Bug#995366: xfce4-pulseaudio-plugin: repeatedly polls D-Bus names, should watch NameOwnerChanged instead

[Simon McVittie <smcv@debian.org>]

Package: xfce4-pulseaudio-plugin
Version: 0.4.3-1
Severity: normal
Tags: upstream

While using debian-live-11.0.0-amd64-xfce.iso to try to reproduce an
unrelated issue, I noticed that the pulseaudio plugin for the panel
calls the ListNames() D-Bus method approximately once per second.
This causes the system to wake up unnecessarily, and is likely to be
a waste of electricity on an otherwise idle system. It also makes it
more difficult to debug the unrelated issue with dbus-monitor :-)

Looking at upstream source code, this seems to be because it calls
pulseaudio_mpris_tick_cb() once per second, which calls
pulseaudio_mpris_get_available_players(), which calls ListNames.

This shouldn't be necessary: D-Bus is designed to be completely idle (no
messages sent, no messages received, no wakeups) if nothing interesting
is happening, similar to other AF_UNIX-based protocols such as X11.

Workaround: remove the Pulseaudio plugin (the volume control) from the
panel while debugging D-Bus-related issues.

I tried to report this upstream, but so far I have been unable to set
up an account on XFCE's Gitlab (presumably the email confirmation has
been greylisted).

For protocols like MPRIS that use an equivalence class of names starting
with a prefix, the intended pattern since dbus-daemon 1.6.0 (stable
release in 2012) is this:

* Add a match rule for the NameOwnerChanged signal with
  arg0namespace="org.mpris.MediaPlayer2"
    - in GDBus, this is something like:
        g_dbus_connection_signal_subscribe (connection,
                                            "org.freedesktop.DBus",
                                            "org.freedesktop.DBus",
                                            "NameOwnerChanged",
                                            "/org/freedesktop/DBus",
                                            "org.mpris.MediaPlayer2",
                                            G_DBUS_SIGNAL_FLAGS_MATCH_ARG0_NAMESPACE,
                                            name_owner_changed_cb, ...);

* When the signal is received, its arguments are the name, the old owner,
  and the new owner
    - if the new owner is "", the name is disappearing from the bus:
      remove it from the list of MPRIS media players
    - otherwise, the name is appearing on the bus: add it to the list of
      MPRIS media players

* After connecting to the signal, call ListNames *once* to populate the
  initial list of MPRIS media players

    smcv