MAPREDUCE-7156. NullPointerException when reaching max shuffle connections. Contributed by Peter Bacsko

(cherry picked from commit ba1f9d66d94ed0b85084d7c40c09a87478b3a05a)

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-shuffle/src/main/java/org/apache/hadoop/mapred/ShuffleHandler.java

@@ -911,6 +911,8 @@ public class ShuffleHandler extends AuxiliaryService {
     @Override
     public void channelOpen(ChannelHandlerContext ctx, ChannelStateEvent evt) 
         throws Exception {
+      super.channelOpen(ctx, evt);
+
       if ((maxShuffleConnections > 0) && (accepted.size() >= maxShuffleConnections)) {
         LOG.info(String.format("Current number of shuffle connections (%d) is " + 
             "greater than or equal to the max allowed shuffle connections (%d)", 
@@ -926,8 +928,6 @@ public class ShuffleHandler extends AuxiliaryService {
         return;
       }
       accepted.add(evt.getChannel());
-      super.channelOpen(ctx, evt);
-     
     }
 
     @Override