Fatal error when trying to get a DBus property with custom type

Question

I'm trying to interface UDisks2 with a Qt application, and reading a custom property crash the program.

For SMART attributes, everything is OK (including the custom properties), but when I try to read the RAID devices property ActiveDevices I get a Qt fatal error and the app crash

Following is a test program which illustrate the behaviour:

#include <QApplication>

#include <QDBusMetaType>
#include <QDBusConnection>
#include <QDBusInterface>

#include <QDebug>


//create the structure as defined in 
// http://udisks.freedesktop.org/docs/latest/gdbus-org.freedesktop.UDisks2.MDRaid.html#gdbus-property-org-freedesktop-UDisks2-MDRaid.ActiveDevices 
struct MDRaidMember {
   QDBusObjectPath block;
   qint32 slot;
   QStringList state;
   qint64 numReadErrors;
   QVariantMap expansion;
};
Q_DECLARE_METATYPE(MDRaidMember)


//marshalling operator
QDBusArgument &operator<<(QDBusArgument &argument, const MDRaidMember& raidMember)
{
    argument.beginStructure();
    argument << raidMember.block;
    argument << raidMember.slot;
    argument << raidMember.state;
    argument << raidMember.numReadErrors;
    argument << raidMember.expansion;
    argument.endStructure();
    return argument;
}


//unmarshall operator
const QDBusArgument &operator>>(const QDBusArgument &argument, MDRaidMember& raidMember)
{
    argument.beginStructure();
    argument >> raidMember.block;
    argument >> raidMember.slot;
    argument >> raidMember.state;
    argument >> raidMember.numReadErrors;
    argument >> raidMember.expansion;
    argument.endStructure();
    return argument;
}



int main(int argc, char *argv[]) {
  QApplication app(argc, argv);

  //register the type, everything looks fine
  qDebug() << qRegisterMetaType<MDRaidMember>("MDRaidMember");
  qDebug() << qDBusRegisterMetaType<MDRaidMember>();


  QDBusInterface iface("org.freedesktop.UDisks2",
                       "/org/freedesktop/UDisks2/mdraid/9aec6784_86ec37d4_8c1c6add_4dc9fe81",
                       "org.freedesktop.UDisks2.MDRaid",
                       QDBusConnection::systemBus());

  //read some properties
  qDebug() << iface.property("UUID");
  qDebug() << iface.property("ActiveDevices"); //crash the test program!

  return app.exec();
}

Executing this code produce the following output

1057
1057
QVariant(QString, "9aec6784:86ec37d4:8c1c6add:4dc9fe81")
Cannot construct placeholder type QDBusRawType
zsh: abort (core dumped)  ./app/test_members

Anyone encoutered such a case ? I don't thing it is related to my code at all, as the result is the same if the custom type aren't declared (keeping only the content of the main function).

Answer 1

I didn't find a correct answer for this, but here is a workaround: calling the Get method on interface org.freedesktop.DBus.Properties and unmarshalling the result manually. The tricky part was figuring the result's content (it's a QVariant containing a QDBusVariant containing a QDBusArgument)

The updated main function:

//...

int main(int argc, char *argv[]) {
  QApplication app(argc, argv);

  //register the type, everything looks fine
  qDebug() << qRegisterMetaType<MDRaidMember>("MDRaidMember");
  qDebug() << qDBusRegisterMetaType<MDRaidMember>();


  QDBusInterface iface("org.freedesktop.UDisks2",
                   "/org/freedesktop/UDisks2/mdraid/9aec6784_86ec37d4_8c1c6add_4dc9fe81",
                   "org.freedesktop.DBus.Properties",
                   QDBusConnection::systemBus());

  QDBusMessage reply = iface.call("Get", "org.freedesktop.UDisks2.MDRaid", "ActiveDevices");
  QVariant v = reply.arguments().first();
  QDBusArgument arg = v.value<QDBusVariant>().variant().value<QDBusArgument>();

  QList<MDRaidMember> members;
  arg.beginArray();
  while(!arg.atEnd()) {
    MDRaidMember m;
    arg >> m;
    members << m;
  }

  qDebug() << members;

  return app.exec();
}