wip use an index approach to avoid O(db_size) complexity

src/backend/core/services/lobby.py

@@ -79,6 +79,14 @@ class LobbyService:
 
     Handles participant entry requests, status management, and notifications
     using cache for state management and LiveKit for real-time updates.
+
+    Participant membership per room is tracked in a native Redis SET (the
+    "room index") so that listing and clearing a room's lobby never scans
+    the shared keyspace. The per-participant cache entries remain the source
+    of truth for state: their TTLs implement liveness (a waiter who stops
+    polling simply expires). The index only records which participant ids
+    may exist for a room; a stale id costs one cache miss and is pruned
+    lazily.
     """
 
     @staticmethod
@@ -86,6 +94,61 @@ class LobbyService:
         """Generate cache key for participant(s) data."""
         return f"{settings.LOBBY_KEY_PREFIX}_{room_id!s}_{participant_id}"
 
+    @staticmethod
+    def _get_index_key(room_id: UUID) -> str:
+        """Raw Redis key of the per-room participant index (a native SET).
+
+        Built through django-redis' make_key so it lives under the same
+        KEY_PREFIX/version namespace as the participant entries.
+        """
+        return cache.client.make_key(f"{settings.LOBBY_KEY_PREFIX}-index_{room_id!s}")
+
+    @staticmethod
+    def _redis(write: bool = True):
+        """Raw redis-py client.
+
+        SADD/SREM/SMEMBERS are not exposed by the Django cache API; this is
+        the documented django-redis escape hatch.
+        """
+        return cache.client.get_client(write=write)
+
+    def _index_add(self, room_id: UUID, participant_id: str) -> None:
+        """Record a participant id in the room index.
+
+        Refreshes a backstop TTL on the index so an abandoned room cannot
+        leak its set beyond the longest participant timeout.
+        """
+        index_key = self._get_index_key(room_id)
+        pipe = self._redis().pipeline(transaction=False)
+        pipe.sadd(index_key, participant_id)
+        pipe.expire(index_key, settings.LOBBY_ACCEPTED_TIMEOUT)
+        pipe.execute()
+
+    def _index_members(self, room_id: UUID) -> List[str]:
+        """All participant ids currently indexed for the room."""
+        members = self._redis(write=False).smembers(self._get_index_key(room_id))
+        return [
+            member.decode() if isinstance(member, bytes) else member
+            for member in members
+        ]
+
+    def _index_touch(self, room_id: UUID) -> None:
+        """Re-arm the room index backstop TTL.
+
+        Called whenever a participant entry is written or refreshed so the
+        index always outlives every entry it references — including a lone
+        waiter whose rolling WAITING TTL would otherwise outlast the
+        backstop set at enter() time.
+        """
+        self._redis().expire(
+            self._get_index_key(room_id), settings.LOBBY_ACCEPTED_TIMEOUT
+        )
+
+    def _index_remove(self, room_id: UUID, *participant_ids: str) -> None:
+        """Drop participant ids from the room index."""
+        if participant_ids:
+            self._redis().srem(self._get_index_key(room_id), *participant_ids)
+
     @staticmethod
     def _get_or_create_participant_id(request) -> str:
         """Extract unique participant identifier from the request."""
@@ -209,14 +272,16 @@ class LobbyService:
         cache.touch(
             self._get_cache_key(room_id, participant_id), settings.LOBBY_WAITING_TIMEOUT
         )
+        self._index_touch(room_id)
 
     def enter(
         self, room_id: UUID, participant_id: str, username: str
     ) -> LobbyParticipant:
         """Add participant to waiting lobby.
 
-        Create a new participant entry in waiting status and notify room
-        participants of the new entry request.
+        Create a new participant entry in waiting status, index the
+        participant id for the room, and notify room participants of the
+        new entry request.
         """
 
         color = utils.generate_color(participant_id)
@@ -245,6 +310,7 @@ class LobbyService:
             participant.to_dict(),
             timeout=settings.LOBBY_WAITING_TIMEOUT,
         )
+        self._index_add(room_id, participant_id)
 
         return participant
 
@@ -267,15 +333,31 @@ class LobbyService:
             return None
 
     def list_waiting_participants(self, room_id: UUID) -> List[dict]:
-        """List all waiting participants for a room."""
+        """List all waiting participants for a room.
 
-        pattern = self._get_cache_key(room_id, "*")
-        keys = list(cache.iter_keys(pattern, itersize=utils.CACHE_SCAN_ITERSIZE))
+        Reads the per-room index (O(participants of this room)) instead of
+        scanning the shared keyspace. Indexed ids whose cache entry has
+        expired are pruned lazily here: the entry TTL is the liveness
+        protocol, so a missing entry means the participant is gone.
+        """
 
-        if not keys:
+        member_ids = self._index_members(room_id)
+
+        if not member_ids:
             return []
 
-        data = cache.get_many(keys)
+        keys_by_id = {
+            participant_id: self._get_cache_key(room_id, participant_id)
+            for participant_id in member_ids
+        }
+        data = cache.get_many(list(keys_by_id.values()))
+
+        dead_ids = [
+            participant_id
+            for participant_id, cache_key in keys_by_id.items()
+            if cache_key not in data
+        ]
+        self._index_remove(room_id, *dead_ids)
 
         waiting_participants = []
         for cache_key, raw_participant in data.items():
@@ -341,16 +423,28 @@ class LobbyService:
 
         participant.status = status
         cache.set(cache_key, participant.to_dict(), timeout=timeout)
+        self._index_touch(room_id)
 
     def clear_room_cache(self, room_id: UUID) -> None:
-        """Clear all participant entries from the cache for a specific room."""
+        """Clear all participant entries from the cache for a specific room.
 
-        cache.delete_pattern(
-            self._get_cache_key(room_id, "*"), itersize=utils.CACHE_SCAN_ITERSIZE
-        )
+        Deletes the indexed participant entries and the index itself with
+        targeted commands instead of a full-keyspace pattern scan.
+        """
+
+        member_ids = self._index_members(room_id)
+        if member_ids:
+            cache.delete_many(
+                [
+                    self._get_cache_key(room_id, participant_id)
+                    for participant_id in member_ids
+                ]
+            )
+        self._redis().delete(self._get_index_key(room_id))
 
     def clear_participant_cache(self, room_id: UUID, participant_id: str) -> None:
         """Clear a given participant entry from the cache for a specific room."""
 
         cache_key = self._get_cache_key(room_id, participant_id)
         cache.delete(cache_key)
+        self._index_remove(room_id, participant_id)