SPARK-1518: FileLogger: Fix compile against Hadoop trunk

In Hadoop trunk (currently Hadoop 3.0.0), the deprecated
FSDataOutputStream#sync() method has been removed.  Instead, we should
call FSDataOutputStream#hflush, which does the same thing as the
deprecated method used to do.

Author: Colin McCabe <cmccabe@cloudera.com>

Closes #898 from cmccabe/SPARK-1518 and squashes the following commits:

752b9d7 [Colin McCabe] FileLogger: Fix compile against Hadoop trunk

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

@@ -61,6 +61,14 @@ private[spark] class FileLogger(
   // Only defined if the file system scheme is not local
   private var hadoopDataStream: Option[FSDataOutputStream] = None
 
+  // The Hadoop APIs have changed over time, so we use reflection to figure out
+  // the correct method to use to flush a hadoop data stream. See SPARK-1518
+  // for details.
+  private val hadoopFlushMethod = {
+    val cls = classOf[FSDataOutputStream]
+    scala.util.Try(cls.getMethod("hflush")).getOrElse(cls.getMethod("sync"))
+  }
+
   private var writer: Option[PrintWriter] = None
 
   /**
@@ -149,13 +157,13 @@ private[spark] class FileLogger(
   /**
    * Flush the writer to disk manually.
    *
-   * If the Hadoop FileSystem is used, the underlying FSDataOutputStream (r1.0.4) must be
-   * sync()'ed manually as it does not support flush(), which is invoked by when higher
-   * level streams are flushed.
+   * When using a Hadoop filesystem, we need to invoke the hflush or sync
+   * method. In HDFS, hflush guarantees that the data gets to all the
+   * DataNodes.
    */
   def flush() {
     writer.foreach(_.flush())
-    hadoopDataStream.foreach(_.sync())
+    hadoopDataStream.foreach(hadoopFlushMethod.invoke(_))
   }
 
   /**