当zk出现session time out可能会导致instance出现多个消费者，从而出现乱序 (#5270)

Author: lwd-coding

server/src/main/java/com/alibaba/otter/canal/server/CanalMQStarter.java

@@ -15,6 +15,7 @@
 import org.slf4j.MDC;
 
 import java.util.Map;
+import java.util.Objects;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -106,7 +107,10 @@ public synchronized void destroy() {
 
     public synchronized void startDestination(String destination) {
         CanalInstance canalInstance = canalServer.getCanalInstances().get(destination);
-        if (canalInstance != null) {
+        CanalMQRunnable canalMQRunable = canalMQWorks.get(destination);
+        //当zk出现session time out可能会导致instance出现多个消费者，从而出现乱序
+        //因此当抢占成功的实例已经启动CanalMQRunnable时就不需要stop然后start了
+        if (canalInstance != null && Objects.isNull(canalMQRunable)) {
             stopDestination(destination);
             CanalMQRunnable canalMQRunnable = new CanalMQRunnable(destination);
             canalMQWorks.put(canalInstance.getDestination(), canalMQRunnable);