YARN-5728. TestMiniYarnClusterNodeUtilization.testUpdateNodeUtilization timeout.

(cherry picked from commit f8bed5e9a7d1ece127fb3da123bbfc26ada0016f)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-tests/src/test/java/org/apache/hadoop/yarn/server/MiniYARNCluster.java

@@ -21,6 +21,7 @@ package org.apache.hadoop.yarn.server;
 import java.io.File;
 import java.io.IOException;
 import java.net.InetAddress;
+import java.net.InetSocketAddress;
 import java.net.UnknownHostException;
 import java.util.Collection;
 import java.util.Map;
@@ -36,6 +37,7 @@ import org.apache.hadoop.fs.FileContext;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.ha.HAServiceProtocol;
 import org.apache.hadoop.metrics2.lib.DefaultMetricsSystem;
+import org.apache.hadoop.net.NetUtils;
 import org.apache.hadoop.security.token.Token;
 import org.apache.hadoop.service.AbstractService;
 import org.apache.hadoop.service.CompositeService;
@@ -445,7 +447,16 @@ public class MiniYARNCluster extends CompositeService {
 
   public static String getHostname() {
     try {
-      return InetAddress.getLocalHost().getHostName();
+      String hostname = InetAddress.getLocalHost().getHostName();
+      // Create InetSocketAddress to see whether it is resolved or not.
+      // If not, just return "localhost".
+      InetSocketAddress addr =
+          NetUtils.createSocketAddrForHost(hostname, 1);
+      if (addr.isUnresolved()) {
+        return "localhost";
+      } else {
+        return hostname;
+      }
     }
     catch (UnknownHostException ex) {
       throw new RuntimeException(ex);