Names require service files to call methods

[acrisci]

Hello, I am not a dbus expert so forgive me if this is programmer error.

I am following the examples and I want to call a method like could be executed from the command line like so:

dbus-send --print-reply --dest=org.mpris.MediaPlayer2.vlc /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player.PlayPause

which should make my media player play.

Here is a program that I would expect to do this with godbus:

package main

import (
    "github.com/godbus/dbus"
)

func main() {
    conn, err := dbus.SessionBus()
    if err != nil {
        panic(err)
    }
    obj := conn.Object("org.mpris.MediaPlayer2.vlc", "/org/mpris/MediaPlayer2")
    call := obj.Call("org.mpris.MediaPlayer2.Player.PlayPause", 0)
    if call.Err != nil {
        panic(call.Err)
    }
}

But I get this error:

panic: The name org.mpris.MediaPlayer2.vlc was not provided by any .service files

with a stack trace. I believe it is correct that there is not any such service file that provides org.mpris.MediaPlayer2.vlc, but vlc is running, and when I list names with dbus-send, it appears in the returned array. However, when I run the example in the examples folder to list names, it does not appear in the returned array.

Could someone help me out with this?

Thanks.

[alexlarsson]

It sounds to me like the your app and dbus-send are talking to different instances of the dbus daemon. Do they get the same DBUS_SESSION_BUS_ADDRESS env var passed in?

[acrisci]

Thanks for the hint. The problem does seem to be related to the fact that the example I provided is talking to a different instance of the dbus daemon.

Without setting DBUS_SESSION_BUS_ADDRESS in the env, dbus-send and the library seem to be choosing different addresses.

The address dbus-send is using can be found by examining the file ~/.dbus/session-bus/[machine id]-[X display]. If I set DBUS_SESSION_BUS_ADDRESS to the address indicated by this file, the library will work as I expect it to and find the vlc name.

I'm not exactly sure what heuristic to use in general to find the "correct" bus address.

I think that the fact that dbus-send is using a different method to find the address could be considered a bug. I have also tried creating a simple dbus service with a python library and the address agrees with dbus-send. Should I file another ticket for this?