HADOOP-15865. ConcurrentModificationException in Configuration.overlay() method. Contributed by Oleksandr Shevchenko.

(cherry picked from commit e872ceb810a343da7fce7185dca78d3b9aad9b7b)
(cherry picked from commit 46e72775f5b2156f49b15beac8bffff9cd5aad7f)

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/conf/Configuration.java

@@ -3395,8 +3395,10 @@ public class Configuration implements Iterable<Map.Entry<String,String>>,
   }
 
   private void overlay(Properties to, Properties from) {
-    for (Entry<Object, Object> entry: from.entrySet()) {
-      to.put(entry.getKey(), entry.getValue());
+    synchronized (from) {
+      for (Entry<Object, Object> entry : from.entrySet()) {
+        to.put(entry.getKey(), entry.getValue());
+      }
     }
   }