remote-follow endpoint (#392)

Adds an endpoint at `users/$user_id/follow-me` to return the follow template for a remote user, to enable following them more easily.

includes/class-webfinger.php

@@ -62,7 +62,7 @@ class Webfinger {
 		$response = \wp_remote_get(
 			$url,
 			array(
-				'headers' => array( 'Accept' => 'application/activity+json' ),
+				'headers' => array( 'Accept' => 'application/jrd+json' ),
 				'redirection' => 0,
 				'timeout' => 2,
 			)
@@ -94,4 +94,110 @@ class Webfinger {
 		\set_transient( $transient_key, $link, HOUR_IN_SECONDS ); // Cache the error for a shorter period.
 		return $link;
 	}
+
+	/**
+	 * Convert a URI string to an identifier and its host.
+	 * Automatically adds acct: if it's missing.
+	 *
+	 * @param string $url The URI (acct:, mailto:, http:, https:)
+	 *
+	 * @return WP_Error|array Error reaction or array with
+	 *                        identifier and host as values
+	 */
+	public static function get_identifier_and_host( $url ) {
+		// remove leading @
+		$url = ltrim( $url, '@' );
+
+		if ( ! preg_match( '/^([a-zA-Z+]+):/', $url, $match ) ) {
+			$identifier = 'acct:' . $url;
+			$scheme = 'acct';
+		} else {
+			$identifier = $url;
+			$scheme = $match[1];
+		}
+
+		$host = null;
+
+		switch ( $scheme ) {
+			case 'acct':
+			case 'mailto':
+			case 'xmpp':
+				if ( strpos( $identifier, '@' ) !== false ) {
+					$host = substr( $identifier, strpos( $identifier, '@' ) + 1 );
+				}
+				break;
+			default:
+				$host = wp_parse_url( $identifier, PHP_URL_HOST );
+				break;
+		}
+
+		if ( empty( $host ) ) {
+			return new WP_Error( 'invalid_identifier', __( 'Invalid Identifier', 'activitypub' ) );
+		}
+
+		return array( $identifier, $host );
+	}
+
+	/**
+	 * Get the WebFinger data for a given URI
+	 *
+	 * @param string $identifier The Identifier: <identifier>@<host>
+	 * @param string $host       The Host: <identifier>@<host>
+	 *
+	 * @return WP_Error|array Error reaction or array with
+	 *                        identifier and host as values
+	 */
+	public static function get_data( $identifier, $host ) {
+		$webfinger_url = 'https://' . $host . '/.well-known/webfinger?resource=' . rawurlencode( $identifier );
+
+		$response = wp_safe_remote_get(
+			$webfinger_url,
+			array(
+				'headers' => array( 'Accept' => 'application/jrd+json' ),
+				'redirection' => 0,
+				'timeout' => 2,
+			)
+		);
+
+		if ( is_wp_error( $response ) ) {
+			return new WP_Error( 'webfinger_url_not_accessible', null, $webfinger_url );
+		}
+
+		$body = wp_remote_retrieve_body( $response );
+
+		return json_decode( $body, true );
+	}
+
+	/**
+	 * Undocumented function
+	 *
+	 * @return void
+	 */
+	public static function get_remote_follow_endpoint( $uri ) {
+		$identifier_and_host = self::get_identifier_and_host( $uri );
+
+		if ( is_wp_error( $identifier_and_host ) ) {
+			return $identifier_and_host;
+		}
+
+		list( $identifier, $host ) = $identifier_and_host;
+
+		$data = self::get_data( $identifier, $host );
+
+		if ( is_wp_error( $data ) ) {
+			return $data;
+		}
+
+		if ( empty( $data['links'] ) ) {
+			return new WP_Error( 'webfinger_url_invalid_response', null, $data );
+		}
+
+		foreach ( $data['links'] as $link ) {
+			if ( 'http://ostatus.org/schema/1.0/subscribe' === $link['rel'] ) {
+				return $link['template'];
+			}
+		}
+
+		return new WP_Error( 'webfinger_remote_follow_endpoint_invalid', null, $data );
+	}
 }

includes/rest/class-users.php

@@ -3,7 +3,9 @@ namespace Activitypub\Rest;
 
 use WP_Error;
 use WP_REST_Server;
+use WP_REST_Request;
 use WP_REST_Response;
+use Activitypub\Webfinger;
 use Activitypub\Activity\Activity;
 use Activitypub\Collection\Users as User_Collection;
 
@@ -40,6 +42,25 @@ class Users {
 				),
 			)
 		);
+
+		\register_rest_route(
+			ACTIVITYPUB_REST_NAMESPACE,
+			'/users/(?P<user_id>[\w\-\.]+)/remote-follow',
+			array(
+				array(
+					'methods'             => WP_REST_Server::READABLE,
+					'callback'            => array( self::class, 'remote_follow_get' ),
+
+					'args'                => array(
+						'resource' => array(
+							'required'          => true,
+							'sanitize_callback' => 'sanitize_text_field',
+						),
+					),
+					'permission_callback' => '__return_true',
+				),
+			)
+		);
 	}
 
 	/**
@@ -80,6 +101,38 @@ class Users {
 		return $response;
 	}
 
+
+	/**
+	 * Endpoint for remote follow UI/Block
+	 *
+	 * @param WP_REST_Request $request The request object.
+	 *
+	 * @return void|string The URL to the remote follow page
+	 */
+	public static function remote_follow_get( WP_REST_Request $request ) {
+		$resource = $request->get_param( 'resource' );
+		$user_id  = $request->get_param( 'user_id' );
+		$user     = User_Collection::get_by_various( $user_id );
+
+		if ( is_wp_error( $user ) ) {
+			return $user;
+		}
+
+		$template = Webfinger::get_remote_follow_endpoint( $resource );
+
+		if ( is_wp_error( $template ) ) {
+			return $template;
+		}
+
+		$resource = $user->get_resource();
+		$url      = str_replace( '{uri}', $resource, $template );
+
+		return new WP_REST_Response(
+			array( 'url' => $url ),
+			200
+		);
+	}
+
 	/**
 	 * The supported parameters
 	 *