message_edit: Ignore duplicates when re-muting new topic name.

This avoids an error when a user has already muted the new topic name.
We do this by ignoring duplicates, rather than catching the
IntegrityError, because this edit happens in a transaction, and that
would abort the transaction.

zerver/lib/actions.py

@@ -7246,6 +7246,7 @@ def do_update_message(
                     muting_user,
                     new_stream if new_stream is not None else stream_being_edited,
                     topic_name if topic_name is not None else orig_topic_name,
+                    ignore_duplicate=True,
                 )
 
     send_event(user_profile.realm, event, users_to_be_notified)

zerver/tests/test_message_edit.py

@@ -1261,9 +1261,11 @@ class EditMessageTest(EditMessageTestCase):
         self.subscribe(aaron, stream_name)
         self.login_user(aaron)
 
+        already_muted_topic = "Already muted topic"
         muted_topics = [
             [stream_name, "Topic1"],
             [stream_name, "Topic2"],
+            [stream_name, already_muted_topic],
         ]
         set_topic_mutes(hamlet, muted_topics)
         set_topic_mutes(cordelia, muted_topics)
@@ -1331,6 +1333,20 @@ class EditMessageTest(EditMessageTestCase):
         self.assertFalse(topic_is_muted(hamlet, stream.id, change_all_topic_name))
         self.assertTrue(topic_is_muted(hamlet, stream.id, change_later_topic_name))
 
+        # Make sure we safely handle the case of the new topic being already muted.
+        check_update_message(
+            user_profile=hamlet,
+            message_id=message_id,
+            stream_id=None,
+            topic_name=already_muted_topic,
+            propagate_mode="change_all",
+            send_notification_to_old_thread=False,
+            send_notification_to_new_thread=False,
+            content=None,
+        )
+        self.assertFalse(topic_is_muted(hamlet, stream.id, change_later_topic_name))
+        self.assertTrue(topic_is_muted(hamlet, stream.id, already_muted_topic))
+
         change_one_topic_name = "Topic 1 edited change_one"
         check_update_message(
             user_profile=hamlet,