From d71843558b52f33cc46215313dba6acc8ad48414 Mon Sep 17 00:00:00 2001
From: Walter Su <waltersu4549@apache.org>
Date: Mon, 28 Mar 2016 15:44:25 +0800
Subject: [PATCH] HDFS-10182. Hedged read might overwrite user's buf.
 Contributed by zhouyingchao.

(cherry picked from commit d8383c687c95dbb37effa307ab2d41497da1cfc2)
---
 .../org/apache/hadoop/hdfs/DFSInputStream.java   | 16 +++++++---------
 1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java b/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
index f4dad121051..fc67aa6b451 100644
--- a/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
+++ b/hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/DFSInputStream.java
@@ -1315,7 +1315,7 @@ private void hedgedFetchBlockByteRange(LocatedBlock block, long start,
         // chooseDataNode is a commitment. If no node, we go to
         // the NN to reget block locations. Only go here on first read.
         chosenNode = chooseDataNode(block, ignored);
-        bb = ByteBuffer.wrap(buf, offset, len);
+        bb = ByteBuffer.allocate(len);
         Callable<ByteBuffer> getFromDataNodeCallable = getFromOneDataNode(
             chosenNode, block, start, end, bb,
             corruptedBlockMap, hedgedReadId++);
@@ -1326,7 +1326,9 @@ private void hedgedFetchBlockByteRange(LocatedBlock block, long start,
           Future<ByteBuffer> future = hedgedService.poll(
               conf.getHedgedReadThresholdMillis(), TimeUnit.MILLISECONDS);
           if (future != null) {
-            future.get();
+            ByteBuffer result = future.get();
+            System.arraycopy(result.array(), result.position(), buf, offset,
+                len);
             return;
           }
           DFSClient.LOG.debug("Waited {}ms to read from {}; spawning hedged "
@@ -1364,13 +1366,9 @@ private void hedgedFetchBlockByteRange(LocatedBlock block, long start,
           ByteBuffer result = getFirstToComplete(hedgedService, futures);
           // cancel the rest.
           cancelAll(futures);
-          if (result.array() != buf) { // compare the array pointers
-            dfsClient.getHedgedReadMetrics().incHedgedReadWins();
-            System.arraycopy(result.array(), result.position(), buf, offset,
-                len);
-          } else {
-            dfsClient.getHedgedReadMetrics().incHedgedReadOps();
-          }
+          dfsClient.getHedgedReadMetrics().incHedgedReadWins();
+          System.arraycopy(result.array(), result.position(), buf, offset,
+              len);
           return;
         } catch (InterruptedException ie) {
           // Ignore and retry