diff -r c7c34916027a -r 177826044cd9 wp/wp-includes/rest-api/endpoints/class-wp-rest-controller.php
--- a/wp/wp-includes/rest-api/endpoints/class-wp-rest-controller.php	Mon Oct 14 18:06:33 2019 +0200
+++ b/wp/wp-includes/rest-api/endpoints/class-wp-rest-controller.php	Mon Oct 14 18:28:13 2019 +0200
@@ -212,12 +212,7 @@
 
 		$data   = (array) $response->get_data();
 		$server = rest_get_server();
-
-		if ( method_exists( $server, 'get_compact_response_links' ) ) {
-			$links = call_user_func( array( $server, 'get_compact_response_links' ), $response );
-		} else {
-			$links = call_user_func( array( $server, 'get_response_links' ), $response );
-		}
+		$links  = $server->get_compact_response_links( $response );
 
 		if ( ! empty( $links ) ) {
 			$data['_links'] = $links;
@@ -305,29 +300,29 @@
 	 */
 	public function get_collection_params() {
 		return array(
-			'context'                => $this->get_context_param(),
-			'page'                   => array(
-				'description'        => __( 'Current page of the collection.' ),
-				'type'               => 'integer',
-				'default'            => 1,
-				'sanitize_callback'  => 'absint',
-				'validate_callback'  => 'rest_validate_request_arg',
-				'minimum'            => 1,
+			'context'  => $this->get_context_param(),
+			'page'     => array(
+				'description'       => __( 'Current page of the collection.' ),
+				'type'              => 'integer',
+				'default'           => 1,
+				'sanitize_callback' => 'absint',
+				'validate_callback' => 'rest_validate_request_arg',
+				'minimum'           => 1,
 			),
-			'per_page'               => array(
-				'description'        => __( 'Maximum number of items to be returned in result set.' ),
-				'type'               => 'integer',
-				'default'            => 10,
-				'minimum'            => 1,
-				'maximum'            => 100,
-				'sanitize_callback'  => 'absint',
-				'validate_callback'  => 'rest_validate_request_arg',
+			'per_page' => array(
+				'description'       => __( 'Maximum number of items to be returned in result set.' ),
+				'type'              => 'integer',
+				'default'           => 10,
+				'minimum'           => 1,
+				'maximum'           => 100,
+				'sanitize_callback' => 'absint',
+				'validate_callback' => 'rest_validate_request_arg',
 			),
-			'search'                 => array(
-				'description'        => __( 'Limit results to those matching a string.' ),
-				'type'               => 'string',
-				'sanitize_callback'  => 'sanitize_text_field',
-				'validate_callback'  => 'rest_validate_request_arg',
+			'search'   => array(
+				'description'       => __( 'Limit results to those matching a string.' ),
+				'type'              => 'string',
+				'sanitize_callback' => 'sanitize_text_field',
+				'validate_callback' => 'rest_validate_request_arg',
 			),
 		);
 	}
@@ -344,10 +339,10 @@
 	 */
 	public function get_context_param( $args = array() ) {
 		$param_details = array(
-			'description'        => __( 'Scope under which the request is made; determines fields present in response.' ),
-			'type'               => 'string',
-			'sanitize_callback'  => 'sanitize_key',
-			'validate_callback'  => 'rest_validate_request_arg',
+			'description'       => __( 'Scope under which the request is made; determines fields present in response.' ),
+			'type'              => 'string',
+			'sanitize_callback' => 'sanitize_key',
+			'validate_callback' => 'rest_validate_request_arg',
 		);
 
 		$schema = $this->get_item_schema();
@@ -385,12 +380,18 @@
 
 		$additional_fields = $this->get_additional_fields();
 
+		$requested_fields = $this->get_fields_for_response( $request );
+
 		foreach ( $additional_fields as $field_name => $field_options ) {
 
 			if ( ! $field_options['get_callback'] ) {
 				continue;
 			}
 
+			if ( ! in_array( $field_name, $requested_fields, true ) ) {
+				continue;
+			}
+
 			$object[ $field_name ] = call_user_func( $field_options['get_callback'], $object, $field_name, $request, $this->get_object_type() );
 		}
 
@@ -518,10 +519,20 @@
 	public function get_fields_for_response( $request ) {
 		$schema = $this->get_item_schema();
 		$fields = isset( $schema['properties'] ) ? array_keys( $schema['properties'] ) : array();
+
+		$additional_fields = $this->get_additional_fields();
+		foreach ( $additional_fields as $field_name => $field_options ) {
+			// For back-compat, include any field with an empty schema
+			// because it won't be present in $this->get_item_schema().
+			if ( is_null( $field_options['schema'] ) ) {
+				$fields[] = $field_name;
+			}
+		}
+
 		if ( ! isset( $request['_fields'] ) ) {
 			return $fields;
 		}
-		$requested_fields = is_array( $request['_fields'] ) ? $request['_fields'] : preg_split( '/[\s,]+/', $request['_fields'] );
+		$requested_fields = wp_parse_list( $request['_fields'] );
 		if ( 0 === count( $requested_fields ) ) {
 			return $fields;
 		}
@@ -585,7 +596,13 @@
 
 				// Only use required / default from arg_options on CREATABLE endpoints.
 				if ( WP_REST_Server::CREATABLE !== $method ) {
-					$params['arg_options'] = array_diff_key( $params['arg_options'], array( 'required' => '', 'default' => '' ) );
+					$params['arg_options'] = array_diff_key(
+						$params['arg_options'],
+						array(
+							'required' => '',
+							'default'  => '',
+						)
+					);
 				}
 
 				$endpoint_args[ $field_id ] = array_merge( $endpoint_args[ $field_id ], $params['arg_options'] );