QUSB will block the main thread and lead to stuck of ui.

[jerry-yuan]

Describe the bug
In qusb.usb@261 a QTimer with timeout of 250ms has connected to a slot checkDevices, in which will invoke the libusb_handle_events_timeout_completed on platforms libusb support hotplug detection (like linux) or manually detect the hotplug by polling the device lists on platrforms libusb that do not support hotplug detect (like windows):

/*!
    Check devices present in system.

    This gets called by the internal timer.
 */
void QUsb::checkDevices()
{
    DbgPrintFuncName();
    Q_D(QUsb);
    QUsb::IdList list;

    timeval t = { 0, 0 };
    qDebug()<<QThread::currentThread();
    if (d->m_has_hotplug) {
        libusb_handle_events_timeout_completed(d->m_ctx, &t, Q_NULLPTR);
    } else {
        list = devices();
        monitorDevices(list);
    }
}

However, this slot function will run in the main thread where the ui will be rendered:

[DEBUG][2021-06-15 17:10:02.338]:main.cpp@11: QThread(0x1e20b2f8)
[DEBUG][2021-06-15 17:10:02.489]:qusb.cpp@286: QThread(0x1e20b2f8)
[DEBUG][2021-06-15 17:10:02.847]:qusb.cpp@286: QThread(0x1e20b2f8)
[DEBUG][2021-06-15 17:10:03.140]:qusb.cpp@286: QThread(0x1e20b2f8)
[DEBUG][2021-06-15 17:10:03.473]:qusb.cpp@286: QThread(0x1e20b2f8)

For this reason, if I instantance a QUsb object there will be a time-consuming process that need to be processed in main thread and the UI rendering process will be disturbed. This problem will lead to an UI stuck.

PLatform:

• OS: Platforms that use polling to detect hotplug, e.g. Windows
• Version: 10

To Reproduce
Steps to reproduce the behavior:

1. Create a Qt Widget Application with default settings.
2. add QtUsb Module to .pro file:

   QT       += core gui usb

3. Instance the QUsb in class MainWindow or MainDialog

   #ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QUsb>
QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
    QUsb usbInstance;
};
#endif // MAINWINDOW_H

3. Compile and Run the program,when the window showed, drag the window you will find the window does not follow the cursor smoothly:
   
   While remove the instance of QUsb just run an empty Qt Widget Application the window could be drag smoothly:
   
   Further more, you can try to instance more QUsb objects to amplify the jam caused by polling.

4. See error

Expected behavior
I'd like this process not running in the main process.

Additional context
I have checked out the sources, I found the timer has been moved to another thread while the QUsb has not moved to the other thread.
In the previous test case, if you add a QThread in MainWindow and move QUsb into it would solve the jam problem, but maybe this thread should be mantainanced by QUsb?

[fpoussin]

Thanks, I'll have a look

[Alvazz]

@jerry-yuan Hi, Jerry. did you finally solve this problem? How was it resolved

[jerry-yuan]

@jerry-yuan Hi, Jerry. did you finally solve this problem? How was it resolved

Yes, I have solved this problem by moving the monitor function to another scanning thread.