feat: Pull message process (doocs#121)

* feat: add pull message process

Author: Oliverwqcwrw

README.md

@@ -283,6 +283,7 @@
 - [RocketMQ IndexFile详解](docs/rocketmq/rocketmq-indexfile.md)
 - [RocketMQ 消费者启动流程](docs/rocketmq/rocketmq-consumer-start.md)
 - [RocketMQ 消息拉取流程](docs/rocketmq/rocketmq-pullmessage.md)
+- [RocketMQ Broker 处理拉取消息请求流程](docs/rocketmq/rocketmq-pullmessage-processor.md)
 
 ## 番外篇（JDK 1.8）
 

docs/rocketmq/rocketmq-pullmessage-processor.md

@@ -0,0 +1,116 @@
+该文所涉及的 RocketMQ 源码版本为 4.9.3。
+
+# RocketMQ broker 处理拉取消息请求流程
+
+org.apache.rocketmq.broker.processor.PullMessageProcessor#processRequest(io.netty.channel.ChannelHandlerContext, org.apache.rocketmq.remoting.protocol.RemotingCommand)
+
+第 1 步、`校验broker是否可读`
+
+```java
+if (!PermName.isReadable(this.brokerController.getBrokerConfig().getBrokerPermission())) {
+    response.setCode(ResponseCode.NO_PERMISSION);
+    response.setRemark(String.format("the broker[%s] pulling message is forbidden", this.brokerController.getBrokerConfig().getBrokerIP1()));
+    return response;
+}
+```
+
+第 2 步、`根据消费组获取订阅信息`
+
+```java
+SubscriptionGroupConfig subscriptionGroupConfig =
+    this.brokerController.getSubscriptionGroupManager().findSubscriptionGroupConfig(requestHeader.getConsumerGroup());
+```
+
+第 3 步、`校验是否允许消费`
+
+```java
+if (!subscriptionGroupConfig.isConsumeEnable()) {
+    response.setCode(ResponseCode.NO_PERMISSION);
+    response.setRemark("subscription group no permission, " + requestHeader.getConsumerGroup());
+    return response;
+}
+```
+
+第 4 步、`根据主题获取对应的配置信息`
+
+```java
+TopicConfig topicConfig = this.brokerController.getTopicConfigManager().selectTopicConfig(requestHeader.getTopic());
+if (null == topicConfig) {
+    log.error("the topic {} not exist, consumer: {}", requestHeader.getTopic(), RemotingHelper.parseChannelRemoteAddr(channel));
+    response.setCode(ResponseCode.TOPIC_NOT_EXIST);
+    response.setRemark(String.format("topic[%s] not exist, apply first please! %s", requestHeader.getTopic(), FAQUrl.suggestTodo(FAQUrl.APPLY_TOPIC_URL)));
+    return response;
+}
+```
+
+第 5 步、`校验主题对应的队列`
+
+```java
+if (requestHeader.getQueueId() < 0 || requestHeader.getQueueId() >= topicConfig.getReadQueueNums()) {
+    String errorInfo = String.format("queueId[%d] is illegal, topic:[%s] topicConfig.readQueueNums:[%d] consumer:[%s]",
+        requestHeader.getQueueId(), requestHeader.getTopic(), topicConfig.getReadQueueNums(), channel.remoteAddress());
+    log.warn(errorInfo);
+    response.setCode(ResponseCode.SYSTEM_ERROR);
+    response.setRemark(errorInfo);
+    return response;
+}
+```
+
+第 6 步、`如果配置了消息过滤表达式，根据表达式进行构建consumerFilterData，如果没有，则根据主题构建`
+
+```java
+consumerFilterData = ConsumerFilterManager.build(
+                        requestHeader.getTopic(), requestHeader.getConsumerGroup(), requestHeader.getSubscription(),
+                        requestHeader.getExpressionType(), requestHeader.getSubVersion()
+
+consumerFilterData = this.brokerController.getConsumerFilterManager().get(requestHeader.getTopic(),
+                    requestHeader.getConsumerGroup());
+```
+
+第 7 步、`校验如果不是Tag过滤，是否开启了自定义属性过滤，如果没有开启，不允许操作 只有使用push推送模式的消费者才能用使用SQL92标准的sql语句，pull拉取模式的消费者是不支持这个功能的。`
+
+```java
+if (!ExpressionType.isTagType(subscriptionData.getExpressionType())
+    && !this.brokerController.getBrokerConfig().isEnablePropertyFilter()) {
+    response.setCode(ResponseCode.SYSTEM_ERROR);
+    response.setRemark("The broker does not support consumer to filter message by " + subscriptionData.getExpressionType());
+    return response;
+}
+```
+
+第 8 步、`根据是否支持重试过滤创建不同的MessageFilter`
+
+```java
+if (this.brokerController.getBrokerConfig().isFilterSupportRetry()) {
+    messageFilter = new ExpressionForRetryMessageFilter(subscriptionData, consumerFilterData,
+        this.brokerController.getConsumerFilterManager());
+} else {
+    messageFilter = new ExpressionMessageFilter(subscriptionData, consumerFilterData,
+        this.brokerController.getConsumerFilterManager());
+}
+```
+
+第 9 步、`根据消费组、主题、队列、偏移量、最大拉取消息数量、消息过滤器查找信息`
+
+```java
+final GetMessageResult getMessageResult =
+    this.brokerController.getMessageStore().getMessage(requestHeader.getConsumerGroup(), requestHeader.getTopic(),
+        requestHeader.getQueueId(), requestHeader.getQueueOffset(), requestHeader.getMaxMsgNums(), messageFilter);
+
+```
+
+第 10 步、`消息为空 设置code为系统错误 返回response`
+
+```java
+response.setCode(ResponseCode.SYSTEM_ERROR);
+response.setRemark("store getMessage return null");
+```
+
+第 11 步、`提交偏移量`
+
+```java
+if (storeOffsetEnable) {
+    this.brokerController.getConsumerOffsetManager().commitOffset(RemotingHelper.parseChannelRemoteAddr(channel),
+        requestHeader.getConsumerGroup(), requestHeader.getTopic(), requestHeader.getQueueId(), requestHeader.getCommitOffset());
+}
+```