QTcpSocket readyRead signal waiting for nothing when writing a file in a separate thread

[SaiyanRiku]

Hi,

I have an annoying bug with my program. The main program is an HTTP web server. Another thread write sometime big amount of data into a file. My problem is when the thread is writing into the file, if i try to send a request to the web server, i got the newConnection signal but the related socket readyRead signal is sometimes coming very later. This problem is randomly happening, some request are very fast and sometimes it may take 5-10 seconds to get the readyRead signal. I use this server locally, so it cannot be a network speed problem. When the thread a finished to write into the file, everything become normal.

Originally, I'm using libqxt-web to do this, but i'm able to reproduce the problem only with a QTcpServer into a simple program. Note that before using libqtx i was using a QTcpServer in a threaded context (using function incomingConnection, waitForReadyRead, waitForByteWritten...) which was not using the default event based system (newConnection, readyread...). This was working nice, but now i need to get it work with libqtx-web system. But, since i can reproduce the problem without libqxt, i think it's not a problem with that library.

If i use a large enough sleep between each write into a file, this seems to solve the problem, but it's not a nice solution.

So my simple program has the following code:

main.cpp
@
int main(int argc, char *argv[])
{
qInstallMsgHandler(log_function);

// Initialize the QT application
QCoreApplication a(argc, argv);

signal(SIGTERM, sig_term);
signal(SIGINT, sig_term);

// Port
quint16 port = 8081;

qDebug("[Main] Initialization server");
WebServer *pWebServer = new WebServer(QHostAddress::Any, port);
a.installEventFilter (pWebServer);

qDebug("[Main] Starting server");
pWebServer->start();

qDebug("[Main] Starting worker");
WorkerThread* pThread = new WorkerThread();
pThread->start();

qDebug("[Main] Starting the main loop");
a.exec();
qDebug("[Main] Exit the main loop");

pThread->abort();
qDebug("[WorkerThread] Wait worker thread");
pThread->wait();
qDebug("[WorkerThread] Destroy worker thread");
delete pThread;

qDebug("[WorkerThread] Destroy service");
delete pWebServer;

return 0;
}@

webserver.cpp
@

WebServer::WebServer(const QHostAddress &host, quint16 port)
{
m_host = host;
m_port = port;
m_count = 0;

m_pServer = new QTcpServer();
connect(m_pServer, SIGNAL(newConnection()), this, SLOT(acceptConnection()));

}

WebServer::~WebServer()
{

}

void WebServer::start()
{
m_pServer->listen(m_host, m_port);
}

void WebServer::acceptConnection()
{
qDebug("[WebServer] ---------------------->");
qDebug("[WebServer] newConnection received");

QTcpSocket* pSocket = m_pServer->nextPendingConnection ();
QObject::connect(pSocket, SIGNAL(readyRead()), this, SLOT(incomingData()));
QObject::connect(pSocket, SIGNAL(disconnected()), this, SLOT(disconnected()));
QObject::connect(pSocket, SIGNAL(destroyed()), this, SLOT(destroyed()));
}

void WebServer::incomingData()
{
qDebug("[WebServer] readyRead received");

QIODevice* pDevice = qobject_cast<QIODevice*>(sender());
if (!pDevice) return;

QString szResponse;
m_count++;
QString szContent = "Count is: " + QString::number(m_count);

szResponse = "HTTP/1.1 200 OK\r\n"
"Content-Type: text/html; charset=utf-8\r\n"
"Content-Length: " + QString::number(szContent.length()) + "\r\n"
"Connection: close\r\n"
"\r\n" + szContent;

qDebug("[WebServer] socket content sent: %s", qPrintable(szContent));

pDevice->write(szResponse.toAscii());
pDevice->close();
}

void WebServer::disconnected()
{
qDebug("[WebServer] socket disconnected");
QIODevice* pDevice = qobject_cast<QIODevice*>(sender());
if (!pDevice) return;

pDevice->deleteLater();
}

void WebServer::destroyed()
{
qDebug("[WebServer] socket destroyed");
qDebug("[WebServer] <----------------------");
}
@

WorkerThread.cpp
@

void WorkerThread::run()
{
qDebug("[WorkerThread] Start worker thread");

char* pBuffer;
int sizeBuf;

sizeBuf = 1000000;

int iCount=4000;

#ifdef MODE_C
qDebug("[WorkerThread] Mode C");
FILE * pFile;
pFile = fopen ("fileout.dat" , "wb");
if(pFile){
while(iCount>0 && !m_bStop){
//qDebug("[WorkerThread] Remaining: %d", iCount);

pBuffer = (char*)malloc(sizeBuf * sizeof(char));

fwrite (pBuffer, 1, sizeBuf, pFile);
fflush (pFile);

free(pBuffer);
pBuffer = NULL;

iCount--;

//usleep(100);
}
}
qDebug("[WorkerThread] Closing the file");
fclose (pFile);
#endif

qDebug("[WorkerThread] End worker thread");
}
@

Here the output log:

@
[lun. déc. 10 14:03:06 2012] [WebServer] ---------------------->
->[lun. déc. 10 14:03:06 2012] [WebServer] newConnection received
->[lun. déc. 10 14:03:10 2012] [WebServer] Event received SockAct(50)
[lun. déc. 10 14:03:10 2012] [WebServer] Socket descriptor 262194
[lun. déc. 10 14:03:10 2012] [WebServer] readyRead received
[lun. déc. 10 14:03:10 2012] [WebServer] socket content sent: Count is: 3
[lun. déc. 10 14:03:10 2012] [WebServer] Event received ChildAdded(68)
[lun. déc. 10 14:03:10 2012] [WebServer] Event received SockAct(50)
[lun. déc. 10 14:03:10 2012] [WebServer] Socket descriptor 262194
[lun. déc. 10 14:03:10 2012] [WebServer] Event received ChildRemoved(71)
[lun. déc. 10 14:03:10 2012] [WebServer] Event received ChildRemoved(71)
[lun. déc. 10 14:03:10 2012] [WebServer] Event received ChildRemoved(71)
[lun. déc. 10 14:03:10 2012] [WebServer] socket disconnected
[lun. déc. 10 14:03:10 2012] [WebServer] Event received DeferredDelete(52)
[lun. déc. 10 14:03:10 2012] [WebServer] socket destroyed
[lun. déc. 10 14:03:10 2012] [WebServer] <----------------------
@

I'm working on Linux 3.2.0-3-amd64 on Mint LMDE. Someone can tell me, if i'm doing something wrong or if it's a bug in the Qt library?
I don't see why Qt is blocking the readyRead signal because the main thread has nothing else to do.

Thank you for you help!