Fixed bug where chord.link_error() throws an exception on a dict type…

… errback object (#8702)

* Fixed bug where _chord.link_error() would call clone() on a dict instead of a signature

* Added unit test: test_flag_allow_error_cb_on_chord_header_with_dict_callback()

celery/canvas.py

@@ -2271,6 +2271,8 @@ def link_error(self, errback):
             ``False`` (the current default), then the error callback will only be
             applied to the body.
         """
+        errback = maybe_signature(errback)
+
         if self.app.conf.task_allow_error_cb_on_chord_header:
             for task in maybe_list(self.tasks) or []:
                 task.link_error(errback.clone(immutable=True))

t/unit/tasks/test_canvas.py

@@ -1688,6 +1688,14 @@ def test_flag_allow_error_cb_on_chord_header_various_header_types(self):
             errback = c.link_error(sig)
             assert errback == sig
 
+    @pytest.mark.usefixtures('depends_on_current_app')
+    def test_flag_allow_error_cb_on_chord_header_with_dict_callback(self):
+        self.app.conf.task_allow_error_cb_on_chord_header = True
+        c = chord(group(signature('th1'), signature('th2')), signature('tbody'))
+        errback_dict = dict(signature('tcb'))
+        errback = c.link_error(errback_dict)
+        assert errback == errback_dict
+
     def test_chord__or__group_of_single_task(self):
         """ Test chaining a chord to a group of a single task. """
         c = chord([signature('header')], signature('body'))