Fix a crash when the return value of a subgenerator is a temporary

object (with a refcount of 1)

Lib/test/test_pep380.py

@@ -364,6 +364,17 @@ class TestPEP380Operation(unittest.TestCase):
         ])
 
 
+    def test_exception_value_crash(self):
+        # There used to be a refcount error when the return value
+        # stored in the StopIteration has a refcount of 1.
+        def g1():
+            yield from g2()
+        def g2():
+            yield "g2"
+            return [42]
+        self.assertEqual(list(g1()), ["g2"])
+
+
     def test_generator_return_value(self):
         """
         Test generator return value

Objects/genobject.c

@@ -475,6 +475,7 @@ PyGen_FetchStopIterationValue(PyObject **pvalue) {
         Py_XDECREF(tb);
         if (ev) {
             value = ((PyStopIterationObject *)ev)->value;
+            Py_INCREF(value);
             Py_DECREF(ev);
         }
     } else if (PyErr_Occurred()) {
@@ -482,8 +483,8 @@ PyGen_FetchStopIterationValue(PyObject **pvalue) {
     }
     if (value == NULL) {
         value = Py_None;
+        Py_INCREF(value);
     }
-    Py_INCREF(value);
     *pvalue = value;
     return 0;
 }