Issue #3275883: Warn if cron updates are enabled and the site is on an unsupported branch

src/Validator/VersionPolicy/SupportedBranchInstalled.php

+<?php
+
+namespace Drupal\automatic_updates\Validator\VersionPolicy;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\DependencyInjection\ContainerInjectionInterface;
+use Drupal\Core\Extension\ExtensionVersion;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\Url;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * A policy rule that requires updating from a supported branch.
+ *
+ * @internal
+ *   This is an internal part of Automatic Updates' version policy for
+ *   Drupal core. It may be changed or removed at any time without warning.
+ *   External code should not interact with this class.
+ */
+class SupportedBranchInstalled implements ContainerInjectionInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The config factory service.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  private $configFactory;
+
+  /**
+   * Constructs a SupportedBranchInstalled object.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory service.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory) {
+    $this->configFactory = $config_factory;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    return new static(
+      $container->get('config.factory')
+    );
+  }
+
+  /**
+   * Checks if the installed version of Drupal is in a supported branch.
+   *
+   * @param string $installed_version
+   *   The installed version of Drupal.
+   *
+   * @return \Drupal\Core\StringTranslation\TranslatableMarkup[]
+   *   The error messages, if any.
+   */
+  public function validate(string $installed_version): array {
+    $available_updates = update_get_available(TRUE);
+
+    $installed = ExtensionVersion::createFromVersionString($installed_version);
+    $installed_major = $installed->getMajorVersion();
+    $installed_minor = $installed->getMinorVersion();
+    $in_supported_major = FALSE;
+
+    $supported_branches = explode(',', $available_updates['drupal']['supported_branches']);
+    foreach ($supported_branches as $supported_branch) {
+      $supported_branch = ExtensionVersion::createFromSupportBranch($supported_branch);
+
+      // Check if this supported branch is in the same major version as what's
+      // installed, since that will influence our messaging.
+      if ($installed_major === $supported_branch->getMajorVersion()) {
+        $in_supported_major = TRUE;
+
+        // If the supported branch's major and minor versions are the same as
+        // the installed ones, this rule is fulfilled.
+        if ($installed_minor === $supported_branch->getMinorVersion()) {
+          return [];
+        }
+      }
+    }
+
+    // By this point, we know the installed version of Drupal is not in a
+    // supported branch, so we'll always show this message.
+    $messages = [
+      $this->t('The currently installed version of Drupal core, @installed_version, is not in a supported minor version. Your site will not be automatically updated during cron until it is updated to a supported minor version.', [
+        '@installed_version' => $installed_version,
+      ]),
+    ];
+
+    // If the installed version of Drupal is in a supported major branch, an
+    // attended update may be possible, depending on configuration.
+    $allow_minor_updates = $this->configFactory->get('automatic_updates.settings')
+      ->get('allow_core_minor_updates');
+
+    if ($in_supported_major && $allow_minor_updates) {
+      $messages[] = $this->t('Use the <a href=":url">update form</a> to update to a supported version.', [
+        ':url' => Url::fromRoute('automatic_updates.module_update')->toString(),
+      ]);
+    }
+    else {
+      $messages[] = $this->t('See the <a href=":url">available updates page</a> for available updates.', [
+        ':url' => Url::fromRoute('update.status')->toString(),
+      ]);
+    }
+    return $messages;
+  }
+
+}