Amend HADOOP-8212 (Improve ActiveStandbyElector's behavior when session expires)

Amendment patch incorporates following feedback from Bikas Saha:
- adds a new functional test for session expiration while in the standby state
- adds a safety check in the StatCallback for session expiration
- improves some comments


git-svn-id: https://svn.apache.org/repos/asf/hadoop/common/branches/HDFS-3042@1307128 13f79535-47bb-0310-9956-ffa450edef68

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/ha/ActiveStandbyElector.java

@@ -470,13 +470,18 @@ public class ActiveStandbyElector implements StatCallback, StringCallback {
       }
       errorMessage = errorMessage
           + ". Not retrying further znode monitoring connection errors.";
+    } else if (isSessionExpired(code)) {
+      // This isn't fatal - the client Watcher will re-join the election
+      LOG.warn("Lock monitoring failed because session was lost");
+      return;
     }
 
     fatalError(errorMessage);
   }
 
   /**
-   * interface implementation of Zookeeper watch events (connection and node)
+   * interface implementation of Zookeeper watch events (connection and node),
+   * proxied by {@link WatcherWithClientRef}.
    */
   synchronized void processWatchEvent(ZooKeeper zk, WatchedEvent event) {
     Event.EventType eventType = event.getType();

hadoop-common-project/hadoop-common/src/test/java/org/apache/hadoop/ha/TestActiveStandbyElectorRealZK.java

@@ -26,6 +26,7 @@ import java.util.UUID;
 
 import org.apache.commons.logging.impl.Log4JLogger;
 import org.apache.hadoop.ha.ActiveStandbyElector.ActiveStandbyElectorCallback;
+import org.apache.hadoop.ha.ActiveStandbyElector.State;
 import org.apache.log4j.Level;
 import org.apache.zookeeper.ZooDefs.Ids;
 import org.apache.zookeeper.server.ZooKeeperServer;
@@ -196,4 +197,48 @@ public class TestActiveStandbyElectorRealZK extends ClientBase {
 
     checkFatalsAndReset();
   }
+  
+  @Test(timeout=15000)
+  public void testHandleSessionExpirationOfStandby() throws Exception {
+    // Let elector 0 be active
+    electors[0].ensureParentZNode();
+    electors[0].joinElection(appDatas[0]);
+    ZooKeeperServer zks = getServer(serverFactory);
+    ActiveStandbyElectorTestUtil.waitForActiveLockData(null,
+        zks, PARENT_DIR, appDatas[0]);
+    Mockito.verify(cbs[0], Mockito.timeout(1000)).becomeActive();
+    checkFatalsAndReset();
+    
+    // Let elector 1 be standby
+    electors[1].joinElection(appDatas[1]);
+    ActiveStandbyElectorTestUtil.waitForElectorState(null, electors[1],
+        State.STANDBY);
+    
+    LOG.info("========================== Expiring standby's session");
+    zks.closeSession(electors[1].getZKSessionIdForTests());
+
+    // Should enter neutral mode when disconnected
+    Mockito.verify(cbs[1], Mockito.timeout(1000)).enterNeutralMode();
+
+    // Should re-join the election and go back to STANDBY
+    ActiveStandbyElectorTestUtil.waitForElectorState(null, electors[1],
+        State.STANDBY);
+    checkFatalsAndReset();
+    
+    LOG.info("========================== Quitting election");
+    electors[1].quitElection(false);
+
+    // Double check that we don't accidentally re-join the election
+    // by quitting elector 0 and ensuring elector 1 doesn't become active
+    electors[0].quitElection(false);
+    
+    // due to receiving the "expired" event.
+    Thread.sleep(1000);
+    Mockito.verify(cbs[1], Mockito.never()).becomeActive();
+    ActiveStandbyElectorTestUtil.waitForActiveLockData(null,
+        zks, PARENT_DIR, null);
+
+    checkFatalsAndReset();
+  }
+
 }