more robust access to local variables;

--- a/src/Pure/System/progress.scala	Sun Aug 27 13:15:32 2023 +0200
+++ b/src/Pure/System/progress.scala	Sun Aug 27 14:22:28 2023 +0200
@@ -370,7 +370,7 @@
     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)