[fix][client] fix retry topic with exclusive mode.

pulsar-broker/src/test/java/org/apache/pulsar/client/api/RetryTopicTest.java

@@ -136,6 +136,46 @@ public void testRetryTopic() throws Exception {
         checkConsumer.close();
     }
 
+    /**
+     * Retry topic feature relies on the delay queue feature when consumer produce a delayed message
+     * to the retry topic. The delay queue feature is only supported in shared and key-shared subscription type.
+     * As a result, the subscription type of the retry topic should be shared or key-shared.
+     * @throws Exception
+     */
+    @Test
+    public void testRetryTopicWithExclusiveMode() throws Exception {
+        final String topic = "persistent://my-property/my-ns/retry-topic-exclusive";
+        final int maxRedeliveryCount = 2;
+
+        Consumer<byte[]> consumer = pulsarClient.newConsumer(Schema.BYTES)
+                .topic(topic)
+                .subscriptionName("my-subscription")
+                .subscriptionType(SubscriptionType.Exclusive)
+                .enableRetry(true)
+                .deadLetterPolicy(DeadLetterPolicy.builder().maxRedeliverCount(maxRedeliveryCount).build())
+                .receiverQueueSize(100)
+                .subscriptionInitialPosition(SubscriptionInitialPosition.Earliest)
+                .subscribe();
+
+        Producer<byte[]> producer = pulsarClient.newProducer(Schema.BYTES)
+                .topic(topic)
+                .create();
+
+        producer.send("Hello Pulsar".getBytes());
+        producer.close();
+
+        // receive message and set delay to 5 seconds
+        Message<byte[]> message = consumer.receive();
+        long timestamp = System.currentTimeMillis();
+        consumer.reconsumeLater(message, 4, TimeUnit.SECONDS);
+
+        // receive message and check the delay is at least 4 seconds
+        consumer.receive();
+        long delay = System.currentTimeMillis() - timestamp;
+        assertTrue(delay >= 2000);
+        consumer.close();
+    }
+
     @Data
     public static class Foo {
         @Nullable

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerBase.java

@@ -48,6 +48,7 @@
 import org.apache.pulsar.client.api.Consumer;
 import org.apache.pulsar.client.api.ConsumerBuilder;
 import org.apache.pulsar.client.api.ConsumerEventListener;
+import org.apache.pulsar.client.api.DeadLetterPolicy;
 import org.apache.pulsar.client.api.Message;
 import org.apache.pulsar.client.api.MessageId;
 import org.apache.pulsar.client.api.MessageIdAdv;
@@ -792,6 +793,13 @@ private boolean isCumulativeAcknowledgementAllowed(SubscriptionType type) {
     }
 
     protected SubType getSubType() {
+        // For retry topic, we always use Shared subscription
+        // Because we will produce delayed messages to retry topic.
+        DeadLetterPolicy deadLetterPolicy = conf.getDeadLetterPolicy();
+        if (deadLetterPolicy != null && topic.equals(deadLetterPolicy.getRetryLetterTopic())) {
+            return SubType.Shared;
+        }
+
         SubscriptionType type = conf.getSubscriptionType();
         switch (type) {
         case Exclusive:

pulsar-client/src/main/java/org/apache/pulsar/client/impl/ConsumerBuilderImpl.java

@@ -151,7 +151,7 @@ public CompletableFuture<Consumer<T>> subscribeAsync() {
                     Math.max(conf.getBatchReceivePolicy().getMaxNumMessages(), conf.getReceiverQueueSize()));
         }
         CompletableFuture<Void> applyDLQConfig;
-        if (conf.isRetryEnable() && conf.getTopicNames().size() > 0) {
+        if (conf.isRetryEnable() && !conf.getTopicNames().isEmpty()) {
             TopicName topicFirst = TopicName.get(conf.getTopicNames().iterator().next());
             //Issue 9327: do compatibility check in case of the default retry and dead letter topic name changed
             String oldRetryLetterTopic = TopicName.get(topicFirst.getDomain().value(), topicFirst.getNamespaceObject(),