[SPARK-11974][CORE] Not all the temp dirs had been deleted when the J…

…VM exits

deleting the temp dir like that

```

scala> import scala.collection.mutable
import scala.collection.mutable

scala> val a = mutable.Set(1,2,3,4,7,0,8,98,9)
a: scala.collection.mutable.Set[Int] = Set(0, 9, 1, 2, 3, 7, 4, 8, 98)

scala> a.foreach(x => {a.remove(x) })

scala> a.foreach(println(_))
98
```

You may not modify a collection while traversing or iterating over it.This can not delete all element of the collection

Author: Zhongshuai Pei <peizhongshuai@huawei.com>

Closes #9951 from DoingDone9/Bug_RemainDir.

core/src/main/scala/org/apache/spark/util/ShutdownHookManager.scala

@@ -57,7 +57,9 @@ private[spark] object ShutdownHookManager extends Logging {
   // Add a shutdown hook to delete the temp dirs when the JVM exits
   addShutdownHook(TEMP_DIR_SHUTDOWN_PRIORITY) { () =>
     logInfo("Shutdown hook called")
-    shutdownDeletePaths.foreach { dirPath =>
+    // we need to materialize the paths to delete because deleteRecursively removes items from
+    // shutdownDeletePaths as we are traversing through it.
+    shutdownDeletePaths.toArray.foreach { dirPath =>
       try {
         logInfo("Deleting directory " + dirPath)
         Utils.deleteRecursively(new File(dirPath))