Make the Make Room Admin API choose to puppet the room creator in v12 rooms (#18805)

Author: anoadragon453

changelog.d/18805.bugfix

@@ -0,0 +1 @@
+Fix bug where the [Make Room Admin API](https://element-hq.github.io/synapse/latest/admin_api/rooms.html#make-room-admin-api) would not treat a room v12's creator power level as the highest in room.

synapse/rest/admin/rooms.py

@@ -634,7 +634,7 @@ async def on_POST(
                 for creator in creators:
                     if self.is_mine_id(creator):
                         # include the creator as they won't be in the PL users map.
-                        admin_users.insert(0, creator)
+                        admin_users.append(creator)
 
             if not admin_users:
                 raise SynapseError(

tests/rest/admin/test_room.py

@@ -2917,6 +2917,39 @@ def test_v12_room(self) -> None:
         )
         self.assertEquals(pl["users"][self.admin_user], 100)
 
+    def test_v12_room_with_many_user_pls(self) -> None:
+        """Test that you can be promoted to the admin user's PL in v12 rooms that contain a range of user PLs."""
+        room_id = self.helper.create_room_as(
+            self.creator,
+            tok=self.creator_tok,
+            room_version=RoomVersions.V12.identifier,
+            is_public=True,
+            extra_content={
+                "power_level_content_override": {
+                    "users": {
+                        self.second_user_id: 50,
+                    },
+                },
+            },
+        )
+
+        self.helper.join(room_id, self.admin_user, tok=self.admin_user_tok)
+        self.helper.join(room_id, self.second_user_id, tok=self.second_tok)
+
+        channel = self.make_request(
+            "POST",
+            f"/_synapse/admin/v1/rooms/{room_id}/make_room_admin",
+            content={},
+            access_token=self.admin_user_tok,
+        )
+
+        self.assertEqual(200, channel.code, msg=channel.json_body)
+
+        pl = self.helper.get_state(
+            room_id, EventTypes.PowerLevels, tok=self.creator_tok
+        )
+        self.assertEquals(pl["users"][self.admin_user], 100)
+
 
 class BlockRoomTestCase(unittest.HomeserverTestCase):
     servlets = [