Jason918 commented on a change in pull request #12654:
URL: https://github.com/apache/pulsar/pull/12654#discussion_r747468886



##########
File path: 
pulsar-broker/src/main/java/org/apache/pulsar/broker/service/SystemTopicBasedTopicPoliciesService.java
##########
@@ -474,7 +475,13 @@ public void registerListener(TopicName topicName, 
TopicPolicyListener<TopicPolic
 
     @Override
     public void unregisterListener(TopicName topicName, 
TopicPolicyListener<TopicPolicies> listener) {
-        listeners.computeIfAbsent(topicName, k -> 
Lists.newCopyOnWriteArrayList()).remove(listener);
+        List<TopicPolicyListener<TopicPolicies>> topicListeners = 
listeners.get(topicName);
+        if (topicListeners != null && topicListeners.remove(listener)) {
+            //delete topic from listeners if it's empty.
+            if (topicListeners.isEmpty()) {

Review comment:
       And I am working on unit test now. Writing unit test for concurrent 
problem is tricky.




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