[SPARK-1886] check executor id existence when executor exit

Author: Zhen Peng <zhenpeng01@baidu.com>

Closes #827 from zhpengg/bugfix-executor-id-not-found and squashes the following commits:

cd8bb65 [Zhen Peng] bugfix: check executor id existence when executor exit

core/src/main/scala/org/apache/spark/deploy/worker/Worker.scala

@@ -263,14 +263,20 @@ private[spark] class Worker(
       }
       val fullId = appId + "/" + execId
       if (ExecutorState.isFinished(state)) {
-        val executor = executors(fullId)
-        logInfo("Executor " + fullId + " finished with state " + state +
-          message.map(" message " + _).getOrElse("") +
-          exitStatus.map(" exitStatus " + _).getOrElse(""))
-        executors -= fullId
-        finishedExecutors(fullId) = executor
-        coresUsed -= executor.cores
-        memoryUsed -= executor.memory
+        executors.get(fullId) match {
+          case Some(executor) => 
+            logInfo("Executor " + fullId + " finished with state " + state +
+              message.map(" message " + _).getOrElse("") +
+              exitStatus.map(" exitStatus " + _).getOrElse(""))
+            executors -= fullId
+            finishedExecutors(fullId) = executor
+            coresUsed -= executor.cores
+            memoryUsed -= executor.memory
+          case None =>
+            logInfo("Unknown Executor " + fullId + " finished with state " + state +
+              message.map(" message " + _).getOrElse("") +
+              exitStatus.map(" exitStatus " + _).getOrElse(""))
+        }
       }
 
     case KillExecutor(masterUrl, appId, execId) =>