MAPREDUCE-7437. MR Fetcher class to use an AtomicInteger to generate IDs. (#5579)

...as until now it wasn't thread safe

Contributed by Steve Loughran

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/java/org/apache/hadoop/mapreduce/task/reduce/Fetcher.java

@@ -31,6 +31,7 @@ import java.util.Collection;
 import java.util.HashSet;
 import java.util.List;
 import java.util.Set;
+import java.util.concurrent.atomic.AtomicInteger;
 
 import javax.crypto.SecretKey;
 import javax.net.ssl.HttpsURLConnection;
@@ -88,7 +89,7 @@ class Fetcher<K,V> extends Thread {
   protected final ShuffleClientMetrics metrics;
   protected final ExceptionReporter exceptionReporter;
   protected final int id;
-  private static int nextId = 0;
+  private static final AtomicInteger NEXT_ID = new AtomicInteger(0);
   protected final int reduce;
   
   private final int connectionTimeout;
@@ -115,7 +116,7 @@ class Fetcher<K,V> extends Thread {
                  Reporter reporter, ShuffleClientMetrics metrics,
                  ExceptionReporter exceptionReporter, SecretKey shuffleKey) {
     this(job, reduceId, scheduler, merger, reporter, metrics,
-        exceptionReporter, shuffleKey, ++nextId);
+        exceptionReporter, shuffleKey, NEXT_ID.incrementAndGet());
   }
 
   @VisibleForTesting