[Interest] Help with QSslSocket race condition?

Israel Brewster ibrewster at flyravn.com
Wed Oct 24 00:07:13 CEST 2018

On Oct 23, 2018, at 1:27 PM, Henry Skoglund <fromqt at tungware.se<mailto:fromqt at tungware.se>> wrote:

On 23/10/2018 20.43, Israel Brewster wrote:
I am using QSslSockets with a QTcpServer to create a simple client/server program (is there a higher level API I could use instead?). I have subclassed QTcpServer to create my server side sockets as QSslSockets and call startServerEncryption(), but otherwise it is just a standard QTcpServer.
When a socket first connects to the server, I make the following connection:
connect(clientConnection, SIGNAL(disconnected()), clientConnection, SLOT(deleteLater()));
where clientConnection is the QSslSocket created by the QTcpServer. Later, when using the socket (again on the server), I have the following function to wait for data from the client:
/// \brief EZPCore::waitForResponseReady
/// \param socket
/// \param msec
/// \return true=data available, false=timeout/error
/// Documentation for QAbstractSocket "waitForReadyRead" says
/// "This function may fail randomly on Windows. Consider using the event loop and the readyRead()
/// signal if your software will run on Windows."
/// so that's what this function does.
bool EZPCore::waitForResponseReady(QSslSocket *socket,int msec){
     if(!socket || !socket->isValid() || !socket->isEncrypted() || socket->state()!=QAbstractSocket::ConnectedState)
         return false; //there is something wrong with the socket, no point in waiting.
        return true;//success! Bytes are available!
    QEventLoop wait;  //don't block other operations while waiting
    QTimer waitTimeout; //only wait a set length of time.
     connect(socket,&QAbstractSocket::disconnected,[&wait]{wait.exit(-2);}); //catch if the socket closes while waiting.
     connect(socket,&QAbstractSocket::readyRead,[&wait]{wait.exit();}); //zero exit
    int result=wait.exec();
     waitTimeout.stop(); //stop the timer if it is still running (probably unneeded?)
        return false; //not connected or timeout
    //should only get here if result is 0, indicating readyRead signal received.
    //if we have bytes available, then return success (0), even if
    //we had a timeout
    if(socket->bytesAvailable()>0) //<--------We frequently crash here, UNLESS I don't call "deleteLater()" on disconnect???
        return true;
    return false; //no bytes available
many times this works flawlessly. However,  I am frequently getting a crash when I call bytesAvailable() on the socket at the end of above function - a crash that is resolved if I don't make the connection to "deleteLater" when the socket is established. This tells me that the socket is getting deleted during this event loop. What I don't understand is why?
- When I come into this function, I check for QAbstractSocket::ConnectedState on the socket, and return immediately if not connected, which should mean that the socket is still connected when this function starts.
- I connect the "disconnected" signal of this function up to exit the event loop with a non-zero status should the socket disconnect during the event loop (entirely possible, of course)
- The function returns immediately if the event loop exits with a non-zero status, and the only way for the event loop to exit with a zero status is for readyRead to be emitted.
As such, the only way for the crashing line of code to be executed should be for a) the socket to be in a connected state (otherwise the function wouldn't run), b) disconnected signal to NOT to be emitted during the event loop (so still connected), and c) the readyRead signal to be emitted. However, the crashing would indicate that the socket *is* being deleted during the event loop, which indicates that the socket is disconnecting. And yet I still get a readyRead signal?
Incidentally, If I connect the destroyed signal to the event loop exit, I *do* catch that. So I can see the socket being destroyed, but not being disconnected. So obviously I have an architecture issue here: the socket is closing and being destroyed while I am still trying to read from it. How can I fix this?

Hi, it could be that a new connection arrives not inside Qt's normal event loop but in your wait.exec() call, i.e. some kind of recursion could occur, I mean EZPCore::waitForResponseReady() calls wait.exec() which calls another EZPCore::waitForResponseReady()...

The above is just a guess, but why not let Qt run its vanilla event loop and refactor your code into 2 separate functions, one that takes care of the disconnected signal and one that handles the readyRead signal. Neither of them should require an eventloop inside them.

If you need to keep track of timeouts, you could use the QTimer but connected to a separate function. Or just let the TCP protocol run its course and disconnect automatically for you...

The function is a replacement for the waitForReadyRead() function, which the documentation states doesn't work properly on windows. I only use it in situations where a function needs to return a value, said value being dependent on the data from the client.

Rgrds Henry

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