actions: Use check_stream_topic when editing message topics.

zerver/lib/actions.py

@@ -155,7 +155,7 @@ from zerver.lib.streams import (
     send_stream_creation_event,
     subscribed_to_stream,
 )
-from zerver.lib.string_validation import check_stream_name
+from zerver.lib.string_validation import check_stream_name, check_stream_topic
 from zerver.lib.timestamp import datetime_to_timestamp, timestamp_to_datetime
 from zerver.lib.timezone import canonicalize_timezone
 from zerver.lib.topic import (
@@ -2975,8 +2975,8 @@ def validate_message_edit_payload(
     if propagate_mode != "change_one" and topic_name is None and stream_id is None:
         raise JsonableError(_("Invalid propagate_mode without topic edit"))
 
-    if topic_name == "":
-        raise JsonableError(_("Topic can't be empty"))
+    if topic_name is not None:
+        check_stream_topic(topic_name)
 
     if stream_id is not None and content is not None:
         raise JsonableError(_("Cannot change message content while changing stream"))

zerver/tests/test_message_edit.py

@@ -192,7 +192,21 @@ class EditMessagePayloadTest(EditMessageTestCase):
                 "topic": " ",
             },
         )
-        self.assert_json_error(result, "Topic can't be empty")
+        self.assert_json_error(result, "Topic can't be empty!")
+
+    def test_edit_message_invalid_topic(self) -> None:
+        self.login("hamlet")
+        msg_id = self.send_stream_message(
+            self.example_user("hamlet"), "Denmark", topic_name="editing", content="before edit"
+        )
+        result = self.client_patch(
+            "/json/messages/" + str(msg_id),
+            {
+                "message_id": msg_id,
+                "topic": "editing\nfun",
+            },
+        )
+        self.assert_json_error(result, "Invalid characters in topic!")
 
     def test_move_message_to_stream_with_content(self) -> None:
         (user_profile, old_stream, new_stream, msg_id, msg_id_later) = self.prepare_move_topics(