Fix QPpdPrintDevice::isDefault

How to test:
 * Have two printers
 * Use lpoptions -d to set the default printer to be one and then the
other
 * Use lpstat -d to check setting the default printer worked
 * Use this simple test program and check the resulting values make
sense
    qDebug() << "DefaultPrinter" << QPrinterInfo::defaultPrinter().printerName();
    const QList<QPrinterInfo> list = QPrinterInfo::availablePrinters();
    for(const QPrinterInfo &pi : list) {
        qDebug() << pi.printerName() << pi.isDefault();
    }

Fixes: QTBUG-70317
Change-Id: I535d11451c568630a374f5c37d8cac32cbb6d3ab
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@qt.io>

src/plugins/printsupport/cups/qcupsprintersupport.cpp

@@ -174,6 +174,11 @@ QStringList QCupsPrinterSupport::availablePrintDeviceIds() const
 }
 
 QString QCupsPrinterSupport::defaultPrintDeviceId() const
+{
+    return staticDefaultPrintDeviceId();
+}
+
+QString QCupsPrinterSupport::staticDefaultPrintDeviceId()
 {
     QString printerId;
     cups_dest_t *dests;

src/plugins/printsupport/cups/qcupsprintersupport_p.h

@@ -71,6 +71,8 @@ public:
     QStringList availablePrintDeviceIds() const override;
     QString defaultPrintDeviceId() const override;
 
+    static QString staticDefaultPrintDeviceId();
+
 private:
     QString cupsOption(int i, const QString &key) const;
 };

src/plugins/printsupport/cups/qppdprintdevice.cpp

@@ -39,6 +39,8 @@
 
 #include "qppdprintdevice.h"
 
+#include "qcupsprintersupport_p.h"
+
 #include <QtCore/QMimeDatabase>
 #include <qdebug.h>
 
@@ -118,7 +120,12 @@ bool QPpdPrintDevice::isValid() const
 
 bool QPpdPrintDevice::isDefault() const
 {
-    return printerTypeFlags() & CUPS_PRINTER_DEFAULT;
+    // There seems to be a bug in cups in which printerTypeFlags
+    // returns CUPS_PRINTER_DEFAULT based only on system values, ignoring user lpoptions
+    // so we can't use that. And also there seems to be a bug in which dests returned
+    // by cupsGetNamedDest don't have is_default set at all so we can't use that either
+    // so go the long route and compare our id against the defaultPrintDeviceId
+    return id() == QCupsPrinterSupport::staticDefaultPrintDeviceId();
 }
 
 QPrint::DeviceState QPpdPrintDevice::state() const