How to pass sba(sv) arguments via qdbus

Question

I'm having a problem with qdbus. I need to pass sba(sv) to the SetUnitProperties method. I am only able to pass sba{sv} by using a dictonary. I can't find any documentation or examples on how to pass an array. Here is the error message : "Invalid arguments 'sba{sv}' to call org.freedesktop.systemd1.Manager.SetUnitProperties(), expecting 'sba(sv)'.".

I have also tried using StartTransienUnit which is even more complicated requiring ssa(sv)a(sa(sv)) !

Here is my code in C:

int QUsbDeviceWrapper::swUpdate() {
    // Initialize the D-Bus connection
    QDBusConnection connection = QDBusConnection::systemBus();

    // Check if the connection is valid
    if (!connection.isConnected()) {
        qWarning("Cannot connect to the D-Bus system bus");
        return 1;
    }

    // Obtain a proxy object for the systemd1 service
    QDBusInterface managerInterface("org.freedesktop.systemd1",
                                    "/org/freedesktop/systemd1",
                                    "org.freedesktop.systemd1.Manager",
                                    connection);

    // Check if the interface is valid
    if (!managerInterface.isValid()) {
        qWarning("Failed to obtain interface to systemd1.Manager");
        return 1;
    }

    // Prepare arguments for the SetUnitProperties method
    QString serviceName = "[email protected]";
    bool runtime = true;
    QVariantMap properties;
    properties.insert("StopWhenUnneeded", QVariant(false)); // Use QVariant for boolean value

    // Call the SetUnitProperties method
    QDBusMessage response = managerInterface.call("SetUnitProperties", serviceName, runtime, properties);

    // Check if the call was successful
    if (response.type() == QDBusMessage::ErrorMessage) {
        qWarning() << "Error calling SetUnitProperties:" << response.errorMessage();
        return 1;
    }

    qDebug() << "SetUnitProperties call successful";

    return 0;
}

I have tried using a QVariantMap to define the array but dbus doesen't accept it. I was expecting dbus to change the unit's properties.

Answer 1

This seems to be making the call, but actually crashes for some reason:

    // Prepare arguments for the SetUnitProperties method
    QString serviceName = "[email protected]";
    bool runtime = true;

    QVariantList propertiesList;
    propertiesList << QVariant::fromValue(Property("StopWhenUnneeded", false));
    // Append other properties...

    // Call the SetUnitProperties method
    QDBusMessage response = managerInterface.call("SetUnitProperties", serviceName, runtime, propertiesList);

Property is defined as:

struct Property {
    Property(const QString &propertyName, bool propertyValue):
        propertyName(propertyName),
        propertyValue(propertyValue) {}
    QString propertyName;
    bool propertyValue;
};

Q_DECLARE_METATYPE(Property)

// Marshall the MyStructure data into a D-Bus argument
QDBusArgument &operator<<(QDBusArgument &argument, const Property &property)
{
    argument.beginStructure();
    argument << property.propertyName << property.propertyValue;
    argument.endStructure();
    return argument;
}

// Retrieve the MyStructure data from the D-Bus argument
const QDBusArgument &operator>>(const QDBusArgument &argument, Property &property)
{
    argument.beginStructure();
    argument >> property.propertyName >> property.propertyValue;
    argument.endStructure();
    return argument;
}

Also don't forget to call qDBusRegisterMetaType<Property>();

Answer 2

According to QDBus documentation, an array can be passed as a QList<something>.

For a(sv) you might need to define a custom type for the {QString, QDBusVariant} structure, then pass it to qDBusRegisterMetaType(), and then you would be able to pass a QList<mySvTupleType> as the argument.

Likewise a(sa(sv)) would involve another custom {QString, QList<mySvTupleType>} structure.

The manual way of creating such a message appears to involve QDBusArgument with its beginArray() and beginStructure() methods – it might actually be easier to build the message by hand this way.

(I don't know offhand why systemd chose to use such a type, but perhaps they specifically needed an ordered dict for this particular method call – the regular dict/map type in many languages wouldn't preserve the ordering.)