[Hot Fix scala#42] Do not stop SparkUI if bind() is not called

This is a bug fix for scala#42 (79d07d6).

In Master, we do not bind() each SparkUI because we do not want to start a server for each finished application. However, when we remove the associated application, we call stop() on the SparkUI, which throws an assertion failure.

This fix ensures we don't call stop() on a SparkUI that was never bind()'ed.

Author: Andrew Or <andrewor14@gmail.com>

Closes scala#188 from andrewor14/ui-fix and squashes the following commits:

94a925f [Andrew Or] Do not stop SparkUI if bind() is not called

core/src/main/scala/org/apache/spark/deploy/master/Master.scala

@@ -149,7 +149,6 @@ private[spark] class Master(
 
   override def postStop() {
     webUi.stop()
-    appIdToUI.values.foreach(_.stop())
     masterMetricsSystem.stop()
     applicationMetricsSystem.stop()
     persistenceEngine.close()
@@ -622,10 +621,7 @@ private[spark] class Master(
       if (completedApps.size >= RETAINED_APPLICATIONS) {
         val toRemove = math.max(RETAINED_APPLICATIONS / 10, 1)
         completedApps.take(toRemove).foreach( a => {
-          appIdToUI.remove(a.id).foreach { ui =>
-            ui.stop()
-            webUi.detachUI(ui)
-          }
+          appIdToUI.remove(a.id).foreach { ui => webUi.detachUI(ui) }
           applicationMetricsSystem.removeSource(a.appSource)
         })
         completedApps.trimStart(toRemove)
@@ -681,10 +677,7 @@ private[spark] class Master(
     // Do not call ui.bind() to avoid creating a new server for each application
     ui.start()
     val success = replayerBus.replay(eventLogDir)
-    if (!success) {
-      ui.stop()
-      None
-    } else Some(ui)
+    if (success) Some(ui) else None
   }
 
   /** Generate a new app ID given a app's submission date */