Fix previewing a preview on reentrance

Make the print preview more tolerant of reentrance by avoiding
previewing itself.

Change-Id: Ia4774cb51bdda7d311414c4e2f9c2bfbed4f187f
Pick-to: 6.7 6.6 6.5
Fixes: QTBUG-122749
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/printsupport/kernel/qprinter.cpp

@@ -129,6 +129,11 @@ QList<const QPicture *> QPrinterPrivate::previewPages() const
     return QList<const QPicture *>();
 }
 
+bool QPrinterPrivate::previewMode() const
+{
+    return (previewEngine != nullptr) && (previewEngine == printEngine);
+}
+
 void QPrinterPrivate::setPreviewMode(bool enable)
 {
     Q_Q(QPrinter);

src/printsupport/kernel/qprinter_p.h

@@ -70,6 +70,7 @@ public:
 #if QT_CONFIG(printpreviewwidget)
     QList<const QPicture *> previewPages() const;
     void setPreviewMode(bool);
+    bool previewMode() const;
 #endif
 
     void setProperty(QPrintEngine::PrintEnginePropertyKey key, const QVariant &value);

src/printsupport/widgets/qprintpreviewwidget.cpp

@@ -361,6 +361,9 @@ void QPrintPreviewWidgetPrivate::generatePreview()
     //### emit paintRequested() until the user changes some parameter
 
     Q_Q(QPrintPreviewWidget);
+    // Avoid previewing a preview
+    if (printer->d_func()->previewMode())
+        return;
     printer->d_func()->setPreviewMode(true);
     emit q->paintRequested(printer);
     printer->d_func()->setPreviewMode(false);