src/Pure/Build/build_schedule.scala

       val cluster_hosts = build_context.build_hosts
 
       val hosts_current =
         cluster_hosts.forall(host => isabelle.Host.read_info(host_database, host.name).isDefined)
       if (!hosts_current) {
-        Build_Cluster.make(build_context, progress = progress)
-          .open().init().benchmark().close()
+        using(Build_Cluster.make(build_context, progress = progress).open())(_.init().benchmark())
       }
 
       val host_infos = Host_Infos.load(cluster_hosts, host_database)
       val timing_data = Timing_Data.load(host_infos, log_database)