[systemd-devel] StartupFinished signal is not sent on 210

[Lennart Poettering]

On Tue, 04.03.14 21:40, Umut Tezduyar (umut at tezduyar.com) wrote:

> >> To compare 208 and 210, I pretty much followed steps @ https://wiki.ubuntu.com/DebuggingDBus and by using
> >> $> dbus-monitor "type=signal,sender='org.freedesktop.systemd1'" --system
> >
> > You have to invoke "Subscribe()" first on the manager bus, to get
> > something. We only send out the messages as long as at least one living
> > client has issued this call.
> 
> Don't you think this is something worth mentioning in the NEWS since
> we were able to retrieve signals without subscribing to them on 208.
> Excuse me if it is documented somewhere and I missed it.

Subscribe() is actually around since day #1 of systemd. And it is
documented here:

http://www.freedesktop.org/wiki/Software/systemd/dbus/

Of course, I must admit it is less than ideal that with logind around
you might be able to get away with never calling it... That certainly
hides bugs...

We changed a couple of times how systemd delivers those signals during
the history, but clients which called Subscribed() should always just
have worked.

Note that 211 actually changes things back to be more like < 209... 

The difference between < 209 + > 210 and 209+210 is the single
subscriber case. With < 209 + > 210 we have the following rule:

1. when there's no subscriber we don't send out any signals
2. when there's one or more subscribers we broadcast signals to everybody

In 209+210 we had this ruleset instead:

1. when there's no subscriber we don't send out any signals
2. when there's one subscriber send all signals as unicast to it
3. when there's two or more subscribers we broadcast signals to everybody

I thought for a while that doing step #2 was smart, but then again, it's
kinda stupid, since it second guesses the dbus filter logic too much,
and the messages look different to the receiver depending on the number
of other subscribers. So I dropped this again...

Lennart

-- 
Lennart Poettering, Red Hat