[SPARK-17022][YARN] Handle potential deadlock in driver handling messages

## What changes were proposed in this pull request?

We directly send RequestExecutors to AM instead of transfer it to yarnShedulerBackend first, to avoid potential deadlock.

## How was this patch tested?

manual tests

Author: WangTaoTheTonic <wangtao111@huawei.com>

Closes #14605 from WangTaoTheTonic/lock.

yarn/src/main/scala/org/apache/spark/scheduler/cluster/YarnSchedulerBackend.scala

@@ -125,8 +125,20 @@ private[spark] abstract class YarnSchedulerBackend(
    * This includes executors already pending or running.
    */
   override def doRequestTotalExecutors(requestedTotal: Int): Boolean = {
-    yarnSchedulerEndpointRef.askWithRetry[Boolean](
-      RequestExecutors(requestedTotal, localityAwareTasks, hostToLocalTaskCount))
+    val r = RequestExecutors(requestedTotal, localityAwareTasks, hostToLocalTaskCount)
+    yarnSchedulerEndpoint.amEndpoint match {
+      case Some(am) =>
+        try {
+          am.askWithRetry[Boolean](r)
+        } catch {
+          case NonFatal(e) =>
+            logError(s"Sending $r to AM was unsuccessful", e)
+            return false
+        }
+      case None =>
+        logWarning("Attempted to request executors before the AM has registered!")
+        return false
+    }
   }
 
   /**
@@ -209,7 +221,7 @@ private[spark] abstract class YarnSchedulerBackend(
    */
   private class YarnSchedulerEndpoint(override val rpcEnv: RpcEnv)
     extends ThreadSafeRpcEndpoint with Logging {
-    private var amEndpoint: Option[RpcEndpointRef] = None
+    var amEndpoint: Option[RpcEndpointRef] = None
 
     private val askAmThreadPool =
       ThreadUtils.newDaemonCachedThreadPool("yarn-scheduler-ask-am-thread-pool")