session_actor: await state assigment of previous change before signalling current change, and avoid crash in overrun situations;

--- a/src/Pure/System/event_bus.scala	Sun Aug 29 19:04:29 2010 +0200
+++ b/src/Pure/System/event_bus.scala	Sun Aug 29 19:48:35 2010 +0200
@@ -32,4 +32,29 @@
   /* event invocation */
 
   def event(x: Event) { synchronized { receivers.foreach(_ ! x) } }
+
+
+  /* await global condition -- triggered via bus events */
+
+  def await(cond: => Boolean)
+  {
+    case object Wait
+    val a = new Actor {
+      def act {
+        if (cond) react { case Wait => reply(()); exit(Wait) }
+        else {
+          loop {
+            react {
+              case trigger if trigger != Wait =>
+                if (cond) { react { case Wait => reply(()); exit(Wait) } }
+            }
+          }
+        }
+      }
+    }
+    this += a
+    a.start
+    a !? Wait
+    this -= a
+  }
 }

--- a/src/Pure/System/session.scala	Sun Aug 29 19:04:29 2010 +0200
+++ b/src/Pure/System/session.scala	Sun Aug 29 19:48:35 2010 +0200
@@ -19,6 +19,7 @@
 
   case object Global_Settings
   case object Perspective
+  case object Assignment
   case class Commands_Changed(set: Set[Command])
 }
 
@@ -44,6 +45,7 @@
   val raw_output = new Event_Bus[Isabelle_Process.Result]
   val commands_changed = new Event_Bus[Session.Commands_Changed]
   val perspective = new Event_Bus[Session.Perspective.type]
+  val assignments = new Event_Bus[Session.Assignment.type]
 
 
   /* unique ids */
@@ -187,7 +189,10 @@
           if (result.is_status) {
             result.body match {
               case List(Isar_Document.Assign(id, edits)) =>
-                try { global_state.change(_.assign(id, edits)) }
+                try {
+                  global_state.change(_.assign(id, edits))
+                  assignments.event(Session.Assignment)
+                }
                 catch { case _: Document.State.Fail => bad_result(result) }
               case List(Keyword.Command_Decl(name, kind)) => syntax += (name, kind)
               case List(Keyword.Keyword_Decl(name)) => syntax += name
@@ -250,7 +255,12 @@
           val previous = global_state.peek().history.tip.current
           val result = Future.fork { Thy_Syntax.text_edits(Session.this, previous.join, edits) }
           val change = global_state.change_yield(_.extend_history(previous, edits, result))
-          val this_actor = self; change.current.map(_ => this_actor ! change)
+
+          val this_actor = self
+          change.current.map(_ => {
+            assignments.await { global_state.peek().is_assigned(previous.get_finished) }
+            this_actor ! change })
+
           reply(())
 
         case change: Document.Change if prover != null =>