[systemd-devel] Why systemd keeps references on passed sockets ?

[Lennart Poettering]

On Di, 08.09.20 17:35, Francis Moreau (francis.moro at gmail.com) wrote:

> On Mon, Sep 7, 2020 at 4:38 PM Lennart Poettering
> <lennart at poettering.net> wrote:
> >
> > "React on the socket close?" — What do you mean by that?
> >
>
> I mean if my service explicitly calls close() then systemd could stop
> the socket on its side so its are freed until the service is
> restarted. I think it is what you described below.

close() just drops a ref to the socket. Only when close() drops the
last ref something actually happens on the socket and it is
destroyed. This means: systemd doesn't get notified about your code
invoking close(), because all you did is drop one ref of many.

> > Note that on Linux you can invoke shutdown() on a listening socket
> > (i.e. not just on the connection socket, but on a listening
> > socket). iirc in that case systemd actually notices and will put the
> > .socket unit in failure mode...
> >
>
> I looked at the code and there is:
>
>     if (state != SOCKET_LISTENING)
>         socket_unwatch_fds(s);
>
> So I'm not sure how systemd can react on shutdown(). And I tried to
> call shutdown() in my service but it has no effects.

Hmm, that suggests we'd have to slightly update our logic then for
this to work: keep the listening fds in the poll, but turn off all
the EPOLLIN bit we listen on.

> > Would that work for you? (Maybe we could even tweak this a bit in
> > systemd, so that when you invoke shutdown() on the socket systemd
> > holds for you we do not consider that a failure anymore, but a clean
> > way to tell systemd to stop the socket).
>
> That is a good idea especially if systemd doesn't consider an error
> when the service closes or shutdown the socket.
>
> But again in the code I can see:
>
>   sd_event_add_io(UNIT(s)->manager->event, &p->event_source, p->fd,
> EPOLLIN, socket_dispatch_io, p);
>
> It seems that it only listens to "EPOLLIN" events. So it doesn't
> listen to "EPOLLUP".

EPOLLHUP is always implied, you don't have to specify it. if you
specifiy zero as mask you will still get notified about EPOLLHUP +
EPOLLERR.

But you are right we currently remove the fd from polling while the
socket's service is running. We'd have to change that (as mentioned
above). happy to review a PR for that.

Lennart

--
Lennart Poettering, Berlin