[PATCH wayland 3/3] server: Add a simple API to find a good default display

[Jason Ekstrand]

On Wed, May 7, 2014 at 9:25 AM, Jasper St. Pierre <jstpierre at mecheye.net>wrote:

> This allows compositors to easily select a good display to listen on.
> ---
>  src/wayland-server.c | 23 +++++++++++++++++++++++
>  src/wayland-server.h |  2 ++
>  2 files changed, 25 insertions(+)
>
> diff --git a/src/wayland-server.c b/src/wayland-server.c
> index 6bc8dc3..5624199 100644
> --- a/src/wayland-server.c
> +++ b/src/wayland-server.c
> @@ -1078,6 +1078,29 @@ open_socket_for_display_name(struct wl_socket *s,
> const char *name)
>         return 0;
>  }
>
> +WL_EXPORT char *
> +wl_choose_default_display(void)
>

I mentioned this on IRC, but this seems like an awkward name.  Perhaps
something like wl_display_choose_default_name?  See also below.


> +{
> +       struct wl_socket s = { 0 };
> +       int displayno = 0;
> +       char display_name[16] = "";
> +
> +       /* A reasonable number of maximum default sockets. If
> +        * you need more than this, set WAYLAND_DISPLAY explicitly. */
> +       const int MAX_DISPLAYNO = 32;
> +
> +       do {
> +               snprintf(display_name, sizeof display_name, "wayland-%d",
> displayno);
> +               if (open_socket_for_display_name(&s, display_name) >= 0) {
> +                       close(s->fd_lock);
> +                       return strdup(display_name);
>

We have a race condition here.  If two compositors start up at the same
time, it's possible that wl_choose_default_display will return "wayland-1"
but then, before we get a chance to actually re-lock and connect to it,
some other compositor opens it.  Honestly, this probably isn't going to be
a huge problem in practice.  However, we may want to re-think the API a bit.

One option would be to do the enumeration in wl_display_add_socket if the
given name is NULL.  This is a small API break, but all it changes is "if
name==NULL and  WAYLAND_DISPLAY is not set and wayland-0 is taken, fail" to
"if name==NULL and WAYLAND_DISPLAY is not set and wayland-0 is taken, try
wayland-1 etc.".  It slightly changes default behavior, but I don't really
see why failing on wayland-0 being taken is a good thing.

Thanks,
--Jason Ekstrand


> +               }
> +       } while (displayno++ < MAX_DISPLAYNO);
> +
> +       errno = EINVAL;
> +       return NULL;
> +}
> +
>  WL_EXPORT int
>  wl_display_add_socket(struct wl_display *display, const char *name)
>  {
> diff --git a/src/wayland-server.h b/src/wayland-server.h
> index 7fc5b47..c9834f1 100644
> --- a/src/wayland-server.h
> +++ b/src/wayland-server.h
> @@ -88,6 +88,8 @@ struct wl_listener *wl_event_loop_get_destroy_listener(
>                                         struct wl_event_loop *loop,
>                                         wl_notify_func_t notify);
>
> +char *wl_choose_default_socket(void);
> +
>  struct wl_display *wl_display_create(void);
>  void wl_display_destroy(struct wl_display *display);
>  struct wl_event_loop *wl_display_get_event_loop(struct wl_display
> *display);
> --
> 1.9.0
>
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>
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