src/Pure/System/scala.scala

     private var futures = Map.empty[String, Future[Unit]]
 
     override def init(session: Session): Unit =
       synchronized { this.session = session }
 
-    override def exit(): Unit = synchronized {
+    override def exit(exit_state: Document.State): Unit = synchronized {
       for ((id, future) <- futures) cancel(id, future)
       futures = Map.empty
     }
 
     private def result(id: String, tag: Scala.Tag, res: List[Bytes]): Unit =