[PATCH] event-loop: add wl_event_loop_has_idle() helper

[Kristian Høgsberg]

On Sun, Sep 09, 2012 at 04:02:45PM +0200, David Herrmann wrote:
> As event-loop uses epoll() as base object we can stick one event-loop into
> another by retrieving the epoll-fd and watching for events on it. However,
> the idle objects aren't based on file-descriptors so if one event-callback
> adds a new idle-source, the possible parent event-loop isn't notified of
> this and will probably go to sleep instead of calling
> wl_event_loop_dispatch() again in the next dispatch-round.
> 
> This wl_event_loop_has_idle() helper can be used to avoid this starvation.
> The parent event loop simply calls this before going to sleep. If it is
> true, it sets the timeout to 0, calls its own events and then calls
> wl_event_loop_dispatch() to dispatch the wayland-event-loop idlers.

Can we just expose dispatch_idle_sources as
wl_event_loop_dispatch_idles() and the you can call that before going
to sleep?

Kristian

> We could also add an "eventfd(2)" file descriptor internally to the
> event-loop and write to it, when an idle source is active, and read from
> it when no idle-source is active, anymore. This guarantees, that this
> eventfd is readable as long as an idle-source is registered. So the parent
> event-loop is notified that the epoll-fd is readable.
> However, on worst case this adds one syscall per idle-source registration
> and one per idle-source execution which can be quite heavy if used
> agressively. So the wl_event_loop_has_idle() approach is the less
> aggressive solution to this.
> 
> This, of course, is only used when stacking an wl_display/event-loop
> object into your own event loop. Weston doesn't need this but other
> compositors might want to stick to their own event-loop implementation
> instead of using "event-loop" for their whole application.
> 
> I used _Bool as I don't know whether stdbool.h is actually compatible with
> all the X-headers and legacy stuff where wayland may be used.
> 
> Signed-off-by: David Herrmann <dh.herrmann at googlemail.com>
> ---
>  src/event-loop.c     | 6 ++++++
>  src/wayland-server.h | 1 +
>  2 files changed, 7 insertions(+)
> 
> diff --git a/src/event-loop.c b/src/event-loop.c
> index a839daf..d0603aa 100644
> --- a/src/event-loop.c
> +++ b/src/event-loop.c
> @@ -424,3 +424,9 @@ wl_event_loop_get_fd(struct wl_event_loop *loop)
>  {
>  	return loop->epoll_fd;
>  }
> +
> +WL_EXPORT _Bool
> +wl_event_loop_has_idle(struct wl_event_loop *loop)
> +{
> +	return !wl_list_empty(&loop->idle_list);
> +}
> diff --git a/src/wayland-server.h b/src/wayland-server.h
> index cd79801..83b6635 100644
> --- a/src/wayland-server.h
> +++ b/src/wayland-server.h
> @@ -71,6 +71,7 @@ struct wl_event_source *wl_event_loop_add_idle(struct wl_event_loop *loop,
>  					       wl_event_loop_idle_func_t func,
>  					       void *data);
>  int wl_event_loop_get_fd(struct wl_event_loop *loop);
> +_Bool wl_event_loop_has_idle(struct wl_event_loop *loop);
>  
>  struct wl_client;
>  struct wl_display;
> -- 
> 1.7.12
>