Defend against a mutation during comparison

diff --git a/Modules/_heapqmodule.c b/Modules/_heapqmodule.c
index d0ddb09fb1593372a9d3ce5ca4a06a9f69e40141..be301a62fc6222a833a8ecd078be1a7040084f23 100644 (file)
--- a/Modules/_heapqmodule.c
+++ b/Modules/_heapqmodule.c
@@ -244,6 +244,11 @@ heappushpop(PyObject *self, PyObject *args)
         return item;
     }
 
+    if (PyList_GET_SIZE(heap) == 0) {
+        PyErr_SetString(PyExc_IndexError, "index out of range");
+        return NULL;
+    }
+
     returnitem = PyList_GET_ITEM(heap, 0);
     Py_INCREF(item);
     PyList_SET_ITEM(heap, 0, item);