Support non-search queries in /json/messages/matches_narrow.

For filters like has:link, where the web app doesn't necessarily
want to guess whether incoming messages meet the criteria of the
filter, the server is asked to query rows that match the query.

Usually these queries are search queries, which have fields for
content_matches and subject_matches.  Our logic was handling those
correctly.

Non-search queries were throwing an exception related to tuple
unpacking.  Now we recognize when those fields are absent and
do the proper thing.

There are probably situations where the web app should stop hitting
this endpoint and just use its own filters.  We are making the most
defensive fix first.

Fixes #6118

zerver/tests/test_narrow.py

@@ -983,6 +983,42 @@ class GetOldMessagesTest(ZulipTestCase):
         self.assertEqual(link_search_result['messages'][0]['match_content'],
                          '<p><a href="https://google.com" target="_blank" title="https://google.com"><span class="highlight">https://google.com</span></a></p>')
 
+    def test_messages_in_narrow_for_non_search(self):
+        # type: () -> None
+        email = self.example_email("cordelia")
+        self.login(email)
+
+        def send(content):
+            # type: (Text) -> int
+            msg_id = self.send_message(
+                sender_name=email,
+                raw_recipients="Verona",
+                message_type=Recipient.STREAM,
+                subject='test_topic',
+                content=content,
+            )
+            return msg_id
+
+        good_id = send('http://foo.com')
+        bad_id = send('no link here')
+        msg_ids = [good_id, bad_id]
+        send('http://bar.com but not in msg_ids')
+
+        narrow = [
+            dict(operator='has', operand='link'),
+        ]
+
+        raw_params = dict(msg_ids=msg_ids, narrow=narrow)
+        params = {k: ujson.dumps(v) for k, v in raw_params.items()}
+        result = self.client_get('/json/messages/matches_narrow', params)
+        self.assert_json_success(result)
+        messages = result.json()['messages']
+        self.assertEqual(len(list(messages.keys())), 1)
+        message = messages[str(good_id)]
+        self.assertIn('a href=', message['match_content'])
+        self.assertIn('http://foo.com', message['match_content'])
+        self.assertEqual(message['match_subject'], 'test_topic')
+
     def test_get_messages_with_only_searching_anchor(self):
         # type: () -> None
         """

zerver/views/messages.py

@@ -1198,8 +1198,19 @@ def messages_in_narrow_backend(request, user_profile,
 
     search_fields = dict()
     for row in query_result:
-        (message_id, subject, rendered_content, content_matches, subject_matches) = row
-        search_fields[message_id] = get_search_fields(rendered_content, subject,
-                                                      content_matches, subject_matches)
+        message_id = row['message_id']
+        subject = row['subject']
+        rendered_content = row['rendered_content']
+
+        if 'content_matches' in row:
+            content_matches = row['content_matches']
+            subject_matches = row['subject_matches']
+            search_fields[message_id] = get_search_fields(rendered_content, subject,
+                                                          content_matches, subject_matches)
+        else:
+            search_fields[message_id] = dict(
+                match_content=rendered_content,
+                match_subject=subject
+            )
 
     return json_success({"messages": search_fields})