Fix incorrect thread summaries when the latest event is edited. (#11992)

If the latest event in a thread was edited than the original
event content was included in bundled aggregation for
threads instead of the edited event content.

tests/rest/client/test_relations.py

@@ -1123,6 +1123,48 @@ class RelationsTestCase(unittest.HomeserverTestCase):
             {"event_id": edit_event_id, "sender": self.user_id}, m_replace_dict
         )
 
+    @unittest.override_config({"experimental_features": {"msc3440_enabled": True}})
+    def test_edit_thread(self):
+        """Test that editing a thread works."""
+
+        # Create a thread and edit the last event.
+        channel = self._send_relation(
+            RelationTypes.THREAD,
+            "m.room.message",
+            content={"msgtype": "m.text", "body": "A threaded reply!"},
+        )
+        self.assertEquals(200, channel.code, channel.json_body)
+        threaded_event_id = channel.json_body["event_id"]
+
+        new_body = {"msgtype": "m.text", "body": "I've been edited!"}
+        channel = self._send_relation(
+            RelationTypes.REPLACE,
+            "m.room.message",
+            content={"msgtype": "m.text", "body": "foo", "m.new_content": new_body},
+            parent_id=threaded_event_id,
+        )
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        # Fetch the thread root, to get the bundled aggregation for the thread.
+        channel = self.make_request(
+            "GET",
+            f"/rooms/{self.room}/event/{self.parent_id}",
+            access_token=self.user_token,
+        )
+        self.assertEquals(200, channel.code, channel.json_body)
+
+        # We expect that the edit message appears in the thread summary in the
+        # unsigned relations section.
+        relations_dict = channel.json_body["unsigned"].get("m.relations")
+        self.assertIn(RelationTypes.THREAD, relations_dict)
+
+        thread_summary = relations_dict[RelationTypes.THREAD]
+        self.assertIn("latest_event", thread_summary)
+        latest_event_in_thread = thread_summary["latest_event"]
+        self.assertEquals(
+            latest_event_in_thread["content"]["body"], "I've been edited!"
+        )
+
     def test_edit_edit(self):
         """Test that an edit cannot be edited."""
         new_body = {"msgtype": "m.text", "body": "Initial edit"}