Fix adding excluded users to the private room sharing tables when joining a room (#11143)

* We only need to fetch users in private rooms

* Filter out `user_id` at the top

* Discard excluded users in the top loop

We weren't doing this in the "First, if they're our user" branch so this
is a bugfix.

* The caller must check that `user_id` is included

This is in the docstring. There are two call sites:
- one in `_handle_room_publicity_change`, which explicitly checks before calling;
- and another in `_handle_room_membership_event`, which returns early if
  the user is excluded.

So this change is safe.

* Test joining a private room with an excluded user

* Tweak an existing test

* Changelog

* test docstring

* lint

synapse/handlers/user_directory.py

@@ -373,31 +373,29 @@ class UserDirectoryHandler(StateDeltasHandler):
         is_public = await self.store.is_room_world_readable_or_publicly_joinable(
             room_id
         )
-        other_users_in_room = await self.store.get_users_in_room(room_id)
-
         if is_public:
             await self.store.add_users_in_public_rooms(room_id, (user_id,))
         else:
+            users_in_room = await self.store.get_users_in_room(room_id)
+            other_users_in_room = [
+                other
+                for other in users_in_room
+                if other != user_id
+                and (
+                    not self.is_mine_id(other)
+                    or await self.store.should_include_local_user_in_dir(other)
+                )
+            ]
             to_insert = set()
 
             # First, if they're our user then we need to update for every user
             if self.is_mine_id(user_id):
-                if await self.store.should_include_local_user_in_dir(user_id):
-                    for other_user_id in other_users_in_room:
-                        if user_id == other_user_id:
-                            continue
-
-                        to_insert.add((user_id, other_user_id))
+                for other_user_id in other_users_in_room:
+                    to_insert.add((user_id, other_user_id))
 
             # Next we need to update for every local user in the room
             for other_user_id in other_users_in_room:
-                if user_id == other_user_id:
-                    continue
-
-                include_other_user = self.is_mine_id(
-                    other_user_id
-                ) and await self.store.should_include_local_user_in_dir(other_user_id)
-                if include_other_user:
+                if self.is_mine_id(other_user_id):
                     to_insert.add((other_user_id, user_id))
 
             if to_insert: