mapper: should it add "default interfaces" and is it valid to implement no interfaces on an object?

[Patrick Williams]

On Wed, Oct 30, 2024 at 03:00:01PM +0800, Lei Yu wrote:

Based on the info from Matt, answering these questions.

> So here are the questions:
> 1. How to explain the above behavior of "D-Bus calls on default
> interfaces", especially for 2 & 3?

#2 seems like a bug in mapper?  When I run this on a sample from systemd
I get good results:

```
$ busctl --user call org.freedesktop.systemd1 /org/freedesktop/systemd1/unit org.freedesktop.DBus.Properties GetAll s org.freedesktop.DBus.Properties
a{sv} 0
```

> 2. Should mapper add "default interfaces" for all the objects and
> their parents? If yes, it adds extra interfaces that are not
> implemented; If no, it misses some interfaces that an object really
> implements.

I don't know that it is adding "default interfaces" really.  This code
you linked to seems to be "assume these default interfaces will exist
because they are a requirement of dbus".  The code appears to be an
optimization to be able to advertise the path (and contained objects)
before the bus introspection is complete.

So, I think this is valid, isn't it?

> 3. Is it valid for a D-Bus service to implement no interfaces on an object?

It appears that "no interfaces" isn't a thing.  "no interfaces other
than the 3" is a thing.

I'm not finding explicitly in the dbus specification[1] where Peer,
Introspectable, and Properties are required (except that it is perhaps
implied by the discussion about the Message Bus itself).  sd-bus appears
to always add these though (you can find multiple references in the
code)[2].

Interestingly, another application I have running (Spotify) does *not*
always have these 3 interfaces.  They're obviously using a different
dbus library than sd-bus:

```
$ busctl --user introspect org.mpris.MediaPlayer2.spotify /org/ayatana/NotificationItem
NAME TYPE SIGNATURE RESULT/VALUE FLAGS
$ busctl --user introspect org.mpris.MediaPlayer2.spotify /org/ayatana/NotificationItem/spotify_client
NAME                                TYPE      SIGNATURE RESULT/VALUE                             FLAGS
org.freedesktop.DBus.Introspectable interface -         -                                        -
...
```

So, the "assumption" that mapper has is perhaps only valid for sd-bus
and not for other dbus libraries.

[1]: https://dbus.freedesktop.org/doc/dbus-specification.html
[2]: https://github.com/systemd/systemd/blob/6e492ae98abd9500a4310653dec0c3fced66c7da/src/libsystemd/sd-bus/bus-objects.c#L2472

-- 
Patrick Williams
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