REST API: Encourage proper usage of `register_rest_route()`.

Calling `register_rest_route()` too early in the loading process has the potential to cause some unintentional problems and pitfalls. Because `register_rest_route()` calls `rest_get_server()` (which creates the `WP_REST_Server` instance), calling the function directly and/or before `rest_api_init` should be discouraged.

For example, if `register_rest_route ()` is called on `init`, the REST API server instance is set up (and all functions added to `rest_api_init` and other related hooks are invoked), even though the current request may not be a REST request. Also, if `register_rest_route()` is called even earlier (say, in an `mu-plugin` file), required endpoints may be missing since normal plugins have not yet been loaded and have not had a chance to register their own action hooks.

This adds a `_doing_it_wrong()` notice the first time `register_rest_route()` is called before `rest_api_init` in a request to encourage best practices for registering REST API routes.

Props kraftbj, desrosj, timothyblynjacobs.
Fixes #45265.
Built from https://develop.svn.wordpress.org/trunk@44568


git-svn-id: http://core.svn.wordpress.org/trunk@44399 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api.php

@@ -17,7 +17,10 @@ define( 'REST_API_VERSION', '2.0' );
 /**
  * Registers a REST API route.
  *
+ * Note: Do not use before the {@see 'rest_api_init'} hook.
+ *
  * @since 4.4.0
+ * @since 5.1.0 Added a _doing_it_wrong() notice when not called on or after the rest_api_init hook.
  *
  * @param string $namespace The first URL segment after core prefix. Should be unique to your package/plugin.
  * @param string $route     The base URL for route you are adding.
@@ -41,6 +44,10 @@ function register_rest_route( $namespace, $route, $args = array(), $override = f
 		return false;
 	}
 
+	if ( ! did_action( 'rest_api_init' ) ) {
+		_doing_it_wrong( 'register_rest_route', __( 'REST API routes must be registered on the rest_api_init action.' ), '5.1.0' );
+	}
+
 	if ( isset( $args['args'] ) ) {
 		$common_args = $args['args'];
 		unset( $args['args'] );

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-beta1-44567';
+$wp_version = '5.1-beta1-44568';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.