client: Safe cast a "wl_object *" to "wl_proxy *"

Client message observers 4/6

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

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

This can be used by client message observers in particular to get the proxy id
and class name, for logging those details.

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

src/wayland-client-core.h

@@ -228,6 +228,9 @@ wl_proxy_set_queue(struct wl_proxy *proxy, struct wl_event_queue *queue);
 struct wl_event_queue *
 wl_proxy_get_queue(const struct wl_proxy *proxy);
 
+struct wl_proxy *
+wl_proxy_from_object(struct wl_object *object);
+
 const char *
 wl_event_queue_get_name(const struct wl_event_queue *queue);
 

src/wayland-client.c

@@ -2720,6 +2720,28 @@ wl_proxy_wrapper_destroy(void *proxy_wrapper)
 	free(wrapper);
 }
 
+/** Safely converts an object into its corresponding proxy
+ *
+ * \param object object to get the proxy for
+ * \return A corresponding proxy, or NULL on failure.
+ *
+ * Safely converts an object into its corresponding proxy.
+ *
+ * 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_proxy
+ */
+WL_EXPORT struct wl_proxy *
+wl_proxy_from_object(struct wl_object *object)
+{
+	struct wl_proxy *proxy;
+	if (object == NULL)
+		return NULL;
+	return wl_container_of(object, proxy, object);
+}
+
 WL_EXPORT void
 wl_log_set_handler_client(wl_log_func_t handler)
 {

tests/protocol-logger-test.c

@@ -294,10 +294,11 @@ client_log_to_stderr_demo(void *user_data, enum wl_client_message_type type,
 			break;
 		case 'o':
 			if (args[i].o) {
-				// Note: server logger should instead cast to
-				// wl_resource, and use wl_resource_get_class
-				// and wl_resource_get_id.
-				arg_proxy = (struct wl_proxy *)(args[i].o);
+				// Note: server logger should instead use
+				// wl_resource_from_object, and then
+				// wl_resource_get_class and
+				// wl_resource_get_id.
+				arg_proxy = wl_proxy_from_object(args[i].o);
 				arg_class = wl_proxy_get_class(arg_proxy);
 
 				fprintf(f, "%s#%u",