fix https://github.com/ctripcorp/apollo/issues/2111 (#2255)

apollo-configservice/src/main/java/com/ctrip/framework/apollo/configservice/controller/NotificationControllerV2.java

@@ -132,24 +132,11 @@ public class NotificationControllerV2 implements ReleaseMessageListener {
 
     Set<String> watchedKeys = Sets.newHashSet(watchedKeysMap.values());
 
-    List<ReleaseMessage> latestReleaseMessages =
-        releaseMessageService.findLatestReleaseMessagesGroupByMessages(watchedKeys);
-
     /**
-     * Manually close the entity manager.
-     * Since for async request, Spring won't do so until the request is finished,
-     * which is unacceptable since we are doing long polling - means the db connection would be hold
-     * for a very long time
+     * 1、set deferredResult before the check, for avoid more waiting
+     * If the check before setting deferredResult,it may receive a notification the next time
+     * when method handleMessage is executed between check and set deferredResult.
      */
-    entityManagerUtil.closeEntityManager();
-
-    List<ApolloConfigNotification> newNotifications =
-        getApolloConfigNotifications(namespaces, clientSideNotifications, watchedKeysMap,
-            latestReleaseMessages);
-
-    if (!CollectionUtils.isEmpty(newNotifications)) {
-      deferredResultWrapper.setResult(newNotifications);
-    } else {
     deferredResultWrapper
           .onTimeout(() -> logWatchedKeys(watchedKeys, "Apollo.LongPoll.TimeOutKeys"));
 
@@ -169,6 +156,27 @@ public class NotificationControllerV2 implements ReleaseMessageListener {
     logWatchedKeys(watchedKeys, "Apollo.LongPoll.RegisteredKeys");
     logger.debug("Listening {} from appId: {}, cluster: {}, namespace: {}, datacenter: {}",
         watchedKeys, appId, cluster, namespaces, dataCenter);
+
+    /**
+     * 2、check new release
+     */
+    List<ReleaseMessage> latestReleaseMessages =
+        releaseMessageService.findLatestReleaseMessagesGroupByMessages(watchedKeys);
+
+    /**
+     * Manually close the entity manager.
+     * Since for async request, Spring won't do so until the request is finished,
+     * which is unacceptable since we are doing long polling - means the db connection would be hold
+     * for a very long time
+     */
+    entityManagerUtil.closeEntityManager();
+
+    List<ApolloConfigNotification> newNotifications =
+        getApolloConfigNotifications(namespaces, clientSideNotifications, watchedKeysMap,
+            latestReleaseMessages);
+
+    if (!CollectionUtils.isEmpty(newNotifications)) {
+      deferredResultWrapper.setResult(newNotifications);
     }
 
     return deferredResultWrapper.getResult();
@@ -308,4 +316,3 @@ public class NotificationControllerV2 implements ReleaseMessageListener {
     }
   }
 }
-