Try to handle errors in context string building

Author: carljm

Objects/codeobject.c

@@ -30,11 +30,18 @@ notify_code_watchers(PyCodeEvent event, PyCodeObject *co)
             if (cb(event, co) < 0) {
                 // Don't risk resurrecting the object if an unraisablehook keeps
                 // a reference; pass a string as context.
+                PyObject *context = NULL;
                 PyObject *repr = code_repr(co);
-                PyObject *context = PyUnicode_FromFormat("watcher callback for %U", repr);
+                if (repr) {
+                    context = PyUnicode_FromFormat("watcher callback for %U", repr);
+                    Py_DECREF(repr);
+                }
+                if (context == NULL) {
+                    context = Py_None;
+                    Py_INCREF(context);
+                }
                 PyErr_WriteUnraisable(context);
                 Py_DECREF(context);
-                Py_DECREF(repr);
             }
         }
         i++;

Objects/dictobject.c

@@ -5756,6 +5756,10 @@ _PyDict_SendEvent(int watcher_bits,
                 // dict as context, just an informative string message.  Dict
                 // repr can call arbitrary code, so we invent a simpler version.
                 PyObject *context = PyUnicode_FromFormat("watcher callback for <dict at %p>", mp);
+                if (context == NULL) {
+                    context = Py_None;
+                    Py_INCREF(context);
+                }
                 PyErr_WriteUnraisable(context);
                 Py_DECREF(context);
             }

Objects/funcobject.c

@@ -25,11 +25,18 @@ notify_func_watchers(PyInterpreterState *interp, PyFunction_WatchEvent event,
             if (cb(event, func, new_value) < 0) {
                 // Don't risk resurrecting the func if an unraisablehook keeps a
                 // reference; pass a string as context.
+                PyObject *context = NULL;
                 PyObject *repr = func_repr(func);
-                PyObject *context = PyUnicode_FromFormat("watcher callback for %U", repr);
+                if (repr != NULL) {
+                    context = PyUnicode_FromFormat("watcher callback for %U", repr);
+                    Py_DECREF(repr);
+                }
+                if (context == NULL) {
+                    context = Py_None;
+                    Py_INCREF(context);
+                }
                 PyErr_WriteUnraisable(context);
                 Py_DECREF(context);
-                Py_DECREF(repr);
             }
         }
         i++;