[systemd-devel] sd_event_run

[Zbigniew Jędrzejewski-Szmek]

On Fri, Mar 13, 2015 at 06:10:17PM +0100, Tom Gundersen wrote:
> On Fri, Mar 13, 2015 at 4:35 PM, Zbigniew Jędrzejewski-Szmek
> <zbyszek at in.waw.pl> wrote:
> > you added sd_event_run a while ago:
> >
> > commit c45a5a74465a39280b855f9d720b2ab4779a47fa
> > Author: Tom Gundersen <teg at jklm.no>
> > Date:   Fri Aug 15 18:49:29 2014 +0200
> >
> >     sd-event: split run into prepare/wait/dispatch
> >
> >     This will allow sd-event to be integrated into an external event loop, which
> >     in turn will allow (say) glib-based applications to use our various libraries,
> >     without manually integrating each of them (bus, rtnl, dhcp, ...).
> >
> >     The external event-loop should integrate sd-event int he following way:
> >
> >     Every iteration must start with a call to sd_event_prepare(), which will
> >     return 0 if no event sources are ready to be processed, a positive value if
> >     they are and a negative value on error. sd_event_prepare() may only be called
> >     following sd_event_dispatch(); a call to sd_event_wait() indicating that no
> >     sources are ready to be dispatched; or a failed call to sd_event_dispatch() or
> >     sd_event_wait().
> >
> >     A successful call to sd_event_prepare() indicating that no event sources are
> >     ready to be dispatched must be followed by a call to sd_event_wait(),
> >     which will return 0 if it timed out without event sources being ready to
> >     be processed, a negative value on error and a positive value otherwise.
> >     sd_event_wait() may only be called following a successful call to
> >     sd_event_prepare() indicating that no event sources are ready to be dispatched.
> >
> >     If sd_event_wait() indicates that some events sources are ready to be
> >     dispatched, it must be followed by a call to sd_event_dispatch(). This
> >     is the only time sd_event_dispatch() may be called.
> >
> > +_public_ int sd_event_run(sd_event *e, uint64_t timeout) {
> > +        int r;
> > +
> > +        assert_return(e, -EINVAL);
> > +        assert_return(!event_pid_changed(e), -ECHILD);
> > +        assert_return(e->state != SD_EVENT_FINISHED, -ESTALE);
> > +        assert_return(e->state == SD_EVENT_PASSIVE, -EBUSY);
> > +
> > +        r = sd_event_prepare(e);
> > +        if (r > 0)
> > +                return sd_event_dispatch(e);
> > +        else if (r < 0)
> > +                return r;
> > +
> > +        r = sd_event_wait(e, timeout);
> > +        if (r > 0)
> > +                return sd_event_dispatch(e);
> > +        else
> > +                return r;
> > +}
> >
> > Your commit description is almost ready to be turned into a man page, but there
> > a hiccup. According to the last paragraph of the commit message, sd_event_dispatch may
> > only be called after sd_event_wait(). This contradict the code in sd_event_run().
> > (sd_event_dispatch calls sd_event_wait internally, but the user does not know
> > this). Can you clarify the intended rules?
> 
> Indeed. Perhaps the best way to explain it is to look at the states:
> 
>         SD_EVENT_PASSIVE,
>         SD_EVENT_PREPARED,
>         SD_EVENT_PENDING,
>         SD_EVENT_FINISHED,
There's also SD_EVENT_RUNNING. It probably should be documented too,
but I didn't include it in the description of
sd_event_{run,wait,prepare,dispatch,loop}.

A draft is attached, please have a look. After writing it I had some thoughts:
1. shouldn't SD_EVENT_PASSIVE become SD_EVENT_INITIAL? "passive" seems strange
   in this context.
   Similarly, SD_EVENT_ARMED seems more self-explanatory than PREPARED.
   (I don't like PREPARED because it is not obvious whether sources are
   prepared to wait on, or events are prepared to be reaped.)

2. shouldn't sd_event_dispatch also return 0/1 to signify e.g.
   loop-continues / loop-finished. Right now other functions return
   the state as return value, but sd_event_dispatch requires querying
   the event object for state.

Zbyszek
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