Fixed 2 bugs in executor UI.

1) UI crashed if the executor UI was loaded before any tasks started.
2) The total tasks was incorrectly reported due to using string (rather
than int) arithmetic.

core/src/main/scala/spark/ui/exec/ExecutorsUI.scala

@@ -38,10 +38,9 @@ private[spark] class ExecutorsUI(val sc: SparkContext) {
   def render(request: HttpServletRequest): Seq[Node] = {
     val storageStatusList = sc.getExecutorStorageStatus
 
-    val maxMem = storageStatusList.map(_.maxMem).reduce(_+_)
-    val memUsed = storageStatusList.map(_.memUsed()).reduce(_+_)
-    val diskSpaceUsed = storageStatusList.flatMap(_.blocks.values.map(_.diskSize))
-      .reduceOption(_+_).getOrElse(0L)
+    val maxMem = storageStatusList.map(_.maxMem).fold(0L)(_+_)
+    val memUsed = storageStatusList.map(_.memUsed()).fold(0L)(_+_)
+    val diskSpaceUsed = storageStatusList.flatMap(_.blocks.values.map(_.diskSize)).fold(0L)(_+_)
 
     val execHead = Seq("Executor ID", "Address", "RDD blocks", "Memory used", "Disk used",
       "Active tasks", "Failed tasks", "Complete tasks", "Total tasks")
@@ -93,10 +92,9 @@ private[spark] class ExecutorsUI(val sc: SparkContext) {
     val memUsed = sc.getExecutorStorageStatus(a).memUsed().toString
     val maxMem = sc.getExecutorStorageStatus(a).maxMem.toString
     val diskUsed = sc.getExecutorStorageStatus(a).diskUsed().toString
-    val activeTasks = listener.executorToTasksActive.get(a.toString).map(l => l.size)
-      .getOrElse(0).toString
-    val failedTasks = listener.executorToTasksFailed.getOrElse(a.toString, 0).toString
-    val completedTasks = listener.executorToTasksComplete.getOrElse(a.toString, 0).toString
+    val activeTasks = listener.executorToTasksActive.get(a.toString).map(l => l.size).getOrElse(0)
+    val failedTasks = listener.executorToTasksFailed.getOrElse(a.toString, 0)
+    val completedTasks = listener.executorToTasksComplete.getOrElse(a.toString, 0)
     val totalTasks = activeTasks + failedTasks + completedTasks
 
     Seq(
@@ -106,10 +104,10 @@ private[spark] class ExecutorsUI(val sc: SparkContext) {
       memUsed,
       maxMem,
       diskUsed,
-      activeTasks,
-      failedTasks,
-      completedTasks,
-      totalTasks
+      activeTasks.toString,
+      failedTasks.toString,
+      completedTasks.toString,
+      totalTasks.toString
     )
   }