TYPO3 Core

I have extended the FrontendUserAuthentication using the XClass-API.

This is everything but a nice solution, because it manipulates $_POST or $_GET afterwards.

But currently I don't see any other solution. And rebuilding the behaviour of FrontendUserAuthentication as I have seen in other extensions is IMHO even less sustainable.

It would be best if there was a hook or API for the frontend user login process so that it could be easily extended.

ext_localconf.php:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['Objects'][TYPO3\CMS\Frontend\Authentication\FrontendUserAuthentication::class] = [
   'className' =>  Cylancer\Usertools\XClass\CyFrontendUserAuthentication::class

\Cylancer\Usertools\XClass\CyFrontendUserAuthentication:

<?php
namespace Cylancer\Usertools\XClass;

use TYPO3\CMS\Core\Utility\GeneralUtility;

/**
 * Extension class for Front End User Authentication.
 *  
 * This extension allows the login via e-mail or via user name.
 * 
 */
class CyFrontendUserAuthentication extends \TYPO3\CMS\Frontend\Authentication\FrontendUserAuthentication
{

        protected $email_column = 'email'; 

    /**
     * Returns an info array with Login/Logout data submitted by a form or params
     *
     * @return array
     * @see FrontendUserAuthentication::getLoginFormData()
     */
    public function getLoginFormData()
    {
                $username = GeneralUtility::_POST($this->formfield_uname);
                if($username != null ){
                    $_POST[$this->formfield_uname] = $this->mapEMailToUsername($username);
                }
                $username = GeneralUtility::_GET($this->formfield_uname);
                if($username != null ){
                    $_GET[$this->formfield_uname] = $this->mapEMailToUsername($username);
                }
                return parent::getLoginFormData(); 
    }

    /**
     * If you can deduce exactly one user name from the given e-mail address, 
     * the determined user name is returned. In all other cases the given $email 
     * will be returned unchanged. 
     * @param String $email is possibly an e-mail address. 
     */
    private function mapEMailToUsername($email){

                if(!filter_var($email, FILTER_VALIDATE_EMAIL)) {
                    return $email; 
                }    

                $qb = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Database\ConnectionPool::class)->getQueryBuilderForTable($this->user_table);
                $p_email = $qb->createNamedParameter($email); 
                $rows =  $qb->select($this->username_column, $this->email_column)
                                    ->from($this->user_table)
                                    ->where( $qb->expr()->eq( $this->username_column , $p_email))
                                    ->orWhere( $qb->expr()->eq( $this->email_column , $p_email))
                                    ->execute()
                                    ->fetchAll();

                if(count($rows) == 1) {
                     return $rows[0][$this->username_column]; 
                } else {
                    return $email; 
                }
        }

}

It might help someone...
Greetings
Clemens

For a simple test use the extension:
https://extensions.typo3.org/extension/loginviaemail

Thank you... I have now implemented a derivation of the FrontendUserAuthentication as an own service. This works. But I didn't dare to derive only from "AbstractUserAuthentication", too many features are implemented in FrontendUserAuthentication for that. :)