HADOOP-17130. Configuration.getValByRegex() shouldn't be updating the results while fetching. (#2142)

Contributed by Mukund Thakur

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

@@ -3869,6 +3869,7 @@ public class Configuration implements Iterable<Map.Entry<String,String>>,
     Pattern p = Pattern.compile(regex);
 
     Map<String,String> result = new HashMap<String,String>();
+    List<String> resultKeys = new ArrayList<>();
     Matcher m;
 
     for(Map.Entry<Object,Object> item: getProps().entrySet()) {
@@ -3876,11 +3877,12 @@ public class Configuration implements Iterable<Map.Entry<String,String>>,
           item.getValue() instanceof String) {
         m = p.matcher((String)item.getKey());
         if(m.find()) { // match
-          result.put((String) item.getKey(),
-              substituteVars(getProps().getProperty((String) item.getKey())));
+          resultKeys.add((String) item.getKey());
         }
       }
     }
+    resultKeys.forEach(item ->
+            result.put(item, substituteVars(getProps().getProperty(item))));
     return result;
   }