src/Pure/General/sql.scala

         val conn = the_postgresql_connection
         val self_pid = conn.getBackendPID
 
         try {
           while (true) {
-            Isabelle_Thread.interruptible { receiver_delay.sleep() }
-            Option(conn.getNotifications()) match {
+            Isabelle_Thread.interruptible { receiver_delay.sleep(); Option(conn.getNotifications())}
+            match {
               case Some(array) if array.nonEmpty =>
                 synchronized {
                   var received = _receiver_buffer.getOrElse(Map.empty)
                   for (a <- array.iterator if a.getPID != self_pid) {
                     val msg = SQL.Notification(a.getName, a.getParameter)