silverstripe-subsites/code/extensions/ErrorPageSubsite.php

<?php

namespace SilverStripe\Subsites\Extensions;


use SilverStripe\ORM\DataExtension;
use SilverStripe\ORM\DataObject;
use SilverStripe\Subsites\Model\Subsite;


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_error_filename()}
     *
     * 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.
     *
     * @param string $name Filename to write to
     * @param int $statusCode Integer error code
     */
    public function updateErrorFilename(&$name, $statusCode)
    {

        // 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::class, $subsiteID);
            }
        }

        // Without subsite, don't rewrite
        if ($subsite) {
            // Add subdomain to end of filename, just before .html
            // This should preserve translatable locale in the filename as well
            $subdomain = $subsite->domain();
            $name = substr($name, 0, -5) . "-{$subdomain}.html";
        }
    }

}