src/Pure/PIDE/session.scala

   object Consumer {
     def apply[A](name: String)(consume: A => Unit): Consumer[A] =
       new Consumer[A](name, consume)
   }
   final class Consumer[-A] private(val name: String, val consume: A => Unit) {
-    def failure(exn: Throwable): Unit =
+    private def failure(exn: Throwable): Unit =
       Output.error_message(
         "Session consumer failure: " + quote(name) + "\n" + Exn.print(exn))
+
+    def consume_robust(a: A): Unit =
+      try { consume(a) }
+      catch { case exn: Throwable => failure(exn) }
   }
 
   class Outlet[A](dispatcher: Consumer_Thread[() => Unit]) {
     private val consumers = Synchronized[List[Consumer[A]]](Nil)
 
     def += (c: Consumer[A]): Unit = consumers.change(Library.update(c))
     def -= (c: Consumer[A]): Unit = consumers.change(Library.remove(c))
 
     def post(a: A): Unit = {
       for (c <- consumers.value.iterator) {
-        dispatcher.send(() =>
-          try { c.consume(a) }
-          catch { case exn: Throwable => c.failure(exn) })
+        dispatcher.send(() => c.consume_robust(a))
       }
     }
   }