
+                    PyErr_Restore(et, ev, tb);
+                    return -1;
+                }
+                value = ((PyStopIterationObject *)ev)->value;
+                Py_INCREF(value);
+                Py_DECREF(ev);
+            }
         }
+        Py_XDECREF(et);
+        Py_XDECREF(tb);
     } else if (PyErr_Occurred()) {
         return -1;
     }