[systemd-devel] sd-varlink: how to loop over connections?

[Lennart Poettering]

On Mi, 20.11.24 14:51, Thorsten Kukuk (kukuk at suse.com) wrote:

> Hi,
>
> I'm currently playing with sd-varlink and trying to replace dbus in
> one of our daemon with sd-varlink. Not that trivial without
> documentation...
>
> My currently biggest question I couldn't solve is: how should a daemon
> listen to connections?

What kind of event loop are you using inside your daemon? Is this a
continously running singleton daemon that shall respond to many
varlink connections simultaneously?

> There is "sd_varlink_server_loop_auto()", but this exits after the
> last connection is closed.

Yeah, that has a different usecase.

> This is nice if you start the daemon via
> systemd socket activation or something similar and the daemon should
> quit afterwards, but I need an always running daemon.
> And you cannot call that in a loop ("Failed to run Varlink event loop:
> Device or resource busy").

Yeah, once that event loop is done it's done.

> Is the expectation to setup an own event loop without "exit_on_idle"?
> Wouldn't it be better if sd_varlink_server_loop_auto() get's an
> argument, if it should exit on idle or not? Or do I miss something?

So individual connections you can integrate into any event loop you
like relatively easily, it works roughly the same way as for sd-bus,
i.e. every time you enter poll() you query the varlink connection via
sd_varlink_get_fd(), sd_varlink_get_events(), sd_varlink_get_timeout()
for the parameters to pass to poll() on. And afterwards you invoke
sd_varlink_process(). The docs for sd_bus_get_fd() should explain this
to some degree.

I guess your use case is not just individual connections though: you
want a full server. For that we currently do not provide such an API
(because we have many fds to poll on, not just one). For that you can
allocate an event loop object via sd_event_new(), and attach the
server to it. You then can either run the event loop directly, or you
can integrate that even loop into your own event loop of choice
relatively easily (that works becuase sd-event is epoll internally,
which itself provides an fd you can poll on). See the
sd_event_get_fd() man page for details on that. There's even example
there that shows how to use that to glue this thing into glib's event
loop.

So you basically can use sd_varlink_server_loop_auto() as blue print
for this, but you drop the exit on idle stuff, and instead of the
sd_event_loop() at the end you do what the sd_event_get_fd() man page
explains.

Lennart

--
Lennart Poettering, Berlin