[ISSUE #9791] Prefer original producer when checking transaction state

When multiple producers in the same group handle different topics,
the transaction check may be routed to an unrelated producer that
cannot determine the transaction state correctly.

Fix: Record the original producer's clientId in the half message
properties. During transaction check-back, prefer the channel
matching that clientId. Fall back to round-robin if the original
producer is offline.

Changes:
- MessageConst: add PROPERTY_TRANSACTION_PRODUCER_CLIENT_ID
- DefaultMQProducerImpl: write clientId into half message
- ProducerManager: add getAvailableChannel(groupId, preferredClientId)
- AbstractTransactionalMessageCheckListener: use new method
- TransactionalMessageRocksDBService: use new method
- ProducerManagerTest: 3 new test cases

Author: wangjiahua.wjh

broker/src/main/java/org/apache/rocketmq/broker/client/ProducerManager.java

@@ -316,6 +316,30 @@ public Channel getAvailableChannel(String groupId) {
         return lastActiveChannel;
     }
 
+    /**
+     * Get an available channel for the given group, preferring the producer that originally sent the message.
+     * Falls back to round-robin if the preferred producer is not found or not available.
+     *
+     * @param groupId producer group
+     * @param preferredClientId the clientId of the original producer (from half message properties), may be null
+     * @return an available channel, or null if none found
+     */
+    public Channel getAvailableChannel(String groupId, String preferredClientId) {
+        if (preferredClientId != null) {
+            ConcurrentMap<Channel, ClientChannelInfo> channelMap = groupChannelTable.get(groupId);
+            if (channelMap != null) {
+                for (Map.Entry<Channel, ClientChannelInfo> entry : channelMap.entrySet()) {
+                    if (preferredClientId.equals(entry.getValue().getClientId())
+                        && entry.getKey().isActive() && entry.getKey().isWritable()) {
+                        return entry.getKey();
+                    }
+                }
+            }
+        }
+        // Fall back to round-robin selection
+        return getAvailableChannel(groupId);
+    }
+
     public Channel findChannel(String clientId) {
         return clientChannelTable.get(clientId);
     }

broker/src/main/java/org/apache/rocketmq/broker/transaction/AbstractTransactionalMessageCheckListener.java

@@ -61,7 +61,8 @@ public void sendCheckMessage(MessageExt msgExt) throws Exception {
         msgExt.setQueueId(Integer.parseInt(msgExt.getUserProperty(MessageConst.PROPERTY_REAL_QUEUE_ID)));
         msgExt.setStoreSize(0);
         String groupId = msgExt.getProperty(MessageConst.PROPERTY_PRODUCER_GROUP);
-        Channel channel = brokerController.getProducerManager().getAvailableChannel(groupId);
+        String producerClientId = msgExt.getUserProperty(MessageConst.PROPERTY_TRANSACTION_PRODUCER_CLIENT_ID);
+        Channel channel = brokerController.getProducerManager().getAvailableChannel(groupId, producerClientId);
         if (channel != null) {
             brokerController.getBroker2Client().checkProducerTransactionState(groupId, channel, checkTransactionStateRequestHeader, msgExt);
         } else {

broker/src/main/java/org/apache/rocketmq/broker/transaction/rocksdb/TransactionalMessageRocksDBService.java

@@ -234,7 +234,8 @@ private void sendCheckMessage(MessageExt msgExt) {
             msgExt.setQueueId(Integer.parseInt(msgExt.getUserProperty(MessageConst.PROPERTY_REAL_QUEUE_ID)));
             msgExt.setStoreSize(0);
             String groupId = msgExt.getProperty(MessageConst.PROPERTY_PRODUCER_GROUP);
-            Channel channel = brokerController.getProducerManager().getAvailableChannel(groupId);
+            String producerClientId = msgExt.getUserProperty(MessageConst.PROPERTY_TRANSACTION_PRODUCER_CLIENT_ID);
+            Channel channel = brokerController.getProducerManager().getAvailableChannel(groupId, producerClientId);
             if (channel != null) {
                 brokerController.getBroker2Client().checkProducerTransactionState(groupId, channel, checkTransactionStateRequestHeader, msgExt);
             } else {

broker/src/test/java/org/apache/rocketmq/broker/client/ProducerManagerTest.java

@@ -225,4 +225,37 @@ public void testGetAvailableChannel() {
         assertThat(c).isNull();
     }
 
+    @Test
+    public void testGetAvailableChannelWithPreferredClientId() {
+        producerManager.registerProducer(group, clientInfo);
+        when(channel.isActive()).thenReturn(true);
+        when(channel.isWritable()).thenReturn(true);
+
+        // Match: preferred clientId matches registered producer
+        Channel c = producerManager.getAvailableChannel(group, "clientId");
+        assertThat(c).isSameAs(channel);
+    }
+
+    @Test
+    public void testGetAvailableChannelWithPreferredClientIdNotFound() {
+        producerManager.registerProducer(group, clientInfo);
+        when(channel.isActive()).thenReturn(true);
+        when(channel.isWritable()).thenReturn(true);
+
+        // No match: falls back to round-robin (returns some channel from group)
+        Channel c = producerManager.getAvailableChannel(group, "nonExistentClientId");
+        assertThat(c).isNotNull(); // should fall back to round-robin
+    }
+
+    @Test
+    public void testGetAvailableChannelWithNullPreferredClientId() {
+        producerManager.registerProducer(group, clientInfo);
+        when(channel.isActive()).thenReturn(true);
+        when(channel.isWritable()).thenReturn(true);
+
+        // null clientId: should behave exactly like original getAvailableChannel
+        Channel c = producerManager.getAvailableChannel(group, null);
+        assertThat(c).isNotNull();
+    }
+
 }

client/src/main/java/org/apache/rocketmq/client/impl/producer/DefaultMQProducerImpl.java

@@ -1444,6 +1444,7 @@ public TransactionSendResult sendMessageInTransaction(final Message msg,
         SendResult sendResult = null;
         MessageAccessor.putProperty(msg, MessageConst.PROPERTY_TRANSACTION_PREPARED, "true");
         MessageAccessor.putProperty(msg, MessageConst.PROPERTY_PRODUCER_GROUP, this.defaultMQProducer.getProducerGroup());
+        MessageAccessor.putProperty(msg, MessageConst.PROPERTY_TRANSACTION_PRODUCER_CLIENT_ID, this.mQClientFactory.getClientId());
         try {
             sendResult = this.send(msg);
         } catch (Exception e) {

common/src/main/java/org/apache/rocketmq/common/message/MessageConst.java

@@ -52,6 +52,7 @@ public class MessageConst {
     public static final String PROPERTY_TRANSACTION_PREPARED_QUEUE_OFFSET = "TRAN_PREPARED_QUEUE_OFFSET";
     public static final String PROPERTY_TRANSACTION_ID = "__transactionId__";
     public static final String PROPERTY_TRANSACTION_CHECK_TIMES = "TRANSACTION_CHECK_TIMES";
+    public static final String PROPERTY_TRANSACTION_PRODUCER_CLIENT_ID = "__TXN_PRODUCER_CID__";
     public static final String PROPERTY_INSTANCE_ID = "INSTANCE_ID";
     public static final String PROPERTY_CORRELATION_ID = "CORRELATION_ID";
     public static final String PROPERTY_MESSAGE_REPLY_TO_CLIENT = "REPLY_TO_CLIENT";