QWebChannel - QWebSocket - threading problem

[RafalNiewinski]

Hi,
I am using QWebChannel to expose easy communication interface to connected web browser clients.

Also I have overridden QWebChannelAbstractTransport interface which sends and receives data from/to QWebSocket.

When my client connects and all of the code runs in event loop of main thread all works great,
but when specific action occurs in the application I need to create new thread and move all of the client resources to this thread.
I have properly implemented parent->child hierarchy for moveToThread() operation.

When moveToThread() operation completes successfully and all object runs in new event loop (<- I think) clients can easily send me a message and I can easily process it in new thread,
but when my application trying to send message to client I am receiving console message error: "QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread".

I tried to debug it by printing threads in which objects or called methods works and it seems that everything is fine, but doesn't work.


TESTCASE
Part of my QWebChannelAbstractTransport implementation:

Output from qDebug line 11 looks like this: "Client in msg from thread >>> QThread(0x6aa270)"
Output from qDebug line 3 looks like this: "Client out msg from thread <<< QThread(0x6aa270) (socket: QThread(0x6aa270) )"
So it looks like all of object runs in the same thread but in line 6 I have a message: "QSocketNotifier: Socket notifiers cannot be enabled or disabled from another thread"
Of course socket is QWebSocket object.

void ClientTransportLayer::sendMessage(const QJsonObject &message)
{
    qDebug() <<"Client out msg from thread <<< " <<QThread::currentThread() <<" (socket: " <<socket->thread() <<")";
 
    QJsonDocument doc(message);
    socket->sendTextMessage(QString::fromUtf8(doc.toJson(QJsonDocument::Compact)));
}
 
void ClientTransportLayer::textMessageReceived(const QString &messageData)
{
    qDebug() <<"Client in msg from thread >>> " <<QThread::currentThread();
 
    QJsonParseError error;
    QJsonDocument message = QJsonDocument::fromJson(messageData.toUtf8(), &error);
 
    if(!error.error && message.isObject())
        emit messageReceived(message.object(), this);
}

void ClientTransportLayer::sendMessage(const QJsonObject &message)
{
    qDebug() <<"Client out msg from thread <<< " <<QThread::currentThread() <<" (socket: " <<socket->thread() <<")";

    QJsonDocument doc(message);
    socket->sendTextMessage(QString::fromUtf8(doc.toJson(QJsonDocument::Compact)));
}

void ClientTransportLayer::textMessageReceived(const QString &messageData)
{
    qDebug() <<"Client in msg from thread >>> " <<QThread::currentThread();

    QJsonParseError error;
    QJsonDocument message = QJsonDocument::fromJson(messageData.toUtf8(), &error);

    if(!error.error && message.isObject())
        emit messageReceived(message.object(), this);
}

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[anda_skoa]

Can you show how the application uses this to send messages?
Does this happen from the main thread? Via signal/slot?

One thing you could try debugging-wise is to find the socket notifier message in Qt, set a break point there and then see which thread causes it.

Cheers,
_

[RafalNiewinski]

Thank you for your interest.

sendMessage is overridden [pure virtual slot] of QWebChannelAbstractTransport
I am using this with QWebChannel so sendMessage is called by QWebChannel mechanism
for example as an automatic confirmation response for method called remotely from client side.
When I need to manually (from my code) send message to client, I simply emit a signal from my object registered for publish in QWebChannel object.


My textMessageReceived is connected to QWebSocket that way in constructor

connect(this->socket, SIGNAL(textMessageReceived(QString)), this, SLOT(textMessageReceived(QString)));

connect(this->socket, SIGNAL(textMessageReceived(QString)), this, SLOT(textMessageReceived(QString)));

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That is simplified backtrace for thread running my objects in case of my first example above

Thread 4 (Thread 0x7fffe73ba700 (LWP 23225)):
#0  ClientTransportLayer::sendMessage (this=0x7fffd8002f90, message=...) at ./src/clienttransportlayer.cpp:22
        __PRETTY_FUNCTION__ = "virtual void ClientTransportLayer::sendMessage(const QJsonObject&)"
        doc = {static BinaryFormatTag = 1936351857, d = 0x40ece8 <ClientTransportLayer::sendMessage(QJsonObject const&)>}
#1  0x00007ffff6fc7bb6 in QMetaObjectPublisher::handleMessage (this=0x7fffd8002fd0, message=..., transport=0x7fffd8002f90) at /home/rafal/Dokumenty/qtwebchannel/src/webchannel/qmetaobjectpublisher.cpp:641
        __PRETTY_FUNCTION__ = "void QMetaObjectPublisher::handleMessage(const QJsonObject&, QWebChannelAbstractTransport*)"
        type = TypeInit
#2  0x00007ffff6fc253a in QtPrivate::FunctorCall<QtPrivate::IndexesList<0, 1>, QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void, void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*)>::call (f=(void (QMetaObjectPublisher::*)(QMetaObjectPublisher * const, const QJsonObject &, QWebChannelAbstractTransport *)) 0x7ffff6fc7852 <QMetaObjectPublisher::handleMessage(QJsonObject const&, QWebChannelAbstractTransport*)>, o=0x7fffd8002fd0, arg=0x7fffe73b9700) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobjectdefs_impl.h:507
#3  0x00007ffff6fc2388 in QtPrivate::FunctionPointer<void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*)>::call<QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void> (f=(void (QMetaObjectPublisher::*)(QMetaObjectPublisher * const, const QJsonObject &, QWebChannelAbstractTransport *)) 0x7ffff6fc7852 <QMetaObjectPublisher::handleMessage(QJsonObject const&, QWebChannelAbstractTransport*)>, o=0x7fffd8002fd0, arg=0x7fffe73b9700) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobjectdefs_impl.h:526
#4  0x00007ffff6fc1ea0 in QtPrivate::QSlotObject<void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*), QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void>::impl (which=1, this_=0x7fffd8004640, r=0x7fffd8002fd0, a=0x7fffe73b9700, ret=0x0) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobject_impl.h:149
#5  0x00007ffff67d677d in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5
#6  0x00007ffff6fd7478 in QWebChannelAbstractTransport::messageReceived (this=0x7fffd8002f90, _t1=..., _t2=0x7fffd8002f90) at .moc/moc_qwebchannelabstracttransport.cpp:150
        _a = {0x0, 0x7fffe73b9770, 0x7fffe73b96e8}
#7  0x000000000040f0bb in ClientTransportLayer::textMessageReceived (this=0x7fffd8002f90, messageData=...) at ./src/clienttransportlayer.cpp:37
        __PRETTY_FUNCTION__ = "void ClientTransportLayer::textMessageReceived(const QString&)"
        error = {offset = 0, error = QJsonParseError::NoError}
        message = {static BinaryFormatTag = 1936351857, d = 0x7fffd8004220}
#8  0x000000000041a2bb in ClientTransportLayer::qt_static_metacall (_o=0x7fffd8002f90, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffe73b9910) at moc_clienttransportlayer.cpp:71
#9  0x00007ffff67d6056 in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5
#10 0x00007ffff7204f65 in QWebSocket::textMessageReceived(QString const&) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5WebSockets.so.5
#11 0x00007ffff67d677d in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5

Thread 4 (Thread 0x7fffe73ba700 (LWP 23225)):
#0  ClientTransportLayer::sendMessage (this=0x7fffd8002f90, message=...) at ./src/clienttransportlayer.cpp:22
        __PRETTY_FUNCTION__ = "virtual void ClientTransportLayer::sendMessage(const QJsonObject&)"
        doc = {static BinaryFormatTag = 1936351857, d = 0x40ece8 <ClientTransportLayer::sendMessage(QJsonObject const&)>}
#1  0x00007ffff6fc7bb6 in QMetaObjectPublisher::handleMessage (this=0x7fffd8002fd0, message=..., transport=0x7fffd8002f90) at /home/rafal/Dokumenty/qtwebchannel/src/webchannel/qmetaobjectpublisher.cpp:641
        __PRETTY_FUNCTION__ = "void QMetaObjectPublisher::handleMessage(const QJsonObject&, QWebChannelAbstractTransport*)"
        type = TypeInit
#2  0x00007ffff6fc253a in QtPrivate::FunctorCall<QtPrivate::IndexesList<0, 1>, QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void, void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*)>::call (f=(void (QMetaObjectPublisher::*)(QMetaObjectPublisher * const, const QJsonObject &, QWebChannelAbstractTransport *)) 0x7ffff6fc7852 <QMetaObjectPublisher::handleMessage(QJsonObject const&, QWebChannelAbstractTransport*)>, o=0x7fffd8002fd0, arg=0x7fffe73b9700) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobjectdefs_impl.h:507
#3  0x00007ffff6fc2388 in QtPrivate::FunctionPointer<void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*)>::call<QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void> (f=(void (QMetaObjectPublisher::*)(QMetaObjectPublisher * const, const QJsonObject &, QWebChannelAbstractTransport *)) 0x7ffff6fc7852 <QMetaObjectPublisher::handleMessage(QJsonObject const&, QWebChannelAbstractTransport*)>, o=0x7fffd8002fd0, arg=0x7fffe73b9700) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobjectdefs_impl.h:526
#4  0x00007ffff6fc1ea0 in QtPrivate::QSlotObject<void (QMetaObjectPublisher::*)(QJsonObject const&, QWebChannelAbstractTransport*), QtPrivate::List<QJsonObject const&, QWebChannelAbstractTransport*>, void>::impl (which=1, this_=0x7fffd8004640, r=0x7fffd8002fd0, a=0x7fffe73b9700, ret=0x0) at /home/rafal/Qt/5.7/gcc_64/include/QtCore/qobject_impl.h:149
#5  0x00007ffff67d677d in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5
#6  0x00007ffff6fd7478 in QWebChannelAbstractTransport::messageReceived (this=0x7fffd8002f90, _t1=..., _t2=0x7fffd8002f90) at .moc/moc_qwebchannelabstracttransport.cpp:150
        _a = {0x0, 0x7fffe73b9770, 0x7fffe73b96e8}
#7  0x000000000040f0bb in ClientTransportLayer::textMessageReceived (this=0x7fffd8002f90, messageData=...) at ./src/clienttransportlayer.cpp:37
        __PRETTY_FUNCTION__ = "void ClientTransportLayer::textMessageReceived(const QString&)"
        error = {offset = 0, error = QJsonParseError::NoError}
        message = {static BinaryFormatTag = 1936351857, d = 0x7fffd8004220}
#8  0x000000000041a2bb in ClientTransportLayer::qt_static_metacall (_o=0x7fffd8002f90, _c=QMetaObject::InvokeMetaMethod, _id=0, _a=0x7fffe73b9910) at moc_clienttransportlayer.cpp:71
#9  0x00007ffff67d6056 in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5
#10 0x00007ffff7204f65 in QWebSocket::textMessageReceived(QString const&) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5WebSockets.so.5
#11 0x00007ffff67d677d in QMetaObject::activate(QObject*, int, int, void**) () from /home/rafal/Qt/5.7/gcc_64/lib/libQt5Core.so.5

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[anda_skoa]

I am using this with QWebChannel so sendMessage is called by QWebChannel mechanism

And the QWebChannel has also been moved to the

When I need to manually (from my code) send message to client, I simply emit a signal from my object registered for publish in QWebChannel object.


My textMessageReceived is connected to QWebSocket that way in constructor

connect(this->socket, SIGNAL(textMessageReceived(QString)), this, SLOT(textMessageReceived(QString)));

connect(this->socket, SIGNAL(textMessageReceived(QString)), this, SLOT(textMessageReceived(QString)));

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That looks ok, the cross thread signal/slot connection handling should have transported the call into the thread of "this".

Cheers,
_

[RafalNiewinski]

Yes of course, all objects was moved to the same (new) thread.
I checked that by calling this on each object and all resides in same thread.

qDebug() <<x.thread();

qDebug() <<x.thread();

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[anda_skoa]

Right, then the best choice is likely to see where the call is coming from that triggers the warning.

Cheers,
_