bbp_get_topic_subscribers( int $topic_id )

function bbp_get_topic_subscribers( $topic_id = 0 ) {
	$topic_id = bbp_get_topic_id( $topic_id );
	if ( empty( $topic_id ) )
		return;

	global $wpdb;

	$key   = $wpdb->prefix . '_bbp_subscriptions';
	$users = wp_cache_get( 'bbp_get_topic_subscribers_' . $topic_id, 'bbpress_users' );
	if ( false === $users ) {
		$users = $wpdb->get_col( "SELECT user_id FROM {$wpdb->usermeta} WHERE meta_key = '{$key}' and FIND_IN_SET('{$topic_id}', meta_value) > 0" );
		wp_cache_set( 'bbp_get_topic_subscribers_' . $topic_id, $users, 'bbpress_users' );
	}

	return apply_filters( 'bbp_get_topic_subscribers', $users );
}