c++中的SQLite.数据库是BUSY(多线程)

我遇到了一个问题。我在我的c++项目中使用SQLite3。在日志中，我有错误:DB是locked error code 5。据我所知，错误码5表示DB忙。为了解决这个问题，我开始使用WAL日志模式。但这没有用。

在我的程序中，我有两个连接到同一个DB。我对两个DB连接都使用互斥锁。我用下面的代码打开连接:

if (sqlite3_open_v2(db_path.c_str(), &this->db, SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_NOMUTEX, 0) ) {
    LOG4CPLUS_FATAL(this->logger, "Can not open/create DB " << sqlite3_errmsg(db));
    sqlite3_close(this->db);
}
if (sqlite3_exec(this->db, "PRAGMA journal_mode = WAL;", 0, 0, &err)) {
    LOG4CPLUS_ERROR(this->logger, "SQL det journal mode error: " << err);
    sqlite3_free(err);
}

第一个连接用于向DB插入数据。它每秒发生4次。
第二个连接用于启动事务、选择、更新、删除数据和提交。每5秒发生一次。

第一个连接出现错误。

请帮我解决这个问题。

更新:

第一次连接:

void readings_collector::flushToDb()
{
    this->db_mutex.lock();
    LOG4CPLUS_DEBUG(this->logger, "Flush to DB start.");
    const char *query = "INSERT INTO `readings` (`sensor_id`, `value`, `error`, `timestamp`) VALUES (?,?,?,?)";
    sqlite3_stmt *stmt = NULL;
    int rc = sqlite3_prepare_v2(this->db, query, -1, &stmt, NULL);
    if (SQLITE_OK != rc) {
        LOG4CPLUS_ERROR(this->logger, "sqlite prepare insert statment error: " << sqlite3_errmsg(this->db));
    }
    LOG4CPLUS_TRACE(this->logger, "--------------------");
    LOG4CPLUS_TRACE(this->logger, this->readings.size());
    while(!this->readings.empty()) {
        sensor::reading temp_reading = this->readings.front();
        this->readings.pop();
        LOG4CPLUS_TRACE(this->logger, "Reading " << temp_reading.sensor_id << " : " << temp_reading.value << " : " << temp_reading.error << " : " << temp_reading.timestamp);
        sqlite3_clear_bindings(stmt);
        sqlite3_bind_int(stmt, 1, temp_reading.sensor_id);
        sqlite3_bind_text(stmt, 2, temp_reading.value.c_str(), sizeof(temp_reading.value.c_str()), NULL);
        sqlite3_bind_int(stmt, 3, temp_reading.error);
        sqlite3_bind_int(stmt, 4, temp_reading.timestamp);
        rc = sqlite3_step(stmt);
        if (SQLITE_DONE != rc) {
            LOG4CPLUS_ERROR(this->logger, "sqlite insert statment exec error: " << sqlite3_errmsg(this->db) << "; status: " << rc);
        }
    }
    sqlite3_finalize(stmt);
    LOG4CPLUS_TRACE(this->logger, "Flush to DB finish.");
    this->db_mutex.unlock();
}

第二连接:

void dataSend_task::sendData()
{
    this->db_mutex.lock();
    char *err = 0;
    LOG4CPLUS_INFO(this->logger, "Send data function");
    if (sqlite3_exec(this->db, "BEGIN TRANSACTION", 0, 0, &err)) {
        LOG4CPLUS_ERROR(this->logger, "SQL exec error: " << err);
        sqlite3_free(err);
    }
    if (sqlite3_exec(this->db, this->SQL_UPDATE_READINGS_QUERY, 0, 0, &err)) {
        LOG4CPLUS_ERROR(this->logger, "SQL exec error: " << err);
        sqlite3_free(err);
    }
    this->json.clear();
    this->readingsCounter = 0;
    if (sqlite3_exec(this->db, this->SQL_SELECT_READINGS_QUERY, +[](void *instance, int x, char **y, char **z) {
                         return static_cast<dataSend_task *>(instance)->callback(0, x, y, z);
                     }, this, &err)) {
        LOG4CPLUS_ERROR(this->logger, "SQL exec error: " << err);
        sqlite3_free(err);
    } else {
        LOG4CPLUS_TRACE(this->logger, "Json data:  " << this->json);
        if (this->curlSend()) {
            if (sqlite3_exec(this->db, this->SQL_DELETE_READINGS_QUERY, 0, 0, &err)) {
                LOG4CPLUS_ERROR(this->logger, "SQL exec error: " << err);
                sqlite3_free(err);
            }
        }
    }
    if (sqlite3_exec(this->db, "COMMIT", 0, 0, &err)) {
        LOG4CPLUS_ERROR(this->logger, "SQL exec error: " << err);
        sqlite3_free(err);
    }
    this->db_mutex.unlock();
    this->json.clear();
}