Slot called multiple times to a blocking function in the same thread

moffa13

@joeQ The slot being called multiple time is not accidental ; it just happens for example if I push a button 2 times.

The code is here : Github

addWhateverToList is the slot being called and FilesEncrypt::getFilesFromDirRecursive is the function that takes time.

@jsulm I used QApplication::processEvents() just to test what happens if my function becames "non-blocking".

VRonin

The correct way to proceed, as mentioned by @jsulm is to move the heavy lifting to a separate thread but,

This is a hack!

You can put a queue slot in between:

private:
Q_SLOT void lenghtyOp(){
lenghtyOpQueued =false;
// do the slow stuff
}
bool lenghtyOpQueued = false;
public:
Q_SLOT void queueLenghty(){
if(lenghtyOpQueued) return;
lenghtyOpQueued =true;
QTimer::singleShot(100,this,SLOT(lenghtyOp()));
}

now connect your button to queueLenghty rather than to lenghtyOp

also, you probably want to add an argument to processEvents: QApplication::processEvents(QEventLoop::ExcludeUserInputEvents);

moffa13

@VRonin said in Slot called multiple times to a blocking function in the same thread:

You can put a queue slot in between:

private:
Q_SLOT void lenghtyOp(){
lenghtyOpQueued =false;
// do the slow stuff
}
bool lenghtyOpQueued = false;
public:
Q_SLOT void queueLenghty(){
if(lenghtyOpQueued) return;
lenghtyOpQueued =true;
QTimer::singleShot(100,this,SLOT(lenghtyOp()));
}

I prefered to use QtConcurrent::run it's much cleaner but thanks for the trick !

Thank you all for your answers :)

VRonin

@moffa13 said in Slot called multiple times to a blocking function in the same thread:

QtConcurrent::run it's much cleaner

I don't want to be that guy, but I'd bet my left eye you are doing multi-threading wrong here. Are you sure you don't have a race-condition on ui->tableWidget if you use that?

moffa13

@VRonin said in Slot called multiple times to a blocking function in the same thread:

@moffa13 said in Slot called multiple times to a blocking function in the same thread:

QtConcurrent::run it's much cleaner

I don't want to be that guy, but I'd bet my left eye you are doing multi-threading wrong here. Are you sure you don't have a race-condition on ui->tableWidget if you use that?

I don't understand why would I have this ?
Here's my final code : link

My lambda function is not doing anything to ui->tableWidget

VRonin

I have to be honest, I did not dig deep in to your code, but at first glance it looks like I lost my left eye in the bet 😉

moffa13

Well, I think there is no problem here because the only thing my thread is changing is a QTableWidgetItem which cannot be changed by something else by the logic of my code.

VRonin

@moffa13 said in Slot called multiple times to a blocking function in the same thread:

my thread is changing is a QTableWidgetItem which cannot be changed by something else

if the QTableWidgetItem is visible in a view then the view will directly call the item for read every repaint, hence race condition

moffa13

@VRonin said in Slot called multiple times to a blocking function in the same thread:

@moffa13 said in Slot called multiple times to a blocking function in the same thread:

my thread is changing is a QTableWidgetItem which cannot be changed by something else

if the QTableWidgetItem is visible in a view then the view will directly call the item for read every repaint, hence race condition

Actually I said something wrong. The part who is modifying the QTableWidgetItem is the QFutureWatcher<>::finished slot, which is accessed by the main thread, I think.
One question : Is it bad to lock and unlock a mutex multiple times ?

For example :

_mutex.lock();
//Some var to be protected
_mutex.unlock();
//Thread-safe heavy function
_mutex.lock();
//Some var to be protected
_mutex.unlock();

Thanks !

VRonin

@moffa13 said in Slot called multiple times to a blocking function in the same thread:

One question : Is it bad to lock and unlock a mutex multiple times ?

No, that's exactly what they are deigned for