ZookeeperDataSource might not be initialized well unless the ZooKeeper server is active(#588)

[zhousiliang163]

Describe what this PR does / why we need it

When the ZookeeperDataSource data source is initialized, if the zk server cannot be used or the connection between the client and the zk server is timed out due to network reasons, the monitoring of the data of the znode node may fail

Does this pull request fix one issue?

Fixes #588

Describe how you did it

NodeCache类本身已经实现了对zk的连接和重连事件进行监听，源码如下：

 private ConnectionStateListener connectionStateListener = new ConnectionStateListener()
    {
        @Override
        public void stateChanged(CuratorFramework client, ConnectionState newState)
        {
            if ( (newState == ConnectionState.CONNECTED) || (newState == ConnectionState.RECONNECTED) )
            {
                if ( isConnected.compareAndSet(false, true) )
                {
                    try
                    {
                        reset();
                    }
                    catch ( Exception e )
                    {
                        ThreadUtils.checkInterrupted(e);
                        log.error("Trying to reset after reconnection", e);
                    }
                }
            }
            else
            {
                isConnected.set(false);
            }
        }
    };

reset()方法会设置nodeCache的data属性，所以用户可以通过nodeCache.getCurrentData()方法获取。

Describe how to verify it

Special notes for reviews

[CLAassistant]

All committers have signed the CLA.

[codecov-io]

Codecov Report

Merging #597 into master will increase coverage by 0.58%.
The diff coverage is 81.81%.

@@             Coverage Diff              @@
##             master     #597      +/-   ##
============================================
+ Coverage     39.28%   39.87%   +0.58%     
- Complexity     1229     1246      +17     
============================================
  Files           275      275              
  Lines          8702     8699       -3     
  Branches       1163     1161       -2     
============================================
+ Hits           3419     3469      +50     
+ Misses         4846     4784      -62     
- Partials        437      446       +9

Impacted Files	Coverage Δ	Complexity Δ
...inel/datasource/zookeeper/ZookeeperDataSource.java	61.11% <81.81%> (-0.23%)	9 <0> (+1)
...ava/com/alibaba/csp/sentinel/node/ClusterNode.java	95.23% <0%> (-4.77%)	7% <0%> (-1%)
...l/cluster/server/connection/ConnectionManager.java	76% <0%> (+2%)	11% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LeapArray.java	70.58% <0%> (+2.94%)	34% <0%> (+1%)	⬆️
...m/alibaba/csp/sentinel/log/DateFileLogHandler.java	57.57% <0%> (+3.03%)	7% <0%> (+1%)	⬆️
...a/csp/sentinel/slots/statistic/base/LongAdder.java	34.04% <0%> (+17.02%)	11% <0%> (+7%)	⬆️
...a/csp/sentinel/slots/statistic/base/Striped64.java	66.66% <0%> (+40.62%)	12% <0%> (+7%)	⬆️

---

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[sczyh30]

Could you please explain your changes?

[zhousiliang163]

Could you please explain your changes?

        Stat stat = this.zkClient.checkExists().forPath(this.path);
          if (stat == null) {
              this.zkClient.create().creatingParentContainersIfNeeded().withMode(CreateMode.PERSISTENT).forPath(this.path, null);
          }

把上面的代码删除有两个原因：
一、ZookeeperDataSource是对数据源的监听，没有必要去判断是否存在这个path,并且可能去创建这个path.
二、当ZookeeperDataSource初始化时，如果zk 服务器异常或者网络问题，会导致对数据源的监听失败。

把代码：

  byte[] data = this.zkClient.getData().forPath(this.path);
     if (data != null) {
         return new String(data);

修改为：

  ChildData childData = nodeCache.getCurrentData();
        if (null != childData && childData.getData() != null) {

            configInfo = new String(childData.getData());
        }

当ZookeeperDataSource初始化时，如果zk 服务器异常或者网络问题this.zkClient.getData().forPath(this.path)会执行得特别慢，最终会报错。
nodeCache.getCurrentData()可以代替它

[zhousiliang163]

Could you please explain your changes?

最终实现初始化时对zk数据源的监听与zk服务器、网络等是否正常无关

[sczyh30]

LGTM

[sczyh30]

Thanks for contributing!