Qt - capturing internal qWarning or "failing makeCurrent()" error - c++

Question

I'm running a QWebEngineView in Qt 5.12 on a linux machine with Wayland, where a bug in Qt (there are a few issues reported about this) causes the app to freeze rendering if the screen is disconnected and reconnected, or if it goes into sleep mode.
The app still runs fine, but no further rendering is done ant it's stuck at last frame. The only errors that are visible when running the app are:
[14:52:07.809] connector 87 disconnected
QOpenGLFunctions created with non-current context
QPaintDevice::metrics: Device has no metric information
QQuickWidget: Cannot render due to failing makeCurrent()
QQuickWidget: Cannot render due to failing makeCurrent()
This is the place in Qt that throws this warning:
// qtdeclarative/src/quickwidgets/qquickwidget.cpp
void QQuickWidgetPrivate::render(bool needsSync)
{
if (!useSoftwareRenderer) {
#if QT_CONFIG(opengl)
// createFramebufferObject() bails out when the size is empty. In this case
// we cannot render either.
if (!fbo)
return;
Q_ASSERT(context);
if (!context->makeCurrent(offscreenSurface)) {
qWarning("QQuickWidget: Cannot render due to failing makeCurrent()");
return;
}
I unfortunately cannot alter the Qt version on this device, so I'd like to know if there is any reliable way of detecting this situation from inside the app to do some proper crash/restart?
I couldn't find any way to capture qWarning messages from inside of Qt or anything like that, and the code that causes this doesn't look as it could emit any interesting signal.
As last resort, I could parse the output of my app and look for "Cannot render due to failing makeCurrent()" but I really don't like that I cannot detect it from my own app that rendering has frozen.
I could add QT_FATAL_WARNINGS environment variable, which causes the app to automatically crash upon any warning, but I'm kinda scared that there may be some non-fatal warnings that would cause unintentional crashes.

Answer 1

You can use qInstallMessageHandler to override the default logging mechanism. And in there, you can check the qWarning messages for the specific string you're looking for.
void myMessageOutput(QtMsgType type, const QMessageLogContext &context, const QString &msg)
{
QByteArray localMsg = msg.toLocal8Bit();
const char *file = context.file ? context.file : "";
const char *function = context.function ? context.function : "";
switch (type) {
case QtWarningMsg:
fprintf(stderr, "Warning: %s (%s:%u, %s)\n", localMsg.constData(), file, context.line, function);
if (localMsg == theBadRenderError) {
// Do something here
}
break;
...
}
}
int main(int argc, char **argv)
{
qInstallMessageHandler(myMessageOutput);
QApplication app(argc, argv);
...
return app.exec();
}