[SPARK-5423] [CORE] Register a TaskCompletionListener to make sure re…

…lease all resources

Make `DiskMapIterator.cleanup` idempotent and register a TaskCompletionListener to make sure call `cleanup`.

Author: zsxwing <zsxwing@gmail.com>

Closes #7529 from zsxwing/SPARK-5423 and squashes the following commits:

3e3c413 [zsxwing] Remove TODO
9556c78 [zsxwing] Fix NullPointerException for tests
3d574d9 [zsxwing] Register a TaskCompletionListener to make sure release all resources

core/src/main/scala/org/apache/spark/util/collection/ExternalAppendOnlyMap.scala

@@ -26,7 +26,7 @@ import scala.collection.mutable.ArrayBuffer
 
 import com.google.common.io.ByteStreams
 
-import org.apache.spark.{Logging, SparkEnv}
+import org.apache.spark.{Logging, SparkEnv, TaskContext}
 import org.apache.spark.annotation.DeveloperApi
 import org.apache.spark.serializer.{DeserializationStream, Serializer}
 import org.apache.spark.storage.{BlockId, BlockManager}
@@ -470,14 +470,27 @@ class ExternalAppendOnlyMap[K, V, C](
       item
     }
 
-    // TODO: Ensure this gets called even if the iterator isn't drained.
     private def cleanup() {
       batchIndex = batchOffsets.length  // Prevent reading any other batch
       val ds = deserializeStream
-      deserializeStream = null
-      fileStream = null
-      ds.close()
-      file.delete()
+      if (ds != null) {
+        ds.close()
+        deserializeStream = null
+      }
+      if (fileStream != null) {
+        fileStream.close()
+        fileStream = null
+      }
+      if (file.exists()) {
+        file.delete()
+      }
+    }
+
+    val context = TaskContext.get()
+    // context is null in some tests of ExternalAppendOnlyMapSuite because these tests don't run in
+    // a TaskContext.
+    if (context != null) {
+      context.addTaskCompletionListener(context => cleanup())
     }
   }