fix concurrent writes to mutable keepalive set

src/main/scala/KeepAlive.scala

@@ -1,5 +1,6 @@
 package lila.ws
 
+import java.util.concurrent.atomic.AtomicReference
 import org.apache.pekko.actor.typed.Scheduler
 
 import ipc.*
@@ -32,11 +33,8 @@ object KeepAlive:
 
   final class AliveRooms:
 
-    private val rooms = collection.mutable.Set[RoomId]()
+    private val rooms: AtomicReference[Set[RoomId]] = AtomicReference(Set.empty)
 
-    def apply(roomId: RoomId) = rooms += roomId
+    def apply(roomId: RoomId): Unit = rooms.getAndUpdate(_ + roomId)
 
-    def getAndClear: LilaIn.KeepAlives =
-      val ret = LilaIn.KeepAlives(rooms.toSet)
-      rooms.clear()
-      ret
+    def getAndClear: LilaIn.KeepAlives = LilaIn.KeepAlives(rooms.getAndUpdate(_ => Set.empty))