ActivityPub Relay Mode - Standalone plugin to add relay functionality to WordPress ActivityPub

activitypub-relay-mode.php

<?php
/**
 * Plugin Name: ActivityPub Relay Mode
 * Description: Adds relay mode to the ActivityPub plugin to forward public activities to all followers.
 * Version: 1.0.0
 * Author: Automattic
 * License: GPL-2.0-or-later
 * Requires Plugins: activitypub
 */

namespace Activitypub_Relay_Mode;

use Activitypub\Activity\Activity;
use Activitypub\Collection\Actors;
use Activitypub\Collection\Outbox;

use function Activitypub\is_activity_public;
use function Activitypub\is_single_user;

defined( 'ABSPATH' ) || exit;

/**
 * Initialize the plugin.
 */
function init() {
	// Register setting and handle option changes.
	add_action( 'admin_init', __NAMESPACE__ . '\register_setting' );
	add_action( 'update_option_activitypub_relay_mode', __NAMESPACE__ . '\relay_mode_changed', 10, 2 );

	// Only load relay functionality if relay mode is enabled.
	if ( get_option( 'activitypub_relay_mode', false ) ) {
		add_action( 'activitypub_handled_create', __NAMESPACE__ . '\handle_activity', 10, 3 );
		add_action( 'activitypub_handled_update', __NAMESPACE__ . '\handle_activity', 10, 3 );
		add_action( 'activitypub_handled_delete', __NAMESPACE__ . '\handle_activity', 10, 3 );
		add_action( 'activitypub_handled_announce', __NAMESPACE__ . '\handle_activity', 10, 3 );
		add_action( 'load-settings_page_activitypub', __NAMESPACE__ . '\unhook_settings_fields', 11 );
		add_filter( 'activitypub_blog_actor_type', __NAMESPACE__ . '\set_actor_type_service' );
	}
}
add_action( 'plugins_loaded', __NAMESPACE__ . '\init' );

/**
 * Set the blog actor type to Service when relay mode is enabled.
 *
 * @param string $type The actor type.
 * @return string The modified actor type.
 */
function set_actor_type_service( $type ) {
	return 'Service';
}

/**
 * Handle incoming activity and relay if needed.
 *
 * @param array $activity The activity data.
 * @param array $user_ids The user IDs that are recipients.
 * @param bool  $success  Whether the activity was handled successfully.
 */
function handle_activity( $activity, $user_ids, $success ) {
	// Only relay if: successfully handled, Blog actor is recipient, activity is public, and in single-user mode.
	if (
		! $success ||
		! in_array( Actors::BLOG_USER_ID, (array) $user_ids, true ) ||
		! is_activity_public( $activity ) ||
		! is_single_user()
	) {
		return;
	}

	// Create Announce wrapper.
	$announce = new Activity();
	$announce->set_type( 'Announce' );
	$announce->set_actor( Actors::BLOG_USER_ID );
	$announce->set_object( $activity );
	$announce->set_published( gmdate( ACTIVITYPUB_DATE_TIME_RFC3339 ) );

	// Add to outbox for distribution. The outbox will generate the ID.
	Outbox::add( $announce, Actors::BLOG_USER_ID );
}

/**
 * Unhook settings fields when relay mode is enabled.
 *
 * Removes all settings sections except moderation when relay mode is active.
 */
function unhook_settings_fields() {
	global $wp_settings_sections;

	if ( ! isset( $wp_settings_sections['activitypub_settings'] ) ) {
		return;
	}

	// Keep only the moderation section.
	foreach ( $wp_settings_sections['activitypub_settings'] as $section_id => $section ) {
		if ( 'activitypub_moderation' !== $section_id ) {
			unset( $wp_settings_sections['activitypub_settings'][ $section_id ] );
		}
	}
}

/**
 * Register the relay mode setting.
 */
function register_setting() {
	\register_setting(
		'activitypub_advanced',
		'activitypub_relay_mode',
		array(
			'type'              => 'integer',
			'description'       => 'Enable relay mode to forward public activities to all followers.',
			'default'           => 0,
			'sanitize_callback' => 'absint',
		)
	);

	// Add settings field to advanced section.
	\add_settings_field(
		'activitypub_relay_mode',
		\__( 'Relay Mode', 'activitypub-relay-mode' ),
		__NAMESPACE__ . '\relay_mode_field_callback',
		'activitypub_settings',
		'activitypub_advanced',
		array( 'label_for' => 'activitypub_relay_mode' )
	);
}

/**
 * Render the relay mode settings field.
 */
function relay_mode_field_callback() {
	$value = get_option( 'activitypub_relay_mode', 0 );
	?>
	<label>
		<input type="checkbox" name="activitypub_relay_mode" id="activitypub_relay_mode" value="1" <?php checked( $value, 1 ); ?>>
		<?php esc_html_e( 'Enable relay mode to forward public activities to all followers.', 'activitypub-relay-mode' ); ?>
	</label>
	<p class="description">
		<?php esc_html_e( 'When enabled, this site will act as a relay, forwarding public activities from followed accounts to all followers. The blog actor will be set to "Service" type with username "relay".', 'activitypub-relay-mode' ); ?>
	</p>
	<?php
}

/**
 * Handle relay mode option changes.
 *
 * When relay mode is enabled, switch to blog-only mode and set username to "relay".
 * When disabled, restore previous settings.
 *
 * @param mixed $old_value The old option value.
 * @param mixed $new_value The new option value.
 */
function relay_mode_changed( $old_value, $new_value ) {
	if ( $new_value && ! $old_value ) {
		// Enabling relay mode.
		// Store previous username and actor mode for restoration.
		update_option( 'activitypub_relay_previous_blog_identifier', get_option( 'activitypub_blog_identifier' ) );
		update_option( 'activitypub_relay_previous_actor_mode', get_option( 'activitypub_actor_mode' ) );

		// Set blog username to "relay".
		update_option( 'activitypub_blog_identifier', 'relay' );

		// Switch to blog-only mode.
		update_option( 'activitypub_actor_mode', ACTIVITYPUB_BLOG_MODE );
	} elseif ( ! $new_value && $old_value ) {
		// Disabling relay mode - restore previous settings.
		$previous_identifier = get_option( 'activitypub_relay_previous_blog_identifier' );
		$previous_actor_mode = get_option( 'activitypub_relay_previous_actor_mode' );

		if ( $previous_identifier ) {
			update_option( 'activitypub_blog_identifier', $previous_identifier );
			delete_option( 'activitypub_relay_previous_blog_identifier' );
		}

		if ( $previous_actor_mode ) {
			update_option( 'activitypub_actor_mode', $previous_actor_mode );
			delete_option( 'activitypub_relay_previous_actor_mode' );
		}
	}
}