REST API: Serve response data that is not an array as returned.

`rest_filter_response_fields()` leaves the response unchanged when its
data is not an array, and keeps list items that are not arrays.
`WP_REST_Server::response_to_data()` returns scalar and object data as
it is instead of adding links to it. Routes that return such data no
longer need the `response_processing` option to avoid type errors.

See #66267.

Author: jorgefilipecosta

src/wp-includes/rest-api.php

@@ -950,7 +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.
+ * @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).
@@ -972,6 +973,11 @@ function rest_filter_response_fields( $response, $server, $request ) {
 
 	$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 ) ) {
@@ -1002,7 +1008,7 @@ 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 );

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

@@ -624,22 +624,25 @@ public function serve_request( $path = null ) {
 	 * @since 4.4.0
 	 * @since 5.4.0 The `$embed` parameter can now contain a list of link relations to include.
 	 * @since 7.2.0 Added the `$request` parameter. The links are not added or embedded when the
-	 *              route turns off the `links` or `embed` response processing.
+	 *              route turns off the `links` or `embed` response processing, and data that is
+	 *              not an array is returned as it is.
 	 *
 	 * @param WP_REST_Response     $response Response object.
 	 * @param bool|string[]        $embed    Whether to embed all links, a filtered list of link relations, or no links.
 	 * @param WP_REST_Request|null $request  Optional. The request that generated the response. It resolves the
 	 *                                       response processing settings when the response has no matched handler.
 	 *                                       Default null.
-	 * @return array {
-	 *     Data with sub-requests embedded.
-	 *
-	 *     @type array $_links    Links.
-	 *     @type array $_embedded Embedded objects.
-	 * }
+	 * @return mixed Data with sub-requests embedded: an array with the links under `_links` and the embedded
+	 *               resources under `_embedded`, or data that is not an array as it is.
 	 */
 	public function response_to_data( $response, $embed, $request = null ) {
-		$data       = $response->get_data();
+		$data = $response->get_data();
+
+		// Scalar and object data has no place for links or embedded resources.
+		if ( ! is_array( $data ) && null !== $data ) {
+			return $data;
+		}
+
 		$processing = $this->get_response_processing( $request, $response );
 		$links      = $processing['links'] ? self::get_compact_response_links( $response ) : array();
 

tests/phpunit/tests/rest-api/rest-server.php

@@ -3475,4 +3475,38 @@ public function test_response_processing_is_listed_in_the_index(): void {
 		);
 		$this->assertArrayNotHasKey( 'response_processing', $data['endpoints'][1], 'An endpoint with every feature on should not list the settings.' );
 	}
+
+	/**
+	 * Data provider for test_non_array_data_is_served_as_returned_by_default().
+	 *
+	 * @return array<string, array{0: mixed, 1: bool, 2: string}>
+	 */
+	public static function data_non_array_data_is_served_as_returned_by_default(): array {
+		return array(
+			'string'          => array( 'value', true, '"value"' ),
+			'integer'         => array( 5, true, '5' ),
+			'object'          => array( (object) array( 'b' => 2 ), true, '{"b":2}' ),
+			'empty object'    => array( new stdClass(), true, '{}' ),
+			'list of scalars' => array( array( 'a', 'b' ), false, '["a","b"]' ),
+			'mixed list'      => array( array( array( 'b' => 2 ), 'c' ), false, '[[],"c"]' ),
+		);
+	}
+
+	/**
+	 * @ticket 66267
+	 * @dataProvider data_non_array_data_is_served_as_returned_by_default
+	 *
+	 * @param mixed  $data      The response data.
+	 * @param bool   $with_link Whether the response has an embeddable link.
+	 * @param string $expected  The expected response body.
+	 */
+	public function test_non_array_data_is_served_as_returned_by_default( $data, bool $with_link, string $expected ): void {
+		$this->register_response_processing_route( $data, $with_link );
+
+		$_GET['_fields'] = 'a';
+		$_GET['_embed']  = '1';
+		rest_get_server()->serve_request( '/test-ns/response-processing' );
+
+		$this->assertSame( $expected, rest_get_server()->sent_body, 'Data without fields, or items without fields, should be served as returned.' );
+	}
 }