HDFS-15585: ViewDFS#getDelegationToken should not throw UnsupportedOperationException. (#2312). Contributed by Uma Maheswara Rao G.

hadoop-hdfs-project/hadoop-hdfs-client/src/main/java/org/apache/hadoop/hdfs/ViewDistributedFileSystem.java

@@ -1032,15 +1032,24 @@ public class ViewDistributedFileSystem extends DistributedFileSystem {
     return super.getDefaultPort();
   }
 
+  /**
+   * If no mount points configured, it works same as
+   * {@link DistributedFileSystem#getDelegationToken(String)}. If
+   * there are mount points configured and if default fs(linkFallback)
+   * configured, then it will return default fs delegation token. Otherwise
+   * it will return null.
+   */
   @Override
   public Token<DelegationTokenIdentifier> getDelegationToken(String renewer)
       throws IOException {
     if (this.vfs == null) {
       return super.getDelegationToken(renewer);
     }
-    //Let applications call getDelegationTokenIssuers and get respective
-    // delegation tokens from child fs.
-    throw new UnsupportedOperationException();
+
+    if (defaultDFS != null) {
+      return defaultDFS.getDelegationToken(renewer);
+    }
+    return null;
   }
 
   @Override

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/TestViewDistributedFileSystem.java

@@ -18,14 +18,17 @@
 package org.apache.hadoop.hdfs;
 
 import org.apache.hadoop.conf.Configuration;
+import org.apache.hadoop.fs.CommonConfigurationKeys;
 import org.apache.hadoop.fs.FileSystem;
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.fs.PathHandle;
+import org.apache.hadoop.fs.viewfs.ConfigUtil;
 import org.apache.hadoop.hdfs.protocol.HdfsConstants;
 import org.apache.hadoop.test.Whitebox;
 import org.junit.Test;
 
 import java.io.IOException;
+import java.net.URI;
 
 public class TestViewDistributedFileSystem extends TestDistributedFileSystem{
   @Override
@@ -67,4 +70,23 @@ public class TestViewDistributedFileSystem extends TestDistributedFileSystem{
       }
     }
   }
+
+  @Override
+  public void testEmptyDelegationToken() throws IOException {
+    Configuration conf = getTestConfiguration();
+    MiniDFSCluster cluster = null;
+    try {
+      cluster = new MiniDFSCluster.Builder(conf).numDataNodes(0).build();
+      URI defaultUri =
+          URI.create(conf.get(CommonConfigurationKeys.FS_DEFAULT_NAME_KEY));
+      ConfigUtil.addLinkFallback(conf, defaultUri.getHost(), defaultUri);
+      try (FileSystem fileSys = FileSystem.get(conf)) {
+        fileSys.getDelegationToken("");
+      }
+    } finally {
+      if (cluster != null) {
+        cluster.shutdown();
+      }
+    }
+  }
 }