BewareMyPower commented on code in PR #23049:
URL: https://github.com/apache/pulsar/pull/23049#discussion_r1682388030


##########
pulsar-broker/src/main/java/org/apache/pulsar/broker/namespace/NamespaceService.java:
##########
@@ -1509,6 +1514,31 @@ public CompletableFuture<List<String>> 
getListOfTopics(NamespaceName namespaceNa
         }
     }
 
+    public CompletableFuture<List<String>> getListOfUserTopics(NamespaceName 
namespaceName, Mode mode) {
+        String key = String.format("%s://%s", mode, namespaceName);
+        CompletableFuture<List<String>> queryRes =
+                inProgressQueryUserTopics.computeIfAbsent(key, k -> {
+            CompletableFuture<List<String>> res = new CompletableFuture<>();
+            // Switch thread to avoid blocking other threads who are calling 
the current method.
+            pulsar.getExecutor().execute(() -> {
+                getListOfTopics(namespaceName, mode).thenApply(list -> {
+                    return TopicList.filterSystemTopic(list);
+                }).whenComplete((topics, ex) -> {
+                    if (ex != null) {
+                        res.completeExceptionally(ex);
+                    } else {
+                        res.complete(topics);
+                    }
+                });
+            });
+            return res;
+        });
+        queryRes.whenComplete((ignore, ex) -> {

Review Comment:
   > the CompletableFuture in the second code never complete.
   
   See the following code and outputs.
   
   ```java
       static class Cache {
           private final Map<Integer, CompletableFuture<Integer>> futures = new 
ConcurrentHashMap<>();
   
           public CompletableFuture<Integer> run(int x) {
               final var inserted = new MutableBoolean(false);
               final var future = futures.computeIfAbsent(x, __ -> {
                   inserted.setTrue();
                   final var innerFuture = new CompletableFuture<Integer>();
                   CompletableFuture.runAsync(() -> {
                       try {
                           Thread.sleep(100);
                       } catch (InterruptedException e) {
                       }
                       innerFuture.complete(100);
                   });
                   return innerFuture;
               });
               if (inserted.isTrue()) {
                   future.whenComplete((__, ___) -> {
                       System.out.println("Remove " + x);
                       futures.remove(x, future);
                   });
               }
               return future;
           }
       }
   
       public static void main(String[] args) throws ExecutionException, 
InterruptedException {
           final var cache = new Cache();
           cache.run(1).thenAccept(System.out::println);
           cache.run(1).thenAccept(System.out::println);
           cache.run(1).thenAccept(System.out::println);
           cache.run(1).thenAccept(System.out::println).get();
   ```
   
   Outputs:
   
   ```
   100
   100
   100
   100
   Remove 1
   ```



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