silverstripe-subsites/code/extensions/ErrorPageSubsite.php

<?php
class ErrorPageSubsite extends DataExtension
{
    /**
     * Alter file path to generated a static (static) error page file to handle error page template on different sub-sites 
     *
     * @see Error::get_filepath_for_errorcode()
     *
     * FIXME since {@link Subsite::currentSubsite()} partly relies on Session, viewing other sub-site (including main site) between 
     * opening ErrorPage in the CMS and publish ErrorPage causes static error page to get generated incorrectly. 
     */
    public function alternateFilepathForErrorcode($statusCode, $locale = null)
    {
        $static_filepath = Config::inst()->get($this->owner->ClassName, 'static_filepath');
        $subdomainPart = "";
        
        // Try to get current subsite from session
        $subsite = Subsite::currentSubsite(false);
        
        // since this function is called from Page class before the controller is created, we have to get subsite from domain instead
        if (!$subsite) {
            $subsiteID = Subsite::getSubsiteIDForDomain();
            if ($subsiteID != 0) {
                $subsite = DataObject::get_by_id("Subsite", $subsiteID);
            } else {
                $subsite = null;
            }
        }
        
        if ($subsite) {
            $subdomain = $subsite->domain();
            $subdomainPart = "-{$subdomain}";
        }
        
        if (singleton('SiteTree')->hasExtension('Translatable') && $locale && $locale != Translatable::default_locale()) {
            $filepath = $static_filepath . "/error-{$statusCode}-{$locale}{$subdomainPart}.html";
        } else {
            $filepath = $static_filepath . "/error-{$statusCode}{$subdomainPart}.html";
        }

        return $filepath;
    }
}