REST API: Let routes turn off _fields, _embed and response links

src/wp-includes/rest-api.php

@@ -950,6 +950,8 @@ function _rest_array_intersect_key_recursive( $array1, $array2 ) {
  * Filters the REST API response to include only an allow-listed set of response object fields.
  *
  * @since 4.8.0
+ * @since 7.2.0 The response is left unchanged when the route turns off the `fields` response
+ *              processing or when the response data is not an array.
  *
  * @param WP_REST_Response $response Current response being served.
  * @param WP_REST_Server   $server   ResponseHandler instance (usually WP_REST_Server).
@@ -961,8 +963,18 @@ function rest_filter_response_fields( $response, $server, $request ) {
 		return $response;
 	}
 
+	$processing = $response->get_data_processing();
+	if ( ! $processing['fields'] ) {
+		return $response;
+	}
+
 	$data = $response->get_data();
 
+	// Only arrays have fields to filter.
+	if ( ! is_array( $data ) ) {
+		return $response;
+	}
+
 	$fields = wp_parse_list( $request['_fields'] );
 
 	if ( 0 === count( $fields ) ) {
@@ -993,7 +1005,7 @@ function rest_filter_response_fields( $response, $server, $request ) {
 	if ( wp_is_numeric_array( $data ) ) {
 		$new_data = array();
 		foreach ( $data as $item ) {
-			$new_data[] = _rest_array_intersect_key_recursive( $item, $fields_as_keyed );
+			$new_data[] = is_array( $item ) ? _rest_array_intersect_key_recursive( $item, $fields_as_keyed ) : $item;
 		}
 	} else {
 		$new_data = _rest_array_intersect_key_recursive( $data, $fields_as_keyed );
@@ -3048,7 +3060,7 @@ function rest_preload_api_request( $memo, $path ) {
 		/** This filter is documented in wp-includes/rest-api/class-wp-rest-server.php */
 		$response = apply_filters( 'rest_post_dispatch', rest_ensure_response( $response ), $server, $request );
 		$embed    = $request->has_param( '_embed' ) ? rest_parse_embed_param( $request['_embed'] ) : false;
-		$data     = (array) $server->response_to_data( $response, $embed );
+		$data     = $server->response_to_data( $response, $embed );
 
 		if ( 'OPTIONS' === $method ) {
 			$memo[ $method ][ $path ] = array(

src/wp-includes/rest-api/class-wp-rest-response.php

@@ -15,6 +15,7 @@
  * @see WP_HTTP_Response
  *
  * @phpstan-import-type Route_Handler from WP_REST_Server
+ * @phpstan-import-type Data_Processing from WP_REST_Server
  *
  * @phpstan-type Target_Hints array{
  *     allow?: list<uppercase-string>,
@@ -69,6 +70,19 @@ class WP_REST_Response extends WP_HTTP_Response {
 	 */
 	protected $matched_handler = null;
 
+	/**
+	 * Whether each data processing feature is on for the response.
+	 *
+	 * @since 7.2.0
+	 * @var array<string, bool> Whether the `fields`, `embed` and `links` features are on. Every feature is on by default.
+	 * @phpstan-var Data_Processing
+	 */
+	protected $data_processing = array(
+		'fields' => true,
+		'embed'  => true,
+		'links'  => true,
+	);
+
 	/**
 	 * Adds a link to the response.
 	 *
@@ -249,6 +263,50 @@ public function set_matched_handler( $handler ) {
 		$this->matched_handler = $handler;
 	}
 
+	/**
+	 * Retrieves whether each data processing feature is on for the response.
+	 *
+	 * The server sets the settings of the matched route when it dispatches a request, for responses
+	 * returned by the `rest_pre_dispatch` filter too. Every feature is on until then.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @see WP_REST_Server::resolve_data_processing()
+	 *
+	 * @return array<string, bool> {
+	 *     Whether each data processing feature is on.
+	 *
+	 *     @type bool $fields Whether the `_fields` parameter filters the response data.
+	 *     @type bool $embed  Whether the `_embed` parameter embeds linked resources in the response data.
+	 *     @type bool $links  Whether the response links are added to the response data as `_links`.
+	 * }
+	 *
+	 * @phpstan-return Data_Processing
+	 */
+	public function get_data_processing() {
+		return $this->data_processing;
+	}
+
+	/**
+	 * Sets whether each data processing feature is on for the response.
+	 *
+	 * The setting takes the form of the `data_processing` route option: `false` turns off every
+	 * feature, and an array turns off, or on, the features it names. Embedding needs the links, so
+	 * `embed` is off whenever `links` is off.
+	 *
+	 * @since 7.2.0
+	 *
+	 * @see WP_REST_Server::resolve_data_processing()
+	 *
+	 * @param bool|array<string, bool> $processing Whether every feature is on, or whether the `fields`, `embed`
+	 *                                             and `links` features are on. A feature not named stays on.
+	 *
+	 * @phpstan-param bool|array{fields?: bool, embed?: bool, links?: bool} $processing
+	 */
+	public function set_data_processing( $processing ) {
+		$this->data_processing = WP_REST_Server::resolve_data_processing( array( 'data_processing' => $processing ) );
+	}
+
 	/**
 	 * Checks if the response is an error, i.e. >= 400 response code.
 	 *