[STREAMING][FLAKY-TEST] Catch execution context race condition in `FileBasedWriteAheadLog.close()`

There is a race condition in `FileBasedWriteAheadLog.close()`, where if delete's of old log files are in progress, the write ahead log may close, and result in a `RejectedExecutionException`. This is okay, and should be handled gracefully.

Example test failures:
https://amplab.cs.berkeley.edu/jenkins/job/Spark-1.6-SBT/AMPLAB_JENKINS_BUILD_PROFILE=hadoop1.0,label=spark-test/95/testReport/junit/org.apache.spark.streaming.util/BatchedWriteAheadLogWithCloseFileAfterWriteSuite/BatchedWriteAheadLog___clean_old_logs/

The reason the test fails is in `afterEach`, `writeAheadLog.close` is called, and there may still be async deletes in flight.

tdas zsxwing

Author: Burak Yavuz <brkyvz@gmail.com>

Closes #9953 from brkyvz/flaky-ss.

streaming/src/main/scala/org/apache/spark/streaming/util/FileBasedWriteAheadLog.scala

@@ -17,7 +17,7 @@
 package org.apache.spark.streaming.util
 
 import java.nio.ByteBuffer
-import java.util.concurrent.ThreadPoolExecutor
+import java.util.concurrent.{RejectedExecutionException, ThreadPoolExecutor}
 import java.util.{Iterator => JIterator}
 
 import scala.collection.JavaConverters._
@@ -176,10 +176,16 @@ private[streaming] class FileBasedWriteAheadLog(
     }
     oldLogFiles.foreach { logInfo =>
       if (!executionContext.isShutdown) {
-        val f = Future { deleteFile(logInfo) }(executionContext)
-        if (waitForCompletion) {
-          import scala.concurrent.duration._
-          Await.ready(f, 1 second)
+        try {
+          val f = Future { deleteFile(logInfo) }(executionContext)
+          if (waitForCompletion) {
+            import scala.concurrent.duration._
+            Await.ready(f, 1 second)
+          }
+        } catch {
+          case e: RejectedExecutionException =>
+            logWarning("Execution context shutdown before deleting old WriteAheadLogs. " +
+              "This would not affect recovery correctness.", e)
         }
       }
     }