HDFS-14961. [SBN read] Prevent ZKFC changing Observer Namenode state. Contributed by Ayush Saxena.

hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/namenode/NameNodeRpcServer.java

@@ -1793,11 +1793,22 @@ public synchronized void transitionToActive(StateChangeRequestInfo req)
     nn.checkHaStateChange(req);
     nn.transitionToActive();
   }
-  
+
   @Override // HAServiceProtocol
-  public synchronized void transitionToStandby(StateChangeRequestInfo req) 
+  public synchronized void transitionToStandby(StateChangeRequestInfo req)
       throws ServiceFailedException, AccessControlException, IOException {
     checkNNStartup();
+    // This is to eliminate any race condition between manual transition of
+    // namenode into Observer, and ZKFC auto failover election, when the
+    // namenode has already participated in the
+    // ZKFC election, before transition to Observer state as Standby Node.
+    // For more details check : HDFS-14961.
+    if (nn.getState().equals(NameNode.OBSERVER_STATE.toString())
+        && req.getSource() == RequestSource.REQUEST_BY_ZKFC) {
+      throw new AccessControlException(
+          "Request from ZK failover controller at " + Server.getRemoteAddress()
+              + " denied since the namenode is in Observer state.");
+    }
     nn.checkHaStateChange(req);
     nn.transitionToStandby();
   }

hadoop-hdfs-project/hadoop-hdfs/src/test/java/org/apache/hadoop/hdfs/tools/TestDFSZKFailoverController.java

@@ -30,6 +30,8 @@
 import org.apache.hadoop.fs.Path;
 import org.apache.hadoop.ha.ClientBaseWithFixes;
 import org.apache.hadoop.ha.HAServiceProtocol.HAServiceState;
+import org.apache.hadoop.ha.HAServiceProtocol.RequestSource;
+import org.apache.hadoop.ha.HAServiceProtocol.StateChangeRequestInfo;
 import org.apache.hadoop.ha.HealthMonitor;
 import org.apache.hadoop.ha.TestNodeFencer.AlwaysSucceedFencer;
 import org.apache.hadoop.ha.ZKFCTestUtil;
@@ -41,8 +43,11 @@
 import org.apache.hadoop.hdfs.server.namenode.MockNameNodeResourceChecker;
 import org.apache.hadoop.hdfs.server.namenode.NameNode;
 import org.apache.hadoop.hdfs.server.namenode.ha.HATestUtil;
+import org.apache.hadoop.hdfs.server.protocol.NamenodeProtocols;
 import org.apache.hadoop.net.ServerSocketUtil;
+import org.apache.hadoop.security.AccessControlException;
 import org.apache.hadoop.test.GenericTestUtils;
+import org.apache.hadoop.test.LambdaTestUtils;
 import org.apache.hadoop.test.MultithreadedTestUtil.TestContext;
 import org.apache.hadoop.test.MultithreadedTestUtil.TestingThread;
 import org.junit.After;
@@ -201,7 +206,25 @@ public void testManualFailover() throws Exception {
     waitForHAState(0, HAServiceState.ACTIVE);
     waitForHAState(1, HAServiceState.STANDBY);
   }
-  
+
+  /**
+   * Tests that a Namenode in Observer state rejects any state transition
+   * request from ZKFC, as a result of namenode's participation in the ZK
+   * election before getting transition to Observer state, since Observer
+   * Namenode isn't supposed to participate in ZKFC elections.
+   */
+  @Test
+  public void testObserverRejectZkfcCall() throws Exception {
+    NamenodeProtocols nn1 = cluster.getNameNode(1).getRpcServer();
+    nn1.transitionToObserver(
+        new StateChangeRequestInfo(RequestSource.REQUEST_BY_USER_FORCED));
+    waitForHAState(1, HAServiceState.OBSERVER);
+    LambdaTestUtils.intercept(AccessControlException.class,
+        "denied since the namenode is in Observer state.",
+        () -> nn1.transitionToStandby(
+            new StateChangeRequestInfo(RequestSource.REQUEST_BY_ZKFC)));
+  }
+
   @Test(timeout=30000)
   public void testManualFailoverWithDFSHAAdmin() throws Exception {
     DFSHAAdmin tool = new DFSHAAdmin();