Consumer cannot get the list of providers when use ZK as registry(sometimes it happens).

[chshyin]

the bug is：when consumer and provider started at the same time(or it happens in runtime,when cosumer and provider can't connect to zk,and then network recover)，sometimes the consumer will lost the provider list，causing the RpcException。restart consumer or provider will recover it。
code is here：
doSubscribe method id ZookeeperRegistry.java(169-182lines)

                List<URL> urls = new ArrayList<URL>();
                for (String path : toCategoriesPath(url)) {
                    ConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);
                    if (listeners == null) {
                        zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());
                        listeners = zkListeners.get(url);
                    }
                    ChildListener zkListener = listeners.get(listener);
                    if (zkListener == null) {
                        listeners.putIfAbsent(listener, new ChildListener() {
                            // A. it will invoke in zk thread,when zk node changed
                            public void childChanged(String parentPath, List<String> currentChilds) {
                                ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));
                            }
                        });
                        zkListener = listeners.get(listener);
                    }
                    zkClient.create(path, false);
                    List<String> children = zkClient.addChildListener(path, zkListener);
                    if (children != null) {
                        urls.addAll(toUrlsWithEmpty(url, path, children));
                    }
                }
                // B.it will invoke in main thread,or failoverRegister thread
                notify(url, listener, urls);

notity(url,listener,urls) method will invoke at:
A：it will invoke in zk thread,when zk node changed
B：it will invoke in main thread,or failoverRegister thread

The cause of the problem：

1. when consumer start，before main thread invoke B, thread completes its running slice on the CPU，and wait the next CPU time.at this time（1. provider not registed in zk,the urls is empty list。 2. it registed the zk watch event,will listener to zk node change event.）
   2.provider started，registed in zk, the zk node is changed
   3.reveive the zk node change event,invoke A in the zk thread。（A will invoke in zk thread，and B will invoke in another thread）
   4.At the end of the A method，the consumer obtain the real provider list.
   5.main thread obtain the CPU time，invoke B method，and this time urls is empty list。and update the consumer's provider list to empty。it cause the problem

the problem happens A invoke before B,If you can control the method execution order(B invoke before A),the bug will fix。

[chickenlj]

Hi chshyin,

Please consider using English, Dubbo is a global community, and all the issues may be viewed and discussed by people all over the world.

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[nzomkxia]

can you give me a procedure on how to reproduce this issue?

[chickenlj]

Please provide more details. Reopen if necessary.