MessageEvent::DoOnWrite应该消除不必要的buffer拷贝

[dyx2025]

MessageEvent::DoOnWrite是发消息的热路径，应该消除不必要的buffer拷贝。
通过本类新增iovec 结构体数组成员变量暂存niBuffLen和poMessage，新增成员变量暂存已发送的字节个数，使用 writev 系统调用，再对逻辑做适当的修改就能消除buffer拷贝 。

原代码路径：
src/communicate/tcp/message_event.cpp

int MessageEvent :: DoOnWrite()
{
    if (m_iLeftWriteLen > 0)
    {
        return WriteLeft();
    }

    m_oMutex.lock();
    if (m_oInQueue.empty())
    {
        m_oMutex.unlock();
        return 0;
    }
    QueueData tData = m_oInQueue.front();
    m_oInQueue.pop();
    m_iQueueMemSize -= tData.psValue->size();
    m_oMutex.unlock();

    std::string * poMessage = tData.psValue;
    uint64_t llNowTime = Time::GetSteadyClockMS();
    int iDelayMs = llNowTime > tData.llEnqueueAbsTime ? (int)(llNowTime - tData.llEnqueueAbsTime) : 0;
    BP->GetNetworkBP()->TcpOutQueue(iDelayMs);
    if (iDelayMs > TCP_OUTQUEUE_DROP_TIMEMS)
    {
        //PLErr("drop request because enqueue timeout, nowtime %lu unqueuetime %lu",
                //llNowTime, tData.llEnqueueAbsTime);
        delete poMessage;
        return 0;
    }

    int iBuffLen = poMessage->size();
    int niBuffLen = htonl(iBuffLen + 4);

    int iLen = iBuffLen + 4;
    // 没有必要拷贝niBuffLen和poMessage到buffer
    // 通过本类的成员变量暂存niBuffLen和poMessage，以及记录发送的字节个数，再对逻辑做适当的修改就能消除buffer拷贝  
    m_oWriteCacheBuffer.Ready(iLen);
    memcpy(m_oWriteCacheBuffer.GetPtr(), &niBuffLen, 4);
    memcpy(m_oWriteCacheBuffer.GetPtr() + 4, poMessage->c_str(), iBuffLen);

    m_iLeftWriteLen = iLen;
    m_iLastWritePos = 0;

    delete poMessage;

    //PLImp("write len %d ip %s port %d", iLen, m_oAddr.getHost().c_str(), m_oAddr.getPort());

    int iWriteLen = m_oSocket.send(m_oWriteCacheBuffer.GetPtr(), iLen);
    if (iWriteLen < 0)
    {
        PLErr("fail, write len %d ip %s port %d",
                iWriteLen, m_oAddr.getHost().c_str(), m_oAddr.getPort());
        return -1;
    }

    if (iWriteLen == 0)
    {
        //need wait next write
        AddEvent(EPOLLOUT);

        return 1;
    }

    //PLImp("real write len %d", iWriteLen);

    if (iWriteLen == iLen)
    {
        m_iLeftWriteLen = 0;
        m_iLastWritePos = 0;
        //write done
    }
    else if (iWriteLen < iLen)
    {
        m_iLastWritePos = iWriteLen;
        m_iLeftWriteLen = iLen - iWriteLen;

        PLImp("write buflen %d smaller than expectlen %d", iWriteLen, iLen);
    }
    else
    {
        PLErr("write buflen %d large than expectlen %d", iWriteLen, iLen);
    }

    return 0;
}