src/Pure/Tools/build_cluster.scala

 
   /* cumulative return code */
 
   private var _rc: Int = Process_Result.RC.ok
 
-  override def rc: Int = synchronized { _rc }
-  override def return_code(rc: Int): Unit = synchronized { _rc = Process_Result.RC.merge(_rc, rc) }
+  override def rc: Int = _rc.synchronized { _rc }
+
+  override def return_code(rc: Int): Unit =
+    _rc.synchronized { _rc = Process_Result.RC.merge(_rc, rc) }
 
   def capture[A](host: Build_Cluster.Host, op: String)(
     e: => A,
     msg: String = host.message(op + " ..."),
     err: Throwable => String = { exn => return_code(exn); host.message("failed to " + op) }