server: Safe cast a "wl_object *" to "wl_resource *"

Client message observers 5/6

When given an array of wl_arguments for a wl_closure, the .o field is an
opaque wl_object pointer, which the server implementation cannot really do
anything with, without a potentially unsafe cast that assumes details about the
internal implementation.

By adding a wl_resource_from_object() function to the client interface, the client
can safely get the wl_resource pointer.

This can be used by server protocol loggers in particular to get the resource id
and class name, for logging those details

Signed-off-by: Lloyd Pique <lpique@google.com>

src/wayland-server-core.h

@@ -628,6 +628,9 @@ struct wl_listener *
 wl_resource_get_destroy_listener(struct wl_resource *resource,
 				 wl_notify_func_t notify);
 
+struct wl_resource *
+wl_resource_from_object(struct wl_object *object);
+
 #define wl_resource_for_each(resource, list)					\
 	for (resource = 0, resource = wl_resource_from_link((list)->next);	\
 	     wl_resource_get_link(resource) != (list);				\

src/wayland-server.c

@@ -905,6 +905,28 @@ wl_resource_get_class(struct wl_resource *resource)
 	return resource->object.interface->name;
 }
 
+/** Safely converts an object into its corresponding resource
+ *
+ * \param object object to get the resource for
+ * \return A corresponding resource, or NULL on failure
+ *
+ * Safely converts an object into its corresponding resource.
+ *
+ * This is useful for implementing functions that are given a \c wl_argument
+ * array, and that need to do further introspection on the ".o" field, as it
+ * is otherwise an opaque type.
+ *
+ * \memberof wl_resource
+ */
+WL_EXPORT struct wl_resource *
+wl_resource_from_object(struct wl_object *object)
+{
+	struct wl_resource *resource;
+	if (object == NULL)
+		return NULL;
+	return wl_container_of(object, resource, object);
+}
+
 /**
  * Add a listener to be called at the beginning of wl_client destruction
  *