lhotari commented on code in PR #25070:
URL: https://github.com/apache/pulsar/pull/25070#discussion_r2686796178


##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/service/TopicListService.java:
##########
@@ -71,24 +102,67 @@ public List<String> getMatchingTopics() {
          */
         @Override
         public void accept(String topicName, NotificationType 
notificationType) {
+            if (closed) {
+                return;
+            }
             String partitionedTopicName = 
TopicName.get(topicName).getPartitionedTopicName();
             String domainLessTopicName = 
TopicList.removeTopicDomainScheme(partitionedTopicName);
 
             if (topicsPattern.matches(domainLessTopicName)) {
-                List<String> newTopics;
-                List<String> deletedTopics;
+                List<String> newTopics = Collections.emptyList();
+                List<String> deletedTopics = Collections.emptyList();
                 if (notificationType == NotificationType.Deleted) {
-                    newTopics = Collections.emptyList();
-                    deletedTopics = Collections.singletonList(topicName);
-                    matchingTopics.remove(topicName);
-                } else {
-                    deletedTopics = Collections.emptyList();
+                    if (matchingTopics.remove(topicName)) {
+                        deletedTopics = Collections.singletonList(topicName);
+                    }
+                } else if (matchingTopics.add(topicName)) {
                     newTopics = Collections.singletonList(topicName);
-                    matchingTopics.add(topicName);
                 }
-                String hash = TopicList.calculateHash(matchingTopics);
-                topicListService.sendTopicListUpdate(id, hash, deletedTopics, 
newTopics);
+                if (!newTopics.isEmpty() || !deletedTopics.isEmpty()) {
+                    String hash = TopicList.calculateHash(matchingTopics);
+                    sendTopicListUpdate(hash, deletedTopics, newTopics);
+                }
+            }
+        }
+
+        // sends updates one-by-one so that ordering is retained
+        private synchronized void sendTopicListUpdate(String hash, 
List<String> deletedTopics, List<String> newTopics) {
+            if (closed) {
+                return;
+            }
+            Runnable task = () -> topicListService.sendTopicListUpdate(id, 
hash, deletedTopics, newTopics,
+                    this::sendingCompleted);
+            if (!sendingInProgress) {
+                sendingInProgress = true;
+                executor.execute(task);
+            } else {
+                // if sendTopicListSuccess hasn't completed, add to a queue to 
be executed after it completes
+                if (!sendTopicListUpdateTasks.offer(task)) {
+                    log.warn("Dropping update for watcher id {} matching {} 
since queue is full.", id,
+                            topicsPattern.inputPattern());
+                }

Review Comment:
   This is now addressed, however the code coverage in tests should be improved.



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