QDBusConnection error: Invalid object path passed in arguments

[jiveaxe]

Hi, I added mpris support to my application. It works well in all tests except one: Now Playing plasmoid of KDE Plasma Workspace. Clicking on the seek widget to change playback position, Plasma Desktop crashes miserably. In console the following message appears:

process 2395: arguments to dbus_message_iter_append_basic() were incorrect, assertion "_dbus_check_is_valid_path (*string_p)" failed in file dbus-message.c line 2603.

Not very explicative, unfortunately. Not knowing how debug dbus I tried a different approach: I recompiled dbus commenting out the offensive line and fortunately I got a more friendly error message:

QDBusConnection: error: could not send message to service "org.mpris.MediaPlayer2.myapp" path "/org/mpris/MediaPlayer2" interface "org.mpris.MediaPlayer2.Player" member "SetPosition": Marshalling failed: Invalid object path passed in arguments

Then I used qdbusviewer to inspect all mpris methods of my application and all work fine except SetPosition, where it gives the error:

Unable to find method SetPosition on path /org/mpris/MediaPlayer2 in interface org.mpris.MediaPlayer2.Player

Here it is an excerpt of the code I'm using:

#include "MprisPlugin.h"
#include "MprisPluginRootAdaptor.h"
#include "MprisPluginPlayerAdaptor.h"
 
namespace mpris
{
 
const char* MprisPlugin::mprisObjectPath = "/org/mpris/MediaPlayer2";
const char* MprisPlugin::serviceName = "org.mpris.MediaPlayer2.myapp";
const char* MprisPlugin::freedesktopPath = "org.freedesktop.DBus.Properties";
 
static QString s_mpInfoIdentifier = QString( "MPRISPLUGIN" );
 
MprisPlugin::MprisPlugin(QObject* parent) :
    QObject(parent)
{
    // DBus connection
    new MprisPluginRootAdaptor( this );
    new MprisPluginPlayerAdaptor( this );
    QDBusConnection dbus = QDBusConnection::sessionBus();
    dbus.registerObject( mprisObjectPath, this );
    dbus.registerService( serviceName );
 
    // Listen to volume changes
    connect( The::audioEngine(), SIGNAL( volumeChanged( int ) ),
                                 SLOT( onVolumeChanged( int ) ) );
 
    // When a track is added or removed, CanGoNext updated signal is sent
    if ( !The::playlist() )
    {
        connect( The::playlist(), SIGNAL( itemCountChanged( unsigned int ) ),
                                  SLOT( onTrackCountChanged( unsigned int ) ) );
    }
 
    // Connect to AudioEngine's seeked signal
    connect( The::audioEngine(), SIGNAL( seeked( qint64, bool ) ),
                                 SLOT( onSeeked( qint64, bool ) ) );
 
    connect( The::audioEngine(), SIGNAL( stateChanged(Phonon::State, Phonon::State ) ),
                                 SLOT( engineStateChanged(Phonon::State, Phonon::State ) ) );
 
    connect( The::audioEngine(), SIGNAL( sourceChanged() ),
                                 SLOT( trackChanged() ) );
 
    connect( The::audioEngine(), SIGNAL( seekableChanged(bool) ),
                                 SLOT( seekableChanged(bool) ) );
 
    connect( The::audioEngine(), SIGNAL( trackLengthChanged(qint64) ),
                                 SLOT( trackLengthChanged(qint64) ) );
}
 
void MprisPlugin::notifyPropertyChanged( const QString& interface, const QString& propertyName )
{
    QDBusMessage signal = QDBusMessage::createSignal(
        mprisObjectPath,
        freedesktopPath,
        "PropertiesChanged" );
    signal << interface;
    QVariantMap changedProps;
    changedProps.insert(propertyName, property(propertyName.toLatin1()));
    signal << changedProps;
    signal << QStringList();
    qDebug() << propertyName;
    qDebug() << changedProps;
    QDBusConnection::sessionBus().send(signal);
}
 
// ...
 
// org.mpris.MediaPlayer2.Player
 
// ...
 
qlonglong MprisPlugin::position() const
{
    return (qlonglong) ( The::audioEngine()->currentTime() * 1000 );
}
 
// ...
 
void MprisPlugin::SetPosition( const QDBusObjectPath& TrackId, qlonglong Position )
{
    if ( !canSeek() )
        return;
 
    if ( TrackId.path() != QString( "/track/" ) + QString::number( The::audioEngine()->currentTrackID() ) )
        return;
 
    if ( ( Position < 0) || ( Position > The::audioEngine()->currentTrackTotalTime() * 1000 )  )
        return;
 
    The::audioEngine()->seek( (qint64) (Position / 1000 ) );
}
 
// ...
 
void MprisPlugin::Seek( qlonglong Offset )
{
    if ( !canSeek() )
        return;
 
    qlonglong seekTime = position() + Offset;
    if ( seekTime < 0 )
        The::audioEngine()->seek( 0 );
    else if ( seekTime > The::audioEngine()->currentTrackTotalTime() * 1000 )
        Next();
    else
        The::audioEngine()->seek( (qint64) ( seekTime / 1000 ) );
 
}
 
void MprisPlugin::onSeeked( qint64 ms, bool userSeek )
{
    if ( userSeek )
        emit Seeked( ms * 1000 );
}
 
void MprisPlugin::engineStateChanged(Phonon::State newState, Phonon::State oldState)
{
    if(newState != Phonon::PlayingState && newState != Phonon::PausedState) {
        m_lastMetadata= QVariantMap();
        notifyPropertyChanged("org.mpris.MediaPlayer2.Player", "Metadata");
    }
 
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "PlaybackStatus" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanGoNext" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanGoPrevious" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanPause" );
}
 
// ...
 
void MprisPlugin::trackChanged()
{
    PlaylistItemPtr track = The::audioEngine()->currentTrack();
    m_lastMetadata = QVariantMap();
    m_lastMetadata.insert( "mpris:trackid", QString( "/track/" ) + QString::number( The::audioEngine()->currentTrackID() ) );
    m_lastMetadata.insert( "mpris:length", static_cast<qlonglong>( The::audioEngine()->currentTrackTotalTime() ) * 1000 );
    m_lastMetadata.insert( "xesam:album", track->albumName() );
    m_lastMetadata.insert( "xesam:artist", QStringList( track->artistName() ) );
    m_lastMetadata.insert( "xesam:title", track->songName() );
 
    m_lastMetadata.insert( "mpris:artUrl", QString( QUrl::fromLocalFile( track->coverArtFilename() ).toEncoded() ) );
 
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "Metadata" );
}
 
void MprisPlugin::seekableChanged( bool seekable )
{
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanSeek");
}
 
void MprisPlugin::trackLengthChanged( qint64 milliseconds )
{
    if ( milliseconds >= 0 )
        trackChanged();
}
 
} //mpris

#include "MprisPlugin.h"
#include "MprisPluginRootAdaptor.h"
#include "MprisPluginPlayerAdaptor.h"

namespace mpris
{

const char* MprisPlugin::mprisObjectPath = "/org/mpris/MediaPlayer2";
const char* MprisPlugin::serviceName = "org.mpris.MediaPlayer2.myapp";
const char* MprisPlugin::freedesktopPath = "org.freedesktop.DBus.Properties";

static QString s_mpInfoIdentifier = QString( "MPRISPLUGIN" );

MprisPlugin::MprisPlugin(QObject* parent) :
    QObject(parent)
{
    // DBus connection
    new MprisPluginRootAdaptor( this );
    new MprisPluginPlayerAdaptor( this );
    QDBusConnection dbus = QDBusConnection::sessionBus();
    dbus.registerObject( mprisObjectPath, this );
    dbus.registerService( serviceName );

    // Listen to volume changes
    connect( The::audioEngine(), SIGNAL( volumeChanged( int ) ),
                                 SLOT( onVolumeChanged( int ) ) );

    // When a track is added or removed, CanGoNext updated signal is sent
    if ( !The::playlist() )
    {
        connect( The::playlist(), SIGNAL( itemCountChanged( unsigned int ) ),
                                  SLOT( onTrackCountChanged( unsigned int ) ) );
    }

    // Connect to AudioEngine's seeked signal
    connect( The::audioEngine(), SIGNAL( seeked( qint64, bool ) ),
                                 SLOT( onSeeked( qint64, bool ) ) );

    connect( The::audioEngine(), SIGNAL( stateChanged(Phonon::State, Phonon::State ) ),
                                 SLOT( engineStateChanged(Phonon::State, Phonon::State ) ) );

    connect( The::audioEngine(), SIGNAL( sourceChanged() ),
                                 SLOT( trackChanged() ) );

    connect( The::audioEngine(), SIGNAL( seekableChanged(bool) ),
                                 SLOT( seekableChanged(bool) ) );

    connect( The::audioEngine(), SIGNAL( trackLengthChanged(qint64) ),
                                 SLOT( trackLengthChanged(qint64) ) );
}

void MprisPlugin::notifyPropertyChanged( const QString& interface, const QString& propertyName )
{
    QDBusMessage signal = QDBusMessage::createSignal(
        mprisObjectPath,
        freedesktopPath,
        "PropertiesChanged" );
    signal << interface;
    QVariantMap changedProps;
    changedProps.insert(propertyName, property(propertyName.toLatin1()));
    signal << changedProps;
    signal << QStringList();
    qDebug() << propertyName;
    qDebug() << changedProps;
    QDBusConnection::sessionBus().send(signal);
}

// ...

// org.mpris.MediaPlayer2.Player

// ...

qlonglong MprisPlugin::position() const
{
    return (qlonglong) ( The::audioEngine()->currentTime() * 1000 );
}

// ...

void MprisPlugin::SetPosition( const QDBusObjectPath& TrackId, qlonglong Position )
{
    if ( !canSeek() )
        return;

    if ( TrackId.path() != QString( "/track/" ) + QString::number( The::audioEngine()->currentTrackID() ) )
        return;

    if ( ( Position < 0) || ( Position > The::audioEngine()->currentTrackTotalTime() * 1000 )  )
        return;

    The::audioEngine()->seek( (qint64) (Position / 1000 ) );
}

// ...

void MprisPlugin::Seek( qlonglong Offset )
{
    if ( !canSeek() )
        return;

    qlonglong seekTime = position() + Offset;
    if ( seekTime < 0 )
        The::audioEngine()->seek( 0 );
    else if ( seekTime > The::audioEngine()->currentTrackTotalTime() * 1000 )
        Next();
    else
        The::audioEngine()->seek( (qint64) ( seekTime / 1000 ) );

}

void MprisPlugin::onSeeked( qint64 ms, bool userSeek )
{
    if ( userSeek )
        emit Seeked( ms * 1000 );
}

void MprisPlugin::engineStateChanged(Phonon::State newState, Phonon::State oldState)
{
    if(newState != Phonon::PlayingState && newState != Phonon::PausedState) {
        m_lastMetadata= QVariantMap();
        notifyPropertyChanged("org.mpris.MediaPlayer2.Player", "Metadata");
    }

    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "PlaybackStatus" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanGoNext" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanGoPrevious" );
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanPause" );
}

// ...

void MprisPlugin::trackChanged()
{
    PlaylistItemPtr track = The::audioEngine()->currentTrack();
    m_lastMetadata = QVariantMap();
    m_lastMetadata.insert( "mpris:trackid", QString( "/track/" ) + QString::number( The::audioEngine()->currentTrackID() ) );
    m_lastMetadata.insert( "mpris:length", static_cast<qlonglong>( The::audioEngine()->currentTrackTotalTime() ) * 1000 );
    m_lastMetadata.insert( "xesam:album", track->albumName() );
    m_lastMetadata.insert( "xesam:artist", QStringList( track->artistName() ) );
    m_lastMetadata.insert( "xesam:title", track->songName() );

    m_lastMetadata.insert( "mpris:artUrl", QString( QUrl::fromLocalFile( track->coverArtFilename() ).toEncoded() ) );

    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "Metadata" );
}

void MprisPlugin::seekableChanged( bool seekable )
{
    notifyPropertyChanged( "org.mpris.MediaPlayer2.Player", "CanSeek");
}

void MprisPlugin::trackLengthChanged( qint64 milliseconds )
{
    if ( milliseconds >= 0 )
        trackChanged();
}

} //mpris

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Can you explain why just SetPosition does not work?

Very thanks

[anda_skoa]

Does the adator for that interface have that method?

Cheers,
_

[jiveaxe]

yep,

http://pastebin.kde.org/pfc6839a8

[anda_skoa]

That's the interface description. Have you checked the adaptor MprisPluginPlayerAdaptor?

Cheers,
_

[jiveaxe]

This is a cmake project so MprisPluginPlayerAdaptor is auto-generated. I'm using the following macros:

qt4_add_dbus_adaptor(myapp_SRCS
                     dbus/MprisPluginRootAdaptor.xml
                     dbus/MprisPlugin.h mpris::MprisPlugin MprisPluginRootAdaptor MprisPluginRootAdaptor)
qt4_add_dbus_adaptor(myapp_SRCS
                     dbus/MprisPluginPlayerAdaptor.xml
                     dbus/MprisPlugin.h mpris::MprisPlugin MprisPluginPlayerAdaptor MprisPluginPlayerAdaptor)

qt4_add_dbus_adaptor(myapp_SRCS
                     dbus/MprisPluginRootAdaptor.xml
                     dbus/MprisPlugin.h mpris::MprisPlugin MprisPluginRootAdaptor MprisPluginRootAdaptor)
qt4_add_dbus_adaptor(myapp_SRCS
                     dbus/MprisPluginPlayerAdaptor.xml
                     dbus/MprisPlugin.h mpris::MprisPlugin MprisPluginPlayerAdaptor MprisPluginPlayerAdaptor)

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in resulting MprisPluginPlayerAdaptor I've searched for "position" and found this:

qlonglong MprisPluginPlayerAdaptor::position() const
{
    // get the value of property Position
    return qvariant_cast< qlonglong >(parent()->property("Position"));
}
 
// ...
 
void MprisPluginPlayerAdaptor::SetPosition(const QDBusObjectPath &TrackId, qlonglong Position)
{
    // handle method call org.mpris.MediaPlayer2.Player.SetPosition
    parent()->SetPosition(TrackId, Position);
}

qlonglong MprisPluginPlayerAdaptor::position() const
{
    // get the value of property Position
    return qvariant_cast< qlonglong >(parent()->property("Position"));
}

// ...

void MprisPluginPlayerAdaptor::SetPosition(const QDBusObjectPath &TrackId, qlonglong Position)
{
    // handle method call org.mpris.MediaPlayer2.Player.SetPosition
    parent()->SetPosition(TrackId, Position);
}

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So it seems that SetPosition is in place.

Thanks for your time

[anda_skoa]

Hmm, very strange.

Can you try to make the call using dbus-send? In contrast to qdbus or qdbusviewer it doesn't use introspection at runtime.
Alternatively you could try to generate interfaces in a small test program and use it to call your object's methods.

The thing with the assert is also puzzling.

Maybe try running the Plasma applet in plasma-windowed and attach a debugger to it and see what the stack looks like when the assert is triggered?

Cheers,
_

[jiveaxe]

Just a quick update: at the end I dropped my implementation and wrote adaptors from scratch (no more qt4_add_dbus_adaptor macro). Now it works.