Attempting to read optional MPRIS properties causes the D-Bus client to hang indefinitely

[azymohliad]

Bug/Feature description

When trying to read optional org.mpris.MediaPlayer2.Player properties such as LoopStatus or Shuffle, or any non-existent property, the client call blocks indefinitely and never returns.

Steps to reproduce

Run Cozy, then try to read optional or any non-existent property from org.mpris.MediaPlayer2.Player interface, for example:

busctl get-property --user org.mpris.MediaPlayer2.Cozy /org/mpris/MediaPlayer2 org.mpris.MediaPlayer2.Player LoopStatus

The command won't return nor print anything. The same happens when trying to read properties with zbus.

The expected behavior would be to return an error.

System Information

• Operating System: ArchLinux
• Installation source: Flathub
• Version of cozy: 1.2.1

Possible root-cause and fix

I believe this is due to errors being handled not according to D-Bus specs: from what I can see, Cozy's MPRIS implementation returns None when the property isn't found or upon other errors. And as I was just told by zbus author, and from what I can see in D-Bus type system spec, None doesn't really map to D-Bus types, which presumably causes clients to block waiting for a supported value.

As I understand, the correct way would be to use return_dbus_error or return_error_literal upon errors instead of invocation.return_value(None). Internally it results in a different D-Bus message type (ERROR instead of METHOD_RETURN).