[systemd-devel] sd-bus object manager question

[David Herrmann]

Hi

On Mon, Jul 20, 2015 at 9:26 PM, David Härdeman <david at hardeman.nu> wrote:
> On Sat, Jun 27, 2015 at 09:22:45AM +0200, David Härdeman wrote:
>>On Thu, Jun 25, 2015 at 04:25:29PM +0200, David Herrmann wrote:
>>>On Tue, Jun 23, 2015 at 2:41 PM, David Härdeman <david at hardeman.nu> wrote:
>>...
>>>> Now, a question...how is an object manager supposed to be implemented in
>>>> sd-bus?
>>>>
>>>> I've seen that there's a sd_bus_add_object_manager() function in sd-bus.h,
>>>> but how would I notify the object manager when objects are added and
>>>> removed?
>>>>
>>>> Two likely candidates seemed to be:
>>>> sd_bus_emit_object_added()
>>>> sd_bus_emit_object_removed()
>>>>
>>>> But calling them did not seem to do the right thing. And there seems to be
>>>> zero usage of sd_bus_add_object_manager() in the systemd tree itself (though
>>>> plenty of calls to sd_bus_add_node_enumerator() which might be related?).
>>>
>>>Not sure where the confusion is. Those calls generate the
>>>InterfacesAdded/Removed signals that the objectmanager API specifies.
>>>They collect the interfaces based on the vtables you registered on a
>>>given path. have a look at test-bus-objects.c which uses these
>>>interfaces.
>>
>>I've done some more experimentation and I think I've got a better idea
>>of what's going on. Sorry if my first email was a bit confusing.
>>
>>My problem is that I couldn't get the sd-bus object manager to properly
>>notify clients when objects are added or removed.
>>
>>I'm testing my sd-bus server with a gdbus client. The
>>GDBusObjectManagerClient should normally generate signals (gobject
>>signals, that is) when objects are added or removed.
>>
>>Anyway, I tried implementing an object manager server with gdbus as
>>well, and when talking to the gdbus server, the client is notified of
>>object additions/removals, so I used used busctl to monitor the signals
>>that were generated. I think I've found the problem.
>>
>>Assume there's an object manager at "/org/example/mgr" and that a new
>>object is created at "/org/example/mgr/object1".
>>
>>With sd-bus, I'd call:
>>sd_bus_emit_object_added(bus, "/org/example/mgr/object1");
>>
>>The generated signal would be:
>>‣ Type=signal  Endian=l  Flags=1  Version=1  Priority=0 Cookie=11
>>  Sender=:1.297  Path=/org/example/mgr/object1
>>Interface=org.freedesktop.DBus.ObjectManager  Member=InterfacesAdded
>>  UniqueName=:1.297
>>  MESSAGE "oa{sa{sv}}" {
>>          OBJECT_PATH "/org/example/mgr/object1";
>>
>>With gdbus, the generated signal is instead:
>>The generated signal would be:
>>‣ Type=signal  Endian=l  Flags=1  Version=1  Priority=0 Cookie=21
>>  Sender=:1.297  Path=/org/example/mgr
>>Interface=org.freedesktop.DBus.ObjectManager  Member=InterfacesAdded
>>  UniqueName=:1.297
>>  MESSAGE "oa{sa{sv}}" {
>>          OBJECT_PATH "/org/example/mgr/object1";
>>
>>Note the difference in the sender path.
>>
>>The GDBusObjectManagerClient seems to ignore the signal sd-bus generates
>>because of the different path.
>>
>>I think that means that sd_bus_emit_object_added() would need to take
>>both the object manager path and the object path as arguments instead of
>>a single path?
>
> Ping?

Thanks for the reminder, almost forgot about this. And sorry for the
delay, many of us are on vacation right now, and the remaining few
have a hard time keeping up with reports.

I pushed a fix here:
    https://github.com/systemd/systemd/pull/641

I'd prefer if we can move this discussion to github.

Thanks
David