[systemd-devel] [PATCH] libsystemd-bus: Add return error msg for unicast signals when well-known name is not available

[Lennart Poettering]

On Tue, 10.12.13 21:27, Lukasz Skalski (lukasz.skalski at op.pl) wrote:

> 
> On 12/10/2013 08:24 PM, Lennart Poettering wrote:
> >On Wed, 04.12.13 14:44, Lukasz Skalski (l.skalski at partner.samsung.com) wrote:
> >
> >>ENXIO, ESRCH and EADDRNOTAVAIL are also returned by ioctl(KDBUS_CMD_MSG_SEND)
> >>when we have unicast signal messages (signals with a DESTINATION
> >>field).
> >
> >Well, but you cannot respond to signals. They are supposed to be these
> >one-time things you send out, where no response is coming back. Method
> >calls otoh are supposed to have responses where either method success or
> >method failure is returned.
> 
> Yes, I know that we cannot respond to signals, but this could be
> useful for further (of course if you'll plan this kind of
> functionality) messages/signals monitoring.

Hmm, printing a log_debug() message when delivery fails would certainly
be a good idea. I have changed the code like that now.

> For example, in GDBUS we can set G_DBUS_DEBUG variable to a list of
> debug options, which cause GLib to print out different types of
> debugging information. When we send unicast signal to non-exist
> destination (in this example test.bus.glib) we can see:

> Above error message is not visible in user application, but is very
> useful for debugging purposes (we can easily check what happen with
> our signal).
> 
> IMHO it would be nice to have similar functionality in
> libsystemd-bus. What is your opinion about this idea?

Hmm, do you need more than the log_debug() bits I added now?

Something else I'd like to see added to kdbus is that we install PID
matches for messages. i.e. a way to subscribe to all messages sent by or
delivered to a specific PID. This could then be used for "busctl
monitor" to give it an almost "strace"-like feel, how one could watch at
any time what specific processes log.

Lennart

-- 
Lennart Poettering, Red Hat