src/Pure/Tools/build_process.scala

     val session_setup: (String, Session) => Unit,
     val uuid: String
   ) {
     def sessions_structure: Sessions.Structure = build_deps.sessions_structure
 
-    def session_context(session: String): Build_Job.Session_Context = sessions(session)
-
-    def sources_shasum(session: String): SHA1.Shasum = session_context(session).sources_shasum
+    def sources_shasum(session: String): SHA1.Shasum = sessions(session).sources_shasum
 
     def old_command_timings(session: String): List[Properties.T] =
       sessions.get(session) match {
         case Some(session_context) =>
           Properties.uncompress(session_context.old_command_timings_blob, cache = store.cache)

     }
     else None
 
   protected def start_job(session_name: String): Unit = {
     val ancestor_results = synchronized {
-      _state.get_results(build_context.session_context(session_name).ancestors)
+      _state.get_results(build_context.sessions(session_name).ancestors)
     }
     val input_heaps =
       if (ancestor_results.isEmpty) {
         SHA1.shasum_meta_info(SHA1.digest(Path.explode("$POLYML_EXE")))
       }