wayland-server: Add API to control globals visibility

Add a new API to let compositor decide whether or not a wl_global
should be advertised to the clients via wl_registry_bind() or
display_get_registry()

By using its own filter, the compositor can decide which wl_global would
be listed to clients.

Compositors can use this mechanism to hide their own private interfaces
that regular clients should not use.

- Hiding interfaces that expose compositor implementation details
  makes it harder for clients to identify the compositor. Therefore
  clients are a little less likely to develop compositor-specific
  workarounds instead of reporting problems upstream.

- Hiding can be used to diminish the problems from missing namespacing:
  if two compositors happen to use the same named global with
  different interfaces for their special-purpose clients, the client
  expecting the different interface would probably never see it
  advertised.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>

src/wayland-server-core.h

@@ -28,6 +28,7 @@
 
 #include <sys/types.h>
 #include <stdint.h>
+#include <stdbool.h>
 #include "wayland-util.h"
 #include "wayland-version.h"
 
@@ -168,6 +169,30 @@ wl_global_create(struct wl_display *display,
 void
 wl_global_destroy(struct wl_global *global);
 
+/** A filter function for wl_global objects
+ *
+ * \param client The client object
+ * \param global The global object to show or hide
+ * \param data   The user data pointer
+ *
+ * A filter function enables the server to decide which globals to
+ * advertise to each client.
+ *
+ * When a wl_global filter is set, the given callback funtion will be
+ * called during wl_global advertisment and binding.
+ *
+ * This function should return true if the global object should be made
+ * visible to the client or false otherwise.
+ */
+typedef bool (*wl_display_global_filter_func_t)(const struct wl_client *client,
+						const struct wl_global *global,
+						void *data);
+
+void
+wl_display_set_global_filter(struct wl_display *display,
+			     wl_display_global_filter_func_t filter,
+			     void *data);
+
 struct wl_client *
 wl_client_create(struct wl_display *display, int fd);