学习open62541 --- [72] client删除subscription时的warning

本文记录一个问题的理解过程，试验条件：

• 使用open62541运行server
• 使用asyncua运行client
• client会向server添加subscription，然后在断开连接前删除subscription

代码和理解

server端基于open62541，

// server.c
#include "open62541.h"

#include <signal.h>
#include <stdlib.h>

UA_Boolean running = true;

static void stopHandler(int sign) {
    UA_LOG_INFO(UA_Log_Stdout, UA_LOGCATEGORY_SERVER, "received ctrl-c");
    running = false;
}

int main(void) 
{
    signal(SIGINT, stopHandler);
    signal(SIGTERM, stopHandler);

    UA_Server *server = UA_Server_new();
    UA_ServerConfig_setDefault(UA_Server_getConfig(server));

    UA_StatusCode retval = UA_Server_run(server, &running);
    
    UA_Server_delete(server);
    
    return retval == UA_STATUSCODE_GOOD ? EXIT_SUCCESS : EXIT_FAILURE;
}

Client代码是基于asyncua，

当删除subscription时，在client端会收到一个warning，

在open62541里对这个错误进行搜索，即UA_STATUSCODE_BADNOSUBSCRIPTION，找到相关代码，如下，

void
UA_Session_detachSubscription(UA_Server *server, UA_Session *session,
                              UA_Subscription *sub, UA_Boolean releasePublishResponses) {
    /* Detach from the session */
    sub->session = NULL;
    TAILQ_REMOVE(&session->subscriptions, sub, sessionListEntry);

    /* Reduce the count */
    UA_assert(session->subscriptionsSize > 0);
    session->subscriptionsSize--;

    /* Reduce the number of outstanding retransmissions */
    session->totalRetransmissionQueueSize -= sub->retransmissionQueueSize;
    
    /* Send remaining publish responses if the last subscription was removed */
    if(!releasePublishResponses || !TAILQ_EMPTY(&session->subscriptions))
        return;
    UA_PublishResponseEntry *pre;
    while((pre = UA_Session_dequeuePublishReq(session))) {
        UA_PublishResponse *response = &pre->response;
        response->responseHeader.serviceResult = UA_STATUSCODE_BADNOSUBSCRIPTION;
        response->responseHeader.timestamp = UA_DateTime_now();
        sendResponse(server, session, session->header.channel, pre->requestId,
                     (UA_Response*)response, &UA_TYPES[UA_TYPES_PUBLISHRESPONSE]);
        UA_PublishResponse_clear(response);
        UA_free(pre);
    }
}

具体原理如下，当client删除subscription时，会发送删除请求，server收到请求后会调用UA_Session_detachSubscription()，代码里会判断这个subscription是否是最后一个subscription，如果是那么就会把剩余的response发送出去，并把这个response的状态置为UA_STATUSCODE_BADNOSUBSCRIPTION

可以看出这个是正常的操作，本文代码里只有一个subscription，所以删除时就会把剩余的response都发出去；但是如果有多个subscription，此时只删除一个subscription就不会收到这个warning。