src/Pure/System/progress.scala

     if (_context == 0) throw new IllegalStateException("Database_Progress after exit")
 
     body
   }
 
-  private def sync_database[A](body: => A): A = {
+  private def sync_database[A](body: => A): A = synchronized {
     Progress.private_data.transaction_lock(db, label = "Database_Progress.sync_database") {
       _stopped_db = Progress.private_data.read_progress_stopped(db, _context)
 
       if (_stopped_db && !base_progress.stopped) base_progress.stop()
       if (!_stopped_db && base_progress.stopped && output_stopped) {