[SPARK-26758][CORE] Idle Executors are not getting killed after spark…

….dynamiAllocation.executorIdleTimeout value

## What changes were proposed in this pull request?

**updateAndSyncNumExecutorsTarget**  API should be called after **initializing** flag is unset
## How was this patch tested?
Added UT and also manually tested

After Fix
![afterfix](https://user-images.githubusercontent.com/35216143/51983136-ed4a5000-24bd-11e9-90c8-c4a562c17a4b.png)

Closes #23697 from sandeep-katta/executorIssue.

Authored-by: sandeep-katta <sandeep.katta2007@gmail.com>
Signed-off-by: Sean Owen <sean.owen@databricks.com>

core/src/main/scala/org/apache/spark/ExecutorAllocationManager.scala

@@ -312,8 +312,6 @@ private[spark] class ExecutorAllocationManager(
   private def schedule(): Unit = synchronized {
     val now = clock.getTimeMillis
 
-    updateAndSyncNumExecutorsTarget(now)
-
     val executorIdsToBeRemoved = ArrayBuffer[String]()
     removeTimes.retain { case (executorId, expireTime) =>
       val expired = now >= expireTime
@@ -323,6 +321,8 @@ private[spark] class ExecutorAllocationManager(
       }
       !expired
     }
+    // Update executor target number only after initializing flag is unset
+    updateAndSyncNumExecutorsTarget(now)
     if (executorIdsToBeRemoved.nonEmpty) {
       removeExecutors(executorIdsToBeRemoved)
     }

core/src/test/scala/org/apache/spark/ExecutorAllocationManagerSuite.scala

@@ -936,12 +936,7 @@ class ExecutorAllocationManagerSuite
 
     assert(maxNumExecutorsNeeded(manager) === 0)
     schedule(manager)
-    // Verify executor is timeout but numExecutorsTarget is not recalculated
-    assert(numExecutorsTarget(manager) === 3)
-
-    // Schedule again to recalculate the numExecutorsTarget after executor is timeout
-    schedule(manager)
-    // Verify that current number of executors should be ramp down when executor is timeout
+    // Verify executor is timeout,numExecutorsTarget is recalculated
     assert(numExecutorsTarget(manager) === 2)
   }
 
@@ -1148,6 +1143,25 @@ class ExecutorAllocationManagerSuite
     verify(mockAllocationClient).killExecutors(Seq("executor-1"), false, false, false)
   }
 
+  test("SPARK-26758 check executor target number after idle time out ") {
+    sc = createSparkContext(1, 5, 3)
+    val manager = sc.executorAllocationManager.get
+    val clock = new ManualClock(10000L)
+    manager.setClock(clock)
+    assert(numExecutorsTarget(manager) === 3)
+    manager.listener.onExecutorAdded(SparkListenerExecutorAdded(
+      clock.getTimeMillis(), "executor-1", new ExecutorInfo("host1", 1, Map.empty)))
+    manager.listener.onExecutorAdded(SparkListenerExecutorAdded(
+      clock.getTimeMillis(), "executor-2", new ExecutorInfo("host1", 2, Map.empty)))
+    manager.listener.onExecutorAdded(SparkListenerExecutorAdded(
+      clock.getTimeMillis(), "executor-3", new ExecutorInfo("host1", 3, Map.empty)))
+    // make all the executors as idle, so that it will be killed
+    clock.advance(executorIdleTimeout * 1000)
+    schedule(manager)
+    // once the schedule is run target executor number should be 1
+    assert(numExecutorsTarget(manager) === 1)
+  }
+
   private def createSparkContext(
       minExecutors: Int = 1,
       maxExecutors: Int = 5,