src/Pure/Tools/build_job.scala

     val session_sources: Sessions.Sources =
       Sessions.Sources.load(session_background.base, cache = store.cache.compress)
 
     private lazy val future_result: Future[Process_Result] =
       Future.thread("build", uninterruptible = true) {
-        Exn.Interrupt.expose()
-
         val parent = info.parent.getOrElse("")
 
         val env =
           Isabelle_System.settings(
             List("ISABELLE_ML_DEBUGGER" -> options.bool("ML_debugger").toString))

         session_setup(session_name, session)
 
         val eval_main = Command_Line.ML_tool("Isabelle_Process.init_build ()" :: eval_store)
 
         val process =
-          Isabelle_Thread.uninterruptible {
-            Isabelle_Process.start(store, options, session, session_background,
-              logic = parent, raw_ml_system = is_pure,
-              use_prelude = use_prelude, eval_main = eval_main,
-              cwd = info.dir.file, env = env)
-          }
+          Isabelle_Process.start(store, options, session, session_background,
+            logic = parent, raw_ml_system = is_pure,
+            use_prelude = use_prelude, eval_main = eval_main,
+            cwd = info.dir.file, env = env)
 
         val build_errors =
           Isabelle_Thread.interrupt_handler(_ => process.terminate()) {
             Exn.capture { process.await_startup() } match {
               case Exn.Res(_) =>