诡异的 QTcpSocket 行为

Question

我在调试接下来的几行代码时一直在做噩梦，一定有什么东西隐藏在我所看到的之外。

这是客户端和服务器之间的连接

QByteArray Client::request(QByteArray cmd)
{
    //DEBUG << "Command: " << cmd.toStdString().c_str();
    QTcpSocket *socket = new QTcpSocket(this);
    socket->connectToHost(hostAddress, port, QIODevice::ReadWrite);
    if (socket->waitForConnected(TIMEOUT))
    {
        socket->write(cmd);
        DEBUG << "Size of bytes written :" << sizeof (cmd);
    }
    else DEBUG << "Couldn't connect to socket";
    if (socket->waitForBytesWritten(TIMEOUT)) DEBUG << "Command sent";
    else DEBUG << "Couldn't write to socket";
    if (socket->waitForReadyRead(TIMEOUT))
    {
        QByteArray data = socket->readAll();
        socket->close();
        return data;
    }
    else DEBUG << "No reply from Server";
    return QByteArray();
}

对于客户端来说基本上就是这样，对于服务器来说，这里是重要的片段。

class ConnectionHandler : public QTcpServer
{
    Q_OBJECT


public:
    explicit ConnectionHandler(QObject *parent = 0);
    void write(QByteArray);

protected:
    void ConnectionHandler::incomingConnection(qintptr descriptor)
{
    DEBUG << "ConnectionHandler:" << "Incoming Connection :" << descriptor;
    ConnectionThread *thread = new ConnectionThread(this, descriptor);
    connect(thread, &QThread::finished, thread, &QThread::deleteLater);
    connect(thread, &ConnectionThread::signalIncomingMessage, this, &ConnectionHandler::slotIncomingMessage);
    connect(this, &ConnectionHandler::signalOutgoingMessage, thread, &ConnectionThread::slotOutgoingMessage);
    thread->start();
}

public slots:
    void slotIncomingMessage(QByteArray);
    void slotListen(bool checked){
    if (checked)
    {
        if (!this->listen(QHostAddress::LocalHost, PORT_NUMBER)) {
            DEBUG << "ConnectionHandler:" << "Could not start the server!";
        } else {
            DEBUG << "ConnectionHandler:" << "Listening...";
        }
    } else {
        this->close();
        DEBUG << "ConnectionHandler:" << "Connection Closed!";
    }
}

signals:
    void signalOutgoingMessage(QByteArray);
};

class ConnectionThread : public QThread
{
    Q_OBJECT
public:
    ConnectionThread(QObject* parent = 0, qintptr descriptor = -1){
    if (descriptor != -1)
    {
        socket = new QTcpSocket();

        DEBUG << "ConnectionThread: Connecting to socket number" << descriptor;
        if (!socket->setSocketDescriptor(descriptor))
        {
            DEBUG << "ConnectionThread: Connection failed.";
            DEBUG << socket->errorString();
        }
        else
        {
            DEBUG << "ConnectionThread: Connected Successfully.";
            connect(socket, &QAbstractSocket::disconnected, this, &ConnectionThread::slotSocketDisconnected);
        }
    }
    else {
        DEBUG << "ConnectionThread: Please provide a descriptor for the connection thread";
    }
}
    void run() override{
    if (socket->state() != QAbstractSocket::ConnectedState)
    {
        DEBUG << "ConnectionThread: Socket is not connected!";
        DEBUG << "ConnectionThread: Closing Thread!";
        emit signalThreadError(socket->errorString());
    }
    else
    {
        DEBUG << "ConnectionThread: Socket is Connected.";
        connect(socket, &QIODevice::readyRead, this, &ConnectionThread::slotThreadReadyRead, Qt::DirectConnection);
        exec();
    }
}

signals:
    void signalThreadError(QString);
    void signalIncomingMessage(QByteArray);

public slots:
    void slotThreadReadyRead(){
    QByteArray msg = socket->readAll();

    if (!msg.isEmpty()) {
        emit signalIncomingMessage(msg);
    }

    DEBUG << "ConnectionThread: Data in:" << msg;
}
    void slotSocketDisconnected();
    void slotOutgoingMessage(QByteArray msg)
{
    if (socket != nullptr) {
        socket->write(msg);
        if (socket->waitForBytesWritten(TIMEOUT)) {
            DEBUG << "ConnectionThread: Outgoing Message: " << msg;
        } else {
            DEBUG << "ConnectionThread: Outgoing Message Timeout";
        }
    }
}

private:
    QTcpSocket *socket = nullptr;
};

基本上会发生什么，它有时会收到来自客户端的传入命令，而大多数时候却不会，不仅如此，它实际上会收到一些命令并忽略一些不同的命令。

这是一个更诡异的行为，请注意 Client::request() 中的 //DEBUG << "Command: " << cmd.toStdString().c_str(); 行，当此行被注释掉时，服务器实际上会收到一些命令，否则，服务器就会失聪。

注释掉该行后的输出：

Size of bytes written : 8
Command sent
target num = 4  (a reply was received)
Size of bytes written : 8
Command sent
No reply from Server

当提到的行未被注释掉时，这是另一个输出：

Command:  NUMB

Size of bytes written : 8
Command sent
No reply from Server
target num = 0
Command:  TRAN

Size of bytes written : 8
Command sent
No reply from Server

更新解决客户端套接字写入和线程打开之间发生的竞争为我解决了问题，在我这里的情况下不需要使用线程，所以我就放弃了它。 @Botje回答

Answer 1

问题是由竞争条件引起的：

（主线程）ConnectionHandler::incomingConnection 创建一个新的 ConnectionThread
(主线程)ConnectionThread构造函数创建一个socket，也属于主线程
（主线程）returns 到事件循环，轮询套接字上的传入数据
(ConnectionThread) 运行方法将槽连接到 readyRead 信号

如果数据到达 2 和 4 之间的 window，readyRead 信号将不会拾取它。有几种可能的解决方案，但最简单的可能是在构建之后将套接字移动到 ConnectionThread，或者将其构建延迟到 ConnectionThread::run 方法。

诡异的 QTcpSocket 行为

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