[HBASE-11018] ZKUtil.getChildDataAndWatchForNewChildren() will not return null as indicated

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 0.96.1, 0.98.1
• Fix Version/s: 0.99.0
• Component/s: Zookeeper
• Hadoop Flags: Reviewed
• Assignee: Jerry He
• Reporter: Jerry He
• Created: 17/Apr/14 20:38
• Updated: 21/Feb/15 23:32
• Resolved: 22/Apr/14 02:30

Description

While working on HBase acl, I found out that ZKUtil.getChildDataAndWatchForNewChildren() will not return null as indicated. Here is the code:

 /**
  ....
   * Returns null if the specified node does not exist.  Otherwise returns a
   * list of children of the specified node.  If the node exists but it has no
   * children, an empty list will be returned.
  ....
   */
  public static List<NodeAndData> getChildDataAndWatchForNewChildren(
      ZooKeeperWatcher zkw, String baseNode) throws KeeperException {
    List<String> nodes =
      ZKUtil.listChildrenAndWatchForNewChildren(zkw, baseNode);
    List<NodeAndData> newNodes = new ArrayList<NodeAndData>();
    if (nodes != null) {
      for (String node : nodes) {
        String nodePath = ZKUtil.joinZNode(baseNode, node);
        byte[] data = ZKUtil.getDataAndWatch(zkw, nodePath);
        newNodes.add(new NodeAndData(nodePath, data));
      }
    }
    return newNodes;
  }

We return 'newNodes' which will never be null.

This is a deprecated method. But it is still used in HBase code.
For example: org.apache.hadoop.hbase.security.access.ZKPermissionWatcher.start()

  public void start() throws KeeperException {
    watcher.registerListener(this);
    if (ZKUtil.watchAndCheckExists(watcher, aclZNode)) {
      List<ZKUtil.NodeAndData> existing =
          ZKUtil.getChildDataAndWatchForNewChildren(watcher, aclZNode);
      if (existing != null) {
        refreshNodes(existing);
      }
    }
  }

We test the 'null' return from the call which becomes the problem.