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I have 2 systemd services.

a.service b.service

b depends on a but I don't want to explicitly mention the dependency in b.service file. Instead I am waiting on the org.freedesktop.DBus.Properties.PropertiesChanged signal from a.service when it starts at boot-up in the code of b.c and poll until I get that signal for it to become active.

           sd_ret = sd_bus_match_signal
                    (
                       bus,
                       NULL,
                       DESTINATION,         
                       A_PATH,           
                       INTERFACE,           
                       "PropertiesChanged", 
                       sd_callback,         
                       NULL                 
                    );

And then use poll to wait on signal from dbusfd.

I did the following experiment:

Experiment 1:

STEP 1:

systemctl mask a.service

systemctl mask b.service

STEP 2: reboot

STEP 3:

systemctl unmask a.service

systemctl unmask b.service

STEP 4: run the 'b' binary

./b

STEP 4:

systemctl start a.service

Observation:

b.service is able to catch the signal:

Signal match triggered! destination=(null) interface=org.freedesktop.DBus.Properties member=PropertiesChanged path=/org/freedesktop/systemd1/unit/a2eservice
Starting to poll
Signal match triggered! destination=(null) interface=org.freedesktop.DBus.Properties member=PropertiesChanged path=/org/freedesktop/systemd1/unit/a_2eservice
Property name: ActiveState
Updated ActiveState: active
a Service is now active.

But in Experiment 2

Both service unmasked during boot-up.

At boot-up the a.service does not seem to emit the PropertiesChanged signal.

So wanted to know if the behavior of a service is same during boot-up and after boot-up and does a service send PropertiesChanged signal during boot-up ?

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2
  • How do you know that the signal wasn't emitted before your listener (b.service) started? Commented Jan 3 at 7:46
  • From the journal logs I can see that the b.service starts first and sees the a.service is not in active state using the sd_bus_call_method(.."Get"..) and then b.service goes into poll blocking call indefinitely. After that in the log I see the a.service is invoked and is ready. But the b.service never comes out of poll() Commented Jan 3 at 15:19

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