[SPARK-24414][UI] Calculate the correct number of tasks for a stage.

This change takes into account all non-pending tasks when calculating
the number of tasks to be shown. This also means that when the stage
is pending, the task table (or, in fact, most of the data in the stage
page) will not be rendered.

I also fixed the label when the known number of tasks is larger than
the recorded number of tasks (it was inverted).

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #21457 from vanzin/SPARK-24414.

Author: Marcelo Vanzin

core/src/main/scala/org/apache/spark/ui/jobs/StagePage.scala

@@ -117,8 +117,7 @@ private[ui] class StagePage(parent: StagesTab, store: AppStatusStore) extends We
 
     val localitySummary = store.localitySummary(stageData.stageId, stageData.attemptId)
 
-    val totalTasks = stageData.numActiveTasks + stageData.numCompleteTasks +
-      stageData.numFailedTasks + stageData.numKilledTasks
+    val totalTasks = taskCount(stageData)
     if (totalTasks == 0) {
       val content =
         <div>
@@ -133,7 +132,7 @@ private[ui] class StagePage(parent: StagesTab, store: AppStatusStore) extends We
     val totalTasksNumStr = if (totalTasks == storedTasks) {
       s"$totalTasks"
     } else {
-      s"$totalTasks, showing ${storedTasks}"
+      s"$storedTasks, showing ${totalTasks}"
     }
 
     val summary =
@@ -686,7 +685,7 @@ private[ui] class TaskDataSource(
 
   private var _tasksToShow: Seq[TaskData] = null
 
-  override def dataSize: Int = stage.numTasks
+  override def dataSize: Int = taskCount(stage)
 
   override def sliceData(from: Int, to: Int): Seq[TaskData] = {
     if (_tasksToShow == null) {
@@ -1052,4 +1051,9 @@ private[ui] object ApiHelper {
     (stage.map(_.name).getOrElse(""), stage.flatMap(_.description).getOrElse(job.name))
   }
 
+  def taskCount(stageData: StageData): Int = {
+    stageData.numActiveTasks + stageData.numCompleteTasks + stageData.numFailedTasks +
+      stageData.numKilledTasks
+  }
+
 }