Don't crash when assigning attributes of the GeneratorExit const singleton

Author: jepler

py/objexcept.c

@@ -218,7 +218,10 @@ void mp_obj_exception_attr(mp_obj_t self_in, qstr attr, mp_obj_t *dest) {
     mp_obj_exception_t *self = MP_OBJ_TO_PTR(self_in);
     if (dest[0] != MP_OBJ_NULL) {
         // store/delete attribute
-        if (attr == MP_QSTR___traceback__) {
+        if (self == &mp_const_GeneratorExit_obj) {
+            // Can't set attributes, but don't want to crash...
+            dest[0] = MP_OBJ_NULL; // indicate success
+        } else if (attr == MP_QSTR___traceback__) {
             if (dest[1] == mp_const_none) {
                 self->traceback = (mp_obj_traceback_t *)&mp_const_empty_traceback_obj;
             } else {

tests/basics/gen_yield_from_close.py

@@ -37,8 +37,9 @@ def gen4():
     yield -1
     try:
         print((yield from gen3()))
-    except GeneratorExit:
+    except GeneratorExit as e:
         print("delegating caught GeneratorExit")
+        e.__traceback__ = None
         raise
     yield 10
     yield 11