markdown: Fix invalid mention bug for user group mention.

Modifies `UserGroupMentionPattern` to inherit from InlineProcessor
instead of Pattern. This change is done because Pattern
stopped checking for matching patterns as soon as it found
a match which was not a valid user group. Due to this all
the subsequent user group mention failed, even if they were
valid. This bug was only present in backend renderring due to
markdown.inlinepatterns.Pattern.

This was reported as issue #17535.

These changes were tested locally in dev server and by adding
some new markdown tests to test these.

zerver/lib/markdown/__init__.py

@@ -1844,11 +1844,13 @@ class UserMentionPattern(markdown.inlinepatterns.InlineProcessor):
         return None, None, None
 
 
-class UserGroupMentionPattern(markdown.inlinepatterns.Pattern):
-    def handleMatch(self, m: Match[str]) -> Optional[Element]:
-        match = m.group(2)
-
+class UserGroupMentionPattern(markdown.inlinepatterns.InlineProcessor):
+    def handleMatch(  # type: ignore[override] # supertype incompatible with supersupertype
+        self, m: Match[str], data: str
+    ) -> Union[Tuple[None, None, None], Tuple[Element, int, int]]:
+        match = m.group(1)
         db_data = self.md.zulip_db_data
+
         if self.md.zulip_message and db_data is not None:
             name = extract_user_group(match)
             user_group = db_data["mention_data"].get_user_group(name)
@@ -1859,15 +1861,15 @@ class UserGroupMentionPattern(markdown.inlinepatterns.Pattern):
             else:
                 # Don't highlight @-mentions that don't refer to a valid user
                 # group.
-                return None
+                return None, None, None
 
             el = Element("span")
             el.set("class", "user-group-mention")
             el.set("data-user-group-id", user_group_id)
             text = f"@{name}"
             el.text = markdown.util.AtomicString(text)
-            return el
-        return None
+            return el, m.start(), m.end()
+        return None, None, None
 
 
 class StreamPattern(CompiledPattern):

zerver/tests/test_markdown.py

@@ -2056,6 +2056,27 @@ class MarkdownTest(ZulipTestCase):
         self.assertEqual(msg.mentions_user_ids, {user_profile.id})
         self.assertEqual(msg.mentions_user_group_ids, {user_group.id})
 
+    def test_invalid_user_group_followed_by_valid_mention_single(self) -> None:
+        sender_user_profile = self.example_user("othello")
+        user_profile = self.example_user("hamlet")
+        msg = Message(sender=sender_user_profile, sending_client=get_client("test"))
+        user_id = user_profile.id
+        user_group = self.create_user_group_for_test("support")
+
+        content = "@**King Hamlet** @*Invalid user group* @*support*"
+        self.assertEqual(
+            render_markdown(msg, content),
+            '<p><span class="user-mention" '
+            f'data-user-id="{user_id}">'
+            "@King Hamlet</span> "
+            "@<em>Invalid user group</em> "
+            '<span class="user-group-mention" '
+            f'data-user-group-id="{user_group.id}">'
+            "@support</span></p>",
+        )
+        self.assertEqual(msg.mentions_user_ids, {user_profile.id})
+        self.assertEqual(msg.mentions_user_group_ids, {user_group.id})
+
     def test_user_group_mention_atomic_string(self) -> None:
         sender_user_profile = self.example_user("othello")
         realm = get_realm("zulip")