[SPARK-10124] [MESOS] Fix removing queued driver in mesos cluster mode.

Currently the spark applications can be queued to the Mesos cluster dispatcher, but when multiple jobs are in queue we don't handle removing jobs from the buffer correctly while iterating and causes null pointer exception.

This patch copies the buffer before iterating them, so exceptions aren't thrown when the jobs are removed.

Author: Timothy Chen <tnachen@gmail.com>

Closes #8322 from tnachen/fix_cluster_mode.

(cherry picked from commit 73431d8)
Signed-off-by: Andrew Or <andrew@databricks.com>

Author: tnachen

core/src/main/scala/org/apache/spark/scheduler/cluster/mesos/MesosClusterScheduler.scala

@@ -507,14 +507,16 @@ private[spark] class MesosClusterScheduler(
       val driversToRetry = pendingRetryDrivers.filter { d =>
         d.retryState.get.nextRetry.before(currentTime)
       }
+
       scheduleTasks(
-        driversToRetry,
+        copyBuffer(driversToRetry),
         removeFromPendingRetryDrivers,
         currentOffers,
         tasks)
+
       // Then we walk through the queued drivers and try to schedule them.
       scheduleTasks(
-        queuedDrivers,
+        copyBuffer(queuedDrivers),
         removeFromQueuedDrivers,
         currentOffers,
         tasks)
@@ -527,13 +529,14 @@ private[spark] class MesosClusterScheduler(
       .foreach(o => driver.declineOffer(o.getId))
   }
 
+  private def copyBuffer(
+      buffer: ArrayBuffer[MesosDriverDescription]): ArrayBuffer[MesosDriverDescription] = {
+    val newBuffer = new ArrayBuffer[MesosDriverDescription](buffer.size)
+    buffer.copyToBuffer(newBuffer)
+    newBuffer
+  }
+
   def getSchedulerState(): MesosClusterSchedulerState = {
-    def copyBuffer(
-        buffer: ArrayBuffer[MesosDriverDescription]): ArrayBuffer[MesosDriverDescription] = {
-      val newBuffer = new ArrayBuffer[MesosDriverDescription](buffer.size)
-      buffer.copyToBuffer(newBuffer)
-      newBuffer
-    }
     stateLock.synchronized {
       new MesosClusterSchedulerState(
         frameworkId,