Authenticate users by username and email.

modules/core/login/farm_login.services.yml

@@ -0,0 +1,5 @@
+services:
+  farm_login.user.auth:
+    class: Drupal\farm_login\UserAuth
+    decorates: user.auth
+    arguments: ['@entity_type.manager', '@password']

modules/core/login/src/UserAuth.php

@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\farm_login;
+
+use Drupal\user\UserAuth as CoreUserAuth;
+
+/**
+ * Extends the core user.auth service to load users by their email.
+ */
+class UserAuth extends CoreUserAuth {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function authenticate($username, $password) {
+    $uid = parent::authenticate($username, $password);
+
+    // If the parent failed to authenticate, try loading the user by email.
+    if (empty($uid) && !empty($username) && strlen($password) > 0) {
+      $account_search = $this->entityTypeManager->getStorage('user')->loadByProperties(['mail' => $username]);
+
+      if ($account = reset($account_search)) {
+        if ($this->passwordChecker->check($password, $account->getPassword())) {
+          // Successful authentication.
+          $uid = $account->id();
+
+          // Update user to new password scheme if needed.
+          if ($this->passwordChecker->needsRehash($account->getPassword())) {
+            $account->setPassword($password);
+            $account->save();
+          }
+        }
+      }
+    }
+
+    return $uid;
+  }
+
+}