Mosquitto客户端on_connect回调持续触发问题排查求助

本地与Mosquitto Broker同主机运行C++应用时，on_connect回调持续触发，Broker日志重复输出：

New Client connected from 10.0.2.37 as publisher-test
Client publisher-test already connected, closing old connection

以下是初始化MQTT客户端的代码：

bool MosquittoAppServer::InitMosquitto() {
    int rc = mosquitto_lib_init();
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #1 - mosquitto_lib_init %s \r\n", rc==MOSQ_ERR_SUCCESS ? "succeeded" : "failed");
    mMosq = mosquitto_new("publisher-test", true, NULL);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #2A %x %s \r\n",&mMosq, mMosq == NULL ? "failed to create mosquitto obj": "successfully created mosquitto obj");
    mosquitto_connect_callback_set(mMosq, on_connect_callback);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #2B mosquitto_connect_callback_set \r\n");
    mosquitto_publish_callback_set(mMosq, on_pub_callback);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #2C mosquitto_publish_callback_set \r\n");
    mosquitto_subscribe_callback_set(mMosq, on_sub_callback);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #2D mosquitto_subscribe_callback_set \r\n");
    mosquitto_message_callback_set(mMosq, on_msg_callback);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #2E mosquitto_message_callback_set \r\n");
    mosquitto_user_data_set(mMosq,  this);
    printf("MosquittoAppServers::InitMosquitto Breeakpoint #2F mosquitto_user_data_set \r\n");
    rc = mosquitto_username_pw_set(mMosq, "mqtt_user", "P@$$#123");
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #3 mosquitto_username_pw_set %s \r\n", rc==MOSQ_ERR_SUCCESS ? "succeeded" : "failed");
    
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #4 about to connect to broker at address: %s \r\n", mBrokerIpAddr);
    
    rc = -1;
    rc = mosquitto_connect(mMosq, mBrokerIpAddr, 1883, 60);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #5 mosquitto_connect %s \r\n", rc==MOSQ_ERR_SUCCESS ? "succeeded" : "failed");
    if(rc != 0) {
        printf("MosquittoAppServer::InitMosquitto Client could not connect to MQTT-Broker!  Error Code: %d\n", rc);
      switch(rc) {
        case MOSQ_ERR_SUCCESS:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_SUCCESS \n");
            break;
        }
        case MOSQ_ERR_INVAL:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_INVAL \n");
            break;
        }
        case MOSQ_ERR_NOMEM:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_NOMEM \n");
            break;
        }
        case MOSQ_ERR_NO_CONN:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_NO_CONN \n");
            break;
        }
        case MOSQ_ERR_PROTOCOL:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_PROTOCOL \n");
            break;
        }
        case MOSQ_ERR_PAYLOAD_SIZE:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_PAYLOAD_SIZE \n");
            break;
        }
        case MOSQ_ERR_MALFORMED_UTF8:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_MALFORMED_UTF8 \n");
            break;
        }
        case MOSQ_ERR_QOS_NOT_SUPPORTED:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_QOS_NOT_SUPPORTED \n");
            break;
        }
        case MOSQ_ERR_OVERSIZE_PACKET:{
            printf("MosquittoAppServer::InitMosquitto failed to connect.  Received Error MOSQ_ERR_OVERSIZE_PACKET \n");
            break;
        }
      }
        mosquitto_destroy(mMosq);
        return;// -1;
    }
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #6 - Now connected to the broker \r\n");  
    rc = mosquitto_publish(mMosq, NULL, "my-mqtt-topic", sizeof("Hello from a cleint!"), "Hello from a client!", 0, false);     
    rc = mosquitto_loop_start(mMosq);
    printf("MosquittoAppServer::InitMosquitto Breeakpoint #8  - mosquitto_loop_start  result: %s \r\n", rc==MOSQ_ERR_SUCCESS ? "succeeded" : "failed");
    
    printf("MosquittoAppServer::InitMosquitto Breeakpoint THE END \r\n");
    
    return rc == 0;
}

1. 重复调用初始化函数：InitMosquitto被多次执行，每次都会创建新的客户端实例并使用固定IDpublisher-test连接Broker，Broker因ID冲突踢掉旧连接，新连接触发on_connect回调，形成循环。
2. 固定客户端ID未处理生命周期：代码使用固定客户端ID，且clean_session设为true，若旧客户端实例未被销毁就发起新连接，Broker会强制关闭旧连接；若旧实例后台仍在尝试通信，会触发重连，进一步加剧循环。
3. 客户端实例未正确销毁：如果mMosq是类成员变量，多次调用InitMosquitto会覆盖旧指针，导致旧实例未被销毁，持续与Broker交互引发冲突。

1. 限制初始化函数仅执行一次
   添加初始化标志位，避免重复创建客户端：

bool MosquittoAppServer::InitMosquitto() {
    static bool isInitialized = false;
    if (isInitialized) {
        printf("MosquittoAppServer already initialized\n");
        return true;
    }
    // 原有初始化代码...
    isInitialized = true;
    return rc == 0;
}

2. 使用动态客户端ID
   让Mosquitto自动生成唯一客户端ID，避免冲突：

mMosq = mosquitto_new(NULL, true, NULL);

3. 正确管理客户端生命周期
   重新初始化前先销毁旧实例：

bool MosquittoAppServer::InitMosquitto() {
    if (mMosq != NULL) {
        mosquitto_loop_stop(mMosq, true);
        mosquitto_destroy(mMosq);
        mMosq = NULL;
    }
    // 原有初始化代码...
}

4. 检查重连逻辑
   若on_connect回调中存在重连代码，需添加状态判断，仅在连接失败时触发重连，并设置合理的重连间隔，避免频繁连接。

内容的提问来源于stack exchange，提问作者user3583535