[systemd-devel] How to add filter for ActiveState property and get to know for which object this property is?

[Tomasz Michalski]

And I have other question. Generally we wonder what will be better design
for monitoring systemd services
1.)
- add-match to ActiveState
- subscribe to systemd1.Manager
- then we have to start in separate thread some infinitive-loop with
sd_bus_wait()
and sd_bus_process() (*)
2.)
- asking for example each 5s all services with the aid of method
sd_bus_get_property about ActiveState. None additional thread is required
in our case as timer when expires send notification to existed infinity
while loop from current handler.

I wonder which solution occupy more processor. From the first glance it
seems like second one, because there might be too much unnecessary
communication through the bus in order to get to know about systemd service
states.
However take care that here (*) we have to have some infinity while loop
probably in separate thread which will block thread waiting for signals
from systemd. As far as timers are concerned in 2nd solution none
additional thread is required. Is it possible somehow in 1.) not to have
additional thread in order to receive signal from systemd?
Moreover 2nd solution seems to be better in case when systemd1.Manager fail
at all (will be killed). In 2nd solution we will get to know it immediately
as there will be none response on sd_bus_get_property. In 1.) solution we
won't know that, thinking services are in active state.
Thanks in advance for your opinions.


2018-01-16 15:41 GMT+01:00 Tomasz Michalski <tmichalski23 at gmail.com>:

> Hi
> I am in the middle of implementation monitor for getting states from
> services. I have some question to my current implementation.
> My current implementation is:
>
> void main()
> {
> .....
>     r = sd_bus_add_match(&bus,
>                          NULL,
>                          "type='signal',"
>                          "sender='org.freedesktop.systemd1',"
>                          "interface='org.freedesktop.DBus.Properties',"
>                          "member='PropertiesChanged',"
>                          "arg0='org.freedesktop.systemd1.Unit'",
>                          propertiesChangedHandler,
>                          NULL); //user data*/
>     if (r < 0)
>     {
>         fprintf(stderr, "Failed to add match for PropertiesChanged",
> strerror(-r));
>         goto finish;
>     }
>
>     r = sd_bus_call_method(
>                      bus,
>                      "org.freedesktop.systemd1",
>                      "/org/freedesktop/systemd1",
>                      "org.freedesktop.systemd1.Manager",
>                      "Subscribe",
>                      &error,
>                      NULL,
>                      NULL);
>     if (r < 0)
>     {
>         fprintf(stderr, "Failed to enable subscription: %s\n", strerror(-r));
>         //TO DO use also error code
>         return r;
>     }
>
> ...
> }
>
> Handler:
> int propertiesChangedHandler(sd_bus_message *message, void *userdata,
> sd_bus_error *error)
> {
>     const char *data;
>     const char *path;
>     int r = 0;
>
>     if(!message)
>         printf("Empty message provided\n");
>
>     path = sd_bus_message_get_path(message);
>     if (!path)
>     {
>         printf("Empty message provided\n");
>         return 0;
>     }
>
>     r = sd_bus_message_read(message, "s", &data); //here problem with
> parsing ActiveState,SubState
>     if (r < 0)
>     {
>         fprintf(stderr, "Failed to parse response message: %s\n",
> strerror(-r));
>         return 0;
>     }
> }
>
>
> Question:
> How to narrow down filter? In the above examples the handler will be
> triggered by any property change. I am only interested in ActiveState,
> eventually SubState trigger. Should I add in sd_bus_add_match:
> "arg1='ActiveState'",
> "arg2='SubState'"
> ?
>
>
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