clean install

web/core/modules/forum/src/Breadcrumb/ForumBreadcrumbBuilderBase.php

@@ -0,0 +1,90 @@
+<?php
+
+namespace Drupal\forum\Breadcrumb;
+
+use Drupal\Core\Breadcrumb\BreadcrumbBuilderInterface;
+use Drupal\Core\Breadcrumb\Breadcrumb;
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Link;
+use Drupal\Core\Routing\RouteMatchInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\forum\ForumManagerInterface;
+
+/**
+ * Provides a forum breadcrumb base class.
+ *
+ * This just holds the dependency-injected config, entity type manager, and
+ * forum manager objects.
+ */
+abstract class ForumBreadcrumbBuilderBase implements BreadcrumbBuilderInterface {
+  use StringTranslationTrait;
+
+  /**
+   * Configuration object for this builder.
+   *
+   * @var \Drupal\Core\Config\Config
+   */
+  protected $config;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The forum manager service.
+   *
+   * @var \Drupal\forum\ForumManagerInterface
+   */
+  protected $forumManager;
+
+  /**
+   * The taxonomy term storage.
+   *
+   * @var \Drupal\taxonomy\TermStorageInterface
+   */
+  protected $termStorage;
+
+  /**
+   * Constructs a forum breadcrumb builder object.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The configuration factory.
+   * @param \Drupal\forum\ForumManagerInterface $forum_manager
+   *   The forum manager service.
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
+   *   The string translation service.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, ConfigFactoryInterface $config_factory, ForumManagerInterface $forum_manager, TranslationInterface $string_translation) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->config = $config_factory->get('forum.settings');
+    $this->forumManager = $forum_manager;
+    $this->setStringTranslation($string_translation);
+    $this->termStorage = $entity_type_manager->getStorage('taxonomy_term');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(RouteMatchInterface $route_match) {
+    $breadcrumb = new Breadcrumb();
+    $breadcrumb->addCacheContexts(['route']);
+
+    $links[] = Link::createFromRoute($this->t('Home'), '<front>');
+
+    $vocabulary = $this->entityTypeManager
+      ->getStorage('taxonomy_vocabulary')
+      ->load($this->config->get('vocabulary'));
+    $breadcrumb->addCacheableDependency($vocabulary);
+    $links[] = Link::createFromRoute($vocabulary->label(), 'forum.index');
+
+    return $breadcrumb->setLinks($links);
+  }
+
+}

web/core/modules/forum/src/Breadcrumb/ForumListingBreadcrumbBuilder.php

@@ -0,0 +1,48 @@
+<?php
+
+namespace Drupal\forum\Breadcrumb;
+
+use Drupal\Core\Link;
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Provides a breadcrumb builder base class for forum listing pages.
+ */
+class ForumListingBreadcrumbBuilder extends ForumBreadcrumbBuilderBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(RouteMatchInterface $route_match) {
+    return $route_match->getRouteName() == 'forum.page' && $route_match->getParameter('taxonomy_term');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(RouteMatchInterface $route_match) {
+    $breadcrumb = parent::build($route_match);
+    $breadcrumb->addCacheContexts(['route']);
+
+    // Add all parent forums to breadcrumbs.
+    /** @var \Drupal\taxonomy\TermInterface $term */
+    $term = $route_match->getParameter('taxonomy_term');
+    $term_id = $term->id();
+    $breadcrumb->addCacheableDependency($term);
+
+    $parents = $this->termStorage->loadAllParents($term_id);
+    if ($parents) {
+      foreach (array_reverse($parents) as $parent) {
+        if ($parent->id() != $term_id) {
+          $breadcrumb->addCacheableDependency($parent);
+          $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page', [
+            'taxonomy_term' => $parent->id(),
+          ]));
+        }
+      }
+    }
+
+    return $breadcrumb;
+  }
+
+}

web/core/modules/forum/src/Breadcrumb/ForumNodeBreadcrumbBuilder.php

@@ -0,0 +1,45 @@
+<?php
+
+namespace Drupal\forum\Breadcrumb;
+
+use Drupal\Core\Link;
+use Drupal\Core\Routing\RouteMatchInterface;
+
+/**
+ * Breadcrumb builder for forum nodes.
+ */
+class ForumNodeBreadcrumbBuilder extends ForumBreadcrumbBuilderBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function applies(RouteMatchInterface $route_match) {
+    return $route_match->getRouteName() == 'entity.node.canonical'
+      && $route_match->getParameter('node')
+      && $this->forumManager->checkNodeType($route_match->getParameter('node'));
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build(RouteMatchInterface $route_match) {
+    $breadcrumb = parent::build($route_match);
+    $breadcrumb->addCacheContexts(['route']);
+
+    $parents = $this->termStorage->loadAllParents($route_match->getParameter('node')->forum_tid);
+    if ($parents) {
+      $parents = array_reverse($parents);
+      foreach ($parents as $parent) {
+        $breadcrumb->addCacheableDependency($parent);
+        $breadcrumb->addLink(Link::createFromRoute($parent->label(), 'forum.page',
+          [
+            'taxonomy_term' => $parent->id(),
+          ]
+        ));
+      }
+    }
+
+    return $breadcrumb;
+  }
+
+}

web/core/modules/forum/src/Controller/ForumController.php

@@ -0,0 +1,348 @@
+<?php
+
+namespace Drupal\forum\Controller;
+
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Controller\ControllerBase;
+use Drupal\Core\Entity\EntityAccessControlHandlerInterface;
+use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
+use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Url;
+use Drupal\forum\ForumManagerInterface;
+use Drupal\taxonomy\TermInterface;
+use Drupal\taxonomy\TermStorageInterface;
+use Drupal\taxonomy\VocabularyStorageInterface;
+use Symfony\Component\DependencyInjection\ContainerInterface;
+
+/**
+ * Controller routines for forum routes.
+ */
+class ForumController extends ControllerBase {
+
+  /**
+   * Forum manager service.
+   *
+   * @var \Drupal\forum\ForumManagerInterface
+   */
+  protected $forumManager;
+
+  /**
+   * Vocabulary storage.
+   *
+   * @var \Drupal\taxonomy\VocabularyStorageInterface
+   */
+  protected $vocabularyStorage;
+
+  /**
+   * Term storage.
+   *
+   * @var \Drupal\taxonomy\TermStorageInterface
+   */
+  protected $termStorage;
+
+  /**
+   * Node access control handler.
+   *
+   * @var \Drupal\Core\Entity\EntityAccessControlHandlerInterface
+   */
+  protected $nodeAccess;
+
+  /**
+   * Field map of existing fields on the site.
+   *
+   * @var array
+   */
+  protected $fieldMap;
+
+  /**
+   * Node type storage handler.
+   *
+   * @var \Drupal\Core\Entity\EntityStorageInterface
+   */
+  protected $nodeTypeStorage;
+
+  /**
+   * The renderer.
+   *
+   * @var \Drupal\Core\Render\RendererInterface
+   */
+  protected $renderer;
+
+  /**
+   * Node entity type, we need to get cache tags from here.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeInterface
+   */
+  protected $nodeEntityTypeDefinition;
+
+  /**
+   * Comment entity type, we need to get cache tags from here.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeInterface
+   */
+  protected $commentEntityTypeDefinition;
+
+  /**
+   * Constructs a ForumController object.
+   *
+   * @param \Drupal\forum\ForumManagerInterface $forum_manager
+   *   The forum manager service.
+   * @param \Drupal\taxonomy\VocabularyStorageInterface $vocabulary_storage
+   *   Vocabulary storage.
+   * @param \Drupal\taxonomy\TermStorageInterface $term_storage
+   *   Term storage.
+   * @param \Drupal\Core\Session\AccountInterface $current_user
+   *   The current logged in user.
+   * @param \Drupal\Core\Entity\EntityAccessControlHandlerInterface $node_access
+   *   Node access control handler.
+   * @param array $field_map
+   *   Array of active fields on the site.
+   * @param \Drupal\Core\Entity\EntityStorageInterface $node_type_storage
+   *   Node type storage handler.
+   * @param \Drupal\Core\Render\RendererInterface $renderer
+   *   The renderer.
+   * @param \Drupal\Core\Entity\EntityTypeInterface $node_entity_type_definition
+   *   Node entity type definition object
+   * @param \Drupal\Core\Entity\EntityTypeInterface $comment_entity_type_definition
+   *   Comment entity type definition object
+   */
+  public function __construct(ForumManagerInterface $forum_manager, VocabularyStorageInterface $vocabulary_storage, TermStorageInterface $term_storage, AccountInterface $current_user, EntityAccessControlHandlerInterface $node_access, array $field_map, EntityStorageInterface $node_type_storage, RendererInterface $renderer, EntityTypeInterface $node_entity_type_definition, EntityTypeInterface $comment_entity_type_definition) {
+    $this->forumManager = $forum_manager;
+    $this->vocabularyStorage = $vocabulary_storage;
+    $this->termStorage = $term_storage;
+    $this->currentUser = $current_user;
+    $this->nodeAccess = $node_access;
+    $this->fieldMap = $field_map;
+    $this->nodeTypeStorage = $node_type_storage;
+    $this->renderer = $renderer;
+    $this->nodeEntityTypeDefinition = $node_entity_type_definition;
+    $this->commentEntityTypeDefinition = $comment_entity_type_definition;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public static function create(ContainerInterface $container) {
+    /** @var \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager */
+    $entity_type_manager = $container->get('entity_type.manager');
+
+    return new static(
+      $container->get('forum_manager'),
+      $entity_type_manager->getStorage('taxonomy_vocabulary'),
+      $entity_type_manager->getStorage('taxonomy_term'),
+      $container->get('current_user'),
+      $entity_type_manager->getAccessControlHandler('node'),
+      $container->get('entity_field.manager')->getFieldMap(),
+      $entity_type_manager->getStorage('node_type'),
+      $container->get('renderer'),
+      $entity_type_manager->getDefinition('node'),
+      $entity_type_manager->getDefinition('comment')
+    );
+  }
+
+  /**
+   * Returns forum page for a given forum.
+   *
+   * @param \Drupal\taxonomy\TermInterface $taxonomy_term
+   *   The forum to render the page for.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function forumPage(TermInterface $taxonomy_term) {
+    // Get forum details.
+    $taxonomy_term->forums = $this->forumManager->getChildren($this->config('forum.settings')->get('vocabulary'), $taxonomy_term->id());
+    $taxonomy_term->parents = $this->termStorage->loadAllParents($taxonomy_term->id());
+
+    if (empty($taxonomy_term->forum_container->value)) {
+      $build = $this->forumManager->getTopics($taxonomy_term->id(), $this->currentUser());
+      $topics = $build['topics'];
+      $header = $build['header'];
+    }
+    else {
+      $topics = [];
+      $header = [];
+    }
+    return $this->build($taxonomy_term->forums, $taxonomy_term, $topics, $taxonomy_term->parents, $header);
+  }
+
+  /**
+   * Returns forum index page.
+   *
+   * @return array
+   *   A render array.
+   */
+  public function forumIndex() {
+    $vocabulary = $this->vocabularyStorage->load($this->config('forum.settings')->get('vocabulary'));
+    $index = $this->forumManager->getIndex();
+    $build = $this->build($index->forums, $index);
+    if (empty($index->forums)) {
+      // Root of empty forum.
+      $build['#title'] = $this->t('No forums defined');
+    }
+    else {
+      // Set the page title to forum's vocabulary name.
+      $build['#title'] = $vocabulary->label();
+      $this->renderer->addCacheableDependency($build, $vocabulary);
+    }
+    return $build;
+  }
+
+  /**
+   * Returns a renderable forum index page array.
+   *
+   * @param array $forums
+   *   A list of forums.
+   * @param \Drupal\taxonomy\TermInterface $term
+   *   The taxonomy term of the forum.
+   * @param array $topics
+   *   The topics of this forum.
+   * @param array $parents
+   *   The parent forums in relation this forum.
+   * @param array $header
+   *   Array of header cells.
+   *
+   * @return array
+   *   A render array.
+   */
+  protected function build($forums, TermInterface $term, $topics = [], $parents = [], $header = []) {
+    $config = $this->config('forum.settings');
+    $build = [
+      '#theme' => 'forums',
+      '#forums' => $forums,
+      '#topics' => $topics,
+      '#parents' => $parents,
+      '#header' => $header,
+      '#term' => $term,
+      '#sortby' => $config->get('topics.order'),
+      '#forums_per_page' => $config->get('topics.page_limit'),
+    ];
+    if (empty($term->forum_container->value)) {
+      $build['#attached']['feed'][] = ['taxonomy/term/' . $term->id() . '/feed', 'RSS - ' . $term->getName()];
+    }
+    $this->renderer->addCacheableDependency($build, $config);
+
+    foreach ($forums as $forum) {
+      $this->renderer->addCacheableDependency($build, $forum);
+    }
+    foreach ($topics as $topic) {
+      $this->renderer->addCacheableDependency($build, $topic);
+    }
+    foreach ($parents as $parent) {
+      $this->renderer->addCacheableDependency($build, $parent);
+    }
+    $this->renderer->addCacheableDependency($build, $term);
+
+    $is_forum = empty($term->forum_container->value);
+    return [
+      'action' => ($is_forum) ? $this->buildActionLinks($config->get('vocabulary'), $term) : [],
+      'forum' => $build,
+      '#cache' => [
+        'tags' => Cache::mergeTags($this->nodeEntityTypeDefinition->getListCacheTags(), $this->commentEntityTypeDefinition->getListCacheTags()),
+      ],
+    ];
+  }
+
+  /**
+   * Returns add forum entity form.
+   *
+   * @return array
+   *   Render array for the add form.
+   */
+  public function addForum() {
+    $vid = $this->config('forum.settings')->get('vocabulary');
+    $taxonomy_term = $this->termStorage->create([
+      'vid' => $vid,
+    ]);
+    return $this->entityFormBuilder()->getForm($taxonomy_term, 'forum');
+  }
+
+  /**
+   * Returns add container entity form.
+   *
+   * @return array
+   *   Render array for the add form.
+   */
+  public function addContainer() {
+    $vid = $this->config('forum.settings')->get('vocabulary');
+    $taxonomy_term = $this->termStorage->create([
+      'vid' => $vid,
+      'forum_container' => 1,
+    ]);
+    return $this->entityFormBuilder()->getForm($taxonomy_term, 'container');
+  }
+
+  /**
+   * Generates an action link to display at the top of the forum listing.
+   *
+   * @param string $vid
+   *   Vocabulary ID.
+   * @param \Drupal\taxonomy\TermInterface $forum_term
+   *   The term for which the links are to be built.
+   *
+   * @return array
+   *   Render array containing the links.
+   */
+  protected function buildActionLinks($vid, ?TermInterface $forum_term = NULL) {
+    $user = $this->currentUser();
+
+    $links = [];
+    // Loop through all bundles for forum taxonomy vocabulary field.
+    foreach ($this->fieldMap['node']['taxonomy_forums']['bundles'] as $type) {
+      if ($this->nodeAccess->createAccess($type)) {
+        $node_type = $this->nodeTypeStorage->load($type);
+        $links[$type] = [
+          '#theme' => 'menu_local_action',
+          '#link' => [
+            'title' => $this->t('Add new @node_type', [
+              '@node_type' => $this->nodeTypeStorage->load($type)->label(),
+            ]),
+            'url' => Url::fromRoute('node.add', ['node_type' => $type]),
+          ],
+          '#cache' => [
+            'tags' => $node_type->getCacheTags(),
+          ],
+        ];
+        if ($forum_term && $forum_term->bundle() == $vid) {
+          // We are viewing a forum term (specific forum), append the tid to
+          // the URL.
+          $links[$type]['#link']['localized_options']['query']['forum_id'] = $forum_term->id();
+        }
+      }
+    }
+    if (empty($links)) {
+      // Authenticated user does not have access to create new topics.
+      if ($user->isAuthenticated()) {
+        $links['disallowed'] = [
+          '#markup' => $this->t('You are not allowed to post new content in the forum.'),
+        ];
+      }
+      // Anonymous user does not have access to create new topics.
+      else {
+        $links['login'] = [
+          '#theme' => 'menu_local_action',
+          '#link' => [
+            'title' => $this->t('Log in to post new content in the forum.'),
+            'url' => Url::fromRoute('user.login', [], ['query' => $this->getDestinationArray()]),
+          ],
+          // Without this workaround, the action links will be rendered as <li>
+          // with no wrapping <ul> element.
+          // @todo Find a better way for this in https://www.drupal.org/node/3181052.
+          '#prefix' => '<ul class="action-links">',
+          '#suffix' => '</ul>',
+        ];
+      }
+    }
+    else {
+      // Without this workaround, the action links will be rendered as <li> with
+      // no wrapping <ul> element.
+      // @todo Find a better way for this in https://www.drupal.org/node/3181052.
+      $links['#prefix'] = '<ul class="action-links">';
+      $links['#suffix'] = '</ul>';
+    }
+    return $links;
+  }
+
+}

web/core/modules/forum/src/Form/ContainerForm.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Drupal\forum\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Base form for container term edit forms.
+ *
+ * @internal
+ */
+class ContainerForm extends ForumForm {
+
+  /**
+   * Reusable URL stub to use in watchdog messages.
+   *
+   * @var string
+   */
+  protected $urlStub = 'container';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    // Build the bulk of the form from the parent forum form.
+    $form = parent::form($form, $form_state);
+
+    // Set the title and description of the name field.
+    $form['name']['#title'] = $this->t('Container name');
+    $form['name']['#description'] = $this->t('Short but meaningful name for this collection of related forums.');
+
+    // Alternate description for the container parent.
+    $form['parent'][0]['#description'] = $this->t('Containers are usually placed at the top (root) level, but may also be placed inside another container or forum.');
+    $this->forumFormType = $this->t('forum container');
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildEntity(array $form, FormStateInterface $form_state) {
+    $entity = parent::buildEntity($form, $form_state);
+    $entity->forum_container = TRUE;
+    return $entity;
+  }
+
+}

web/core/modules/forum/src/Form/DeleteForm.php

@@ -0,0 +1,71 @@
+<?php
+
+namespace Drupal\forum\Form;
+
+use Drupal\Core\Form\ConfirmFormBase;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Url;
+use Drupal\taxonomy\TermInterface;
+
+/**
+ * Builds the form to delete a forum term.
+ *
+ * @internal
+ */
+class DeleteForm extends ConfirmFormBase {
+
+  /**
+   * The taxonomy term being deleted.
+   *
+   * @var \Drupal\taxonomy\TermInterface
+   */
+  protected $taxonomyTerm;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'forum_confirm_delete';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getQuestion() {
+    return $this->t('Are you sure you want to delete the forum %label?', ['%label' => $this->taxonomyTerm->label()]);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCancelUrl() {
+    return new Url('forum.overview');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getConfirmText() {
+    return $this->t('Delete');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?TermInterface $taxonomy_term = NULL) {
+    $this->taxonomyTerm = $taxonomy_term;
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->taxonomyTerm->delete();
+    $this->messenger()->addStatus($this->t('The forum %label and all sub-forums have been deleted.', ['%label' => $this->taxonomyTerm->label()]));
+    $this->logger('forum')->notice('forum: deleted %label and all its sub-forums.', ['%label' => $this->taxonomyTerm->label()]);
+    $form_state->setRedirectUrl($this->getCancelUrl());
+  }
+
+}

web/core/modules/forum/src/Form/ForumForm.php

@@ -0,0 +1,170 @@
+<?php
+
+namespace Drupal\forum\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Link;
+use Drupal\Core\Url;
+use Drupal\taxonomy\TermForm;
+
+/**
+ * Base form for forum term edit forms.
+ *
+ * @internal
+ */
+class ForumForm extends TermForm {
+
+  /**
+   * Reusable type field to use in status messages.
+   *
+   * @var string
+   */
+  protected $forumFormType;
+
+  /**
+   * Reusable URL stub to use in watchdog messages.
+   *
+   * @var string
+   */
+  protected $urlStub = 'forum';
+
+  /**
+   * {@inheritdoc}
+   */
+  public function form(array $form, FormStateInterface $form_state) {
+    // Build the bulk of the form from the parent taxonomy term form.
+    $form = parent::form($form, $form_state);
+
+    // Set the title and description of the name field.
+    $form['name']['#title'] = $this->t('Forum name');
+    $form['name']['#description'] = $this->t('Short but meaningful name for this collection of threaded discussions.');
+
+    // Change the description.
+    $form['description']['#description'] = $this->t('Description and guidelines for discussions within this forum.');
+
+    // Re-use the weight field.
+    $form['weight'] = $form['relations']['weight'];
+
+    // Remove the remaining relations fields.
+    unset($form['relations']);
+
+    // Our parent field is different to the taxonomy term.
+    $form['parent']['#tree'] = TRUE;
+    $form['parent'][0] = $this->forumParentSelect($this->entity->id(), $this->t('Parent'));
+
+    $form['#theme_wrappers'] = ['form__forum'];
+    $this->forumFormType = $this->t('forum');
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildEntity(array $form, FormStateInterface $form_state) {
+    $term = parent::buildEntity($form, $form_state);
+
+    // Assign parents from forum parent select field.
+    $term->parent = [$form_state->getValue(['parent', 0])];
+
+    return $term;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function save(array $form, FormStateInterface $form_state) {
+    $term = $this->entity;
+    $term_storage = $this->entityTypeManager->getStorage('taxonomy_term');
+    $status = $term_storage->save($term);
+
+    $route_name = $this->urlStub == 'container' ? 'entity.taxonomy_term.forum_edit_container_form' : 'entity.taxonomy_term.forum_edit_form';
+    $route_parameters = ['taxonomy_term' => $term->id()];
+    $link = Link::fromTextAndUrl($this->t('Edit'), new Url($route_name, $route_parameters))->toString();
+    $view_link = $term->toLink($term->getName())->toString();
+    switch ($status) {
+      case SAVED_NEW:
+        $this->messenger()->addStatus($this->t('Created new @type %term.', ['%term' => $view_link, '@type' => $this->forumFormType]));
+        $this->logger('forum')->notice('Created new @type %term.', ['%term' => $term->getName(), '@type' => $this->forumFormType, 'link' => $link]);
+        $form_state->setValue('tid', $term->id());
+        break;
+
+      case SAVED_UPDATED:
+        $this->messenger()->addStatus($this->t('The @type %term has been updated.', ['%term' => $term->getName(), '@type' => $this->forumFormType]));
+        $this->logger('forum')->notice('Updated @type %term.', ['%term' => $term->getName(), '@type' => $this->forumFormType, 'link' => $link]);
+        break;
+    }
+
+    $form_state->setRedirect('forum.overview');
+    return $term;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function actions(array $form, FormStateInterface $form_state) {
+    $actions = parent::actions($form, $form_state);
+
+    if (!$this->entity->isNew() && $this->entity->hasLinkTemplate('forum-delete-form')) {
+      $actions['delete']['#url'] = $this->entity->toUrl('forum-delete-form');
+    }
+    else {
+      unset($actions['delete']);
+    }
+
+    return $actions;
+  }
+
+  /**
+   * Returns a select box for available parent terms.
+   *
+   * @param int $tid
+   *   ID of the term that is being added or edited.
+   * @param string $title
+   *   Title for the select box.
+   *
+   * @return array
+   *   A select form element.
+   */
+  protected function forumParentSelect($tid, $title) {
+    $taxonomy_storage = $this->entityTypeManager->getStorage('taxonomy_term');
+    $parents = $taxonomy_storage->loadParents($tid);
+    if ($parents) {
+      $parent = array_shift($parents);
+      $parent = $parent->id();
+    }
+    else {
+      $parent = 0;
+    }
+
+    $vid = $this->config('forum.settings')->get('vocabulary');
+    $children = $taxonomy_storage->loadTree($vid, $tid, NULL, TRUE);
+
+    // A term can't be the child of itself, nor of its children.
+    foreach ($children as $child) {
+      $exclude[] = $child->tid;
+    }
+    $exclude[] = $tid;
+
+    $tree = $taxonomy_storage->loadTree($vid, 0, NULL, TRUE);
+    $options[0] = '<' . $this->t('root') . '>';
+    if ($tree) {
+      foreach ($tree as $term) {
+        if (!in_array($term->id(), $exclude)) {
+          $options[$term->id()] = str_repeat(' -- ', $term->depth) . $term->getName();
+        }
+      }
+    }
+
+    $description = $this->t('Forums may be placed at the top (root) level, or inside another container or forum.');
+
+    return [
+      '#type' => 'select',
+      '#title' => $title,
+      '#default_value' => $parent,
+      '#options' => $options,
+      '#description' => $description,
+      '#required' => TRUE,
+    ];
+  }
+
+}

web/core/modules/forum/src/Form/Overview.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace Drupal\forum\Form;
+
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Render\Element;
+use Drupal\Core\Url;
+use Drupal\taxonomy\Form\OverviewTerms;
+use Drupal\taxonomy\VocabularyInterface;
+use Symfony\Component\HttpKernel\Exception\NotFoundHttpException;
+
+/**
+ * Provides forum overview form for the forum vocabulary.
+ *
+ * @internal
+ */
+class Overview extends OverviewTerms {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'forum_overview';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state, ?VocabularyInterface $taxonomy_vocabulary = NULL) {
+    $forum_config = $this->config('forum.settings');
+    $vid = $forum_config->get('vocabulary');
+    $vocabulary = $this->entityTypeManager->getStorage('taxonomy_vocabulary')->load($vid);
+    if (!$vocabulary) {
+      throw new NotFoundHttpException();
+    }
+
+    // Build base taxonomy term overview.
+    $form = parent::buildForm($form, $form_state, $vocabulary);
+
+    foreach (Element::children($form['terms']) as $key) {
+      if (isset($form['terms'][$key]['#term'])) {
+        /** @var \Drupal\taxonomy\TermInterface $term */
+        $term = $form['terms'][$key]['#term'];
+        $form['terms'][$key]['term']['#url'] = Url::fromRoute('forum.page', ['taxonomy_term' => $term->id()]);
+
+        if (!empty($term->forum_container->value)) {
+          $title = $this->t('edit container');
+          $url = Url::fromRoute('entity.taxonomy_term.forum_edit_container_form', ['taxonomy_term' => $term->id()]);
+        }
+        else {
+          $title = $this->t('edit forum');
+          $url = Url::fromRoute('entity.taxonomy_term.forum_edit_form', ['taxonomy_term' => $term->id()]);
+        }
+
+        // Re-create the operations column and add only the edit link.
+        $form['terms'][$key]['operations'] = [
+          '#type' => 'operations',
+          '#links' => [
+            'edit' => [
+              'title' => $title,
+              'url' => $url,
+            ],
+          ],
+        ];
+
+      }
+    }
+
+    // Remove the alphabetical reset.
+    unset($form['actions']['reset_alphabetical']);
+
+    // Use the existing taxonomy overview submit handler.
+    $form['terms']['#empty'] = $this->t('No containers or forums available. <a href=":container">Add container</a> or <a href=":forum">Add forum</a>.', [
+      ':container' => Url::fromRoute('forum.add_container')->toString(),
+      ':forum' => Url::fromRoute('forum.add_forum')->toString(),
+    ]);
+    return $form;
+  }
+
+}

web/core/modules/forum/src/ForumIndexStorage.php

@@ -0,0 +1,164 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\comment\CommentInterface;
+use Drupal\Core\Database\Connection;
+use Drupal\node\NodeInterface;
+
+/**
+ * Handles CRUD operations to {forum_index} table.
+ */
+class ForumIndexStorage implements ForumIndexStorageInterface {
+
+  /**
+   * The active database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $database;
+
+  /**
+   * Constructs a ForumIndexStorage object.
+   *
+   * @param \Drupal\Core\Database\Connection $database
+   *   The current database connection.
+   */
+  public function __construct(Connection $database) {
+    $this->database = $database;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getOriginalTermId(NodeInterface $node) {
+    return $this->database->queryRange("SELECT [f].[tid] FROM {forum} [f] INNER JOIN {node} [n] ON [f].[vid] = [n].[vid] WHERE [n].[nid] = :nid ORDER BY [f].[vid] DESC", 0, 1, [':nid' => $node->id()])->fetchField();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function create(NodeInterface $node) {
+    $this->database->insert('forum')
+      ->fields([
+        'tid' => $node->forum_tid,
+        'vid' => $node->getRevisionId(),
+        'nid' => $node->id(),
+      ])
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function read(array $vids) {
+    return $this->database->select('forum', 'f')
+      ->fields('f', ['nid', 'tid'])
+      ->condition('f.vid', $vids, 'IN')
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function delete(NodeInterface $node) {
+    $this->database->delete('forum')
+      ->condition('nid', $node->id())
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteRevision(NodeInterface $node) {
+    $this->database->delete('forum')
+      ->condition('nid', $node->id())
+      ->condition('vid', $node->getRevisionId())
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function update(NodeInterface $node) {
+    $this->database->update('forum')
+      ->fields(['tid' => $node->forum_tid])
+      ->condition('vid', $node->getRevisionId())
+      ->execute();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function updateIndex(NodeInterface $node) {
+    $nid = $node->id();
+    $count = $this->database->query("SELECT COUNT([cid]) FROM {comment_field_data} [c] INNER JOIN {forum_index} [i] ON [c].[entity_id] = [i].[nid] WHERE [c].[entity_id] = :nid AND [c].[field_name] = 'comment_forum' AND [c].[entity_type] = 'node' AND [c].[status] = :status AND [c].[default_langcode] = 1", [
+      ':nid' => $nid,
+      ':status' => CommentInterface::PUBLISHED,
+    ])->fetchField();
+
+    if ($count > 0) {
+      // Comments exist.
+      $last_reply = $this->database->queryRange("SELECT [cid], [name], [created], [uid] FROM {comment_field_data} WHERE [entity_id] = :nid AND [field_name] = 'comment_forum' AND [entity_type] = 'node' AND [status] = :status AND [default_langcode] = 1 ORDER BY [cid] DESC", 0, 1, [
+        ':nid' => $nid,
+        ':status' => CommentInterface::PUBLISHED,
+      ])->fetchObject();
+      $this->database->update('forum_index')
+        ->fields([
+          'comment_count' => $count,
+          'last_comment_timestamp' => $last_reply->created,
+        ])
+        ->condition('nid', $nid)
+        ->execute();
+    }
+    else {
+      // Comments do not exist.
+      // @todo This should be actually filtering on the desired node language
+      $this->database->update('forum_index')
+        ->fields([
+          'comment_count' => 0,
+          'last_comment_timestamp' => $node->getCreatedTime(),
+        ])
+        ->condition('nid', $nid)
+        ->execute();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createIndex(NodeInterface $node) {
+    $query = $this->database->insert('forum_index')
+      ->fields(['nid', 'title', 'tid', 'sticky', 'created', 'comment_count', 'last_comment_timestamp']);
+    foreach ($node->getTranslationLanguages() as $langcode => $language) {
+      $translation = $node->getTranslation($langcode);
+      foreach ($translation->taxonomy_forums as $item) {
+        $query->values([
+          'nid' => $node->id(),
+          'title' => $translation->label(),
+          'tid' => $item->target_id,
+          'sticky' => (int) $node->isSticky(),
+          'created' => $node->getCreatedTime(),
+          'comment_count' => 0,
+          'last_comment_timestamp' => $node->getCreatedTime(),
+        ]);
+      }
+    }
+    $query->execute();
+    // The logic for determining last_comment_count is fairly complex, so
+    // update the index too.
+    if ($node->isNew()) {
+      $this->updateIndex($node);
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function deleteIndex(NodeInterface $node) {
+    $this->database->delete('forum_index')
+      ->condition('nid', $node->id())
+      ->execute();
+  }
+
+}

web/core/modules/forum/src/ForumIndexStorageInterface.php

@@ -0,0 +1,90 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\node\NodeInterface;
+
+/**
+ * Handles CRUD operations to {forum_index} table.
+ */
+interface ForumIndexStorageInterface {
+
+  /**
+   * Returns the forum term id associated with an existing forum node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The existing forum node.
+   *
+   * @return int
+   *   The forum term id currently associated with the node.
+   */
+  public function getOriginalTermId(NodeInterface $node);
+
+  /**
+   * Creates a record in {forum} table for the given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the record is to be created.
+   */
+  public function create(NodeInterface $node);
+
+  /**
+   * Reads an array of {forum} records for the given revision ids.
+   *
+   * @param array $vids
+   *   An array of node revision ids.
+   *
+   * @return \Drupal\Core\Database\StatementInterface
+   *   The records from {forum} for the given vids.
+   */
+  public function read(array $vids);
+
+  /**
+   * Updates the {forum} table for the given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the record is to be updated.
+   */
+  public function update(NodeInterface $node);
+
+  /**
+   * Deletes the records in {forum} table for the given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the records are to be deleted.
+   */
+  public function delete(NodeInterface $node);
+
+  /**
+   * Deletes the records in {forum} table for a given node revision.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node revision for which the records are to be deleted.
+   */
+  public function deleteRevision(NodeInterface $node);
+
+  /**
+   * Creates a {forum_index} entry for the given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the index records are to be created.
+   */
+  public function createIndex(NodeInterface $node);
+
+  /**
+   * Updates the {forum_index} records for a given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the index records are to be updated.
+   */
+  public function updateIndex(NodeInterface $node);
+
+  /**
+   * Deletes the {forum_index} records for a given node.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   The node for which the index records are to be deleted.
+   */
+  public function deleteIndex(NodeInterface $node);
+
+}

web/core/modules/forum/src/ForumManager.php

@@ -0,0 +1,525 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Database\Connection;
+use Drupal\Core\Database\Query\PagerSelectExtender;
+use Drupal\Core\Database\Query\TableSortExtender;
+use Drupal\Core\DependencyInjection\DependencySerializationTrait;
+use Drupal\Core\Entity\EntityFieldManagerInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\comment\CommentManagerInterface;
+use Drupal\node\NodeInterface;
+
+/**
+ * Provides forum manager service.
+ */
+class ForumManager implements ForumManagerInterface {
+  use StringTranslationTrait;
+  use DependencySerializationTrait {
+    __wakeup as defaultWakeup;
+    __sleep as defaultSleep;
+  }
+
+  /**
+   * Forum sort order, newest first.
+   */
+  const NEWEST_FIRST = 1;
+
+  /**
+   * Forum sort order, oldest first.
+   */
+  const OLDEST_FIRST = 2;
+
+  /**
+   * Forum sort order, posts with most comments first.
+   */
+  const MOST_POPULAR_FIRST = 3;
+
+  /**
+   * Forum sort order, posts with the least comments first.
+   */
+  const LEAST_POPULAR_FIRST = 4;
+
+  /**
+   * Forum settings config object.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * Entity field manager.
+   *
+   * @var \Drupal\Core\Entity\EntityFieldManagerInterface
+   */
+  protected $entityFieldManager;
+
+  /**
+   * Entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * Database connection.
+   *
+   * @var \Drupal\Core\Database\Connection
+   */
+  protected $connection;
+
+  /**
+   * The comment manager service.
+   *
+   * @var \Drupal\comment\CommentManagerInterface
+   */
+  protected $commentManager;
+
+  /**
+   * Array of last post information keyed by forum (term) id.
+   *
+   * @var array
+   */
+  protected $lastPostData = [];
+
+  /**
+   * Array of forum statistics keyed by forum (term) id.
+   *
+   * @var array
+   */
+  protected $forumStatistics = [];
+
+  /**
+   * Array of forum children keyed by parent forum (term) id.
+   *
+   * @var array
+   */
+  protected $forumChildren = [];
+
+  /**
+   * Array of history keyed by nid.
+   *
+   * @var array
+   */
+  protected $history = [];
+
+  /**
+   * Cached forum index.
+   *
+   * @var \Drupal\taxonomy\TermInterface
+   */
+  protected $index;
+
+  /**
+   * Constructs the forum manager service.
+   *
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory service.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Database\Connection $connection
+   *   The current database connection.
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
+   *   The translation manager service.
+   * @param \Drupal\comment\CommentManagerInterface $comment_manager
+   *   The comment manager service.
+   * @param \Drupal\Core\Entity\EntityFieldManagerInterface $entity_field_manager
+   *   The entity field manager.
+   */
+  public function __construct(ConfigFactoryInterface $config_factory, EntityTypeManagerInterface $entity_type_manager, Connection $connection, TranslationInterface $string_translation, CommentManagerInterface $comment_manager, EntityFieldManagerInterface $entity_field_manager) {
+    $this->configFactory = $config_factory;
+    $this->entityTypeManager = $entity_type_manager;
+    $this->connection = $connection;
+    $this->stringTranslation = $string_translation;
+    $this->commentManager = $comment_manager;
+    $this->entityFieldManager = $entity_field_manager;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getTopics($tid, AccountInterface $account) {
+    $config = $this->configFactory->get('forum.settings');
+    $forum_per_page = $config->get('topics.page_limit');
+    $sortby = $config->get('topics.order');
+
+    $header = [
+      ['data' => $this->t('Topic'), 'field' => 'f.title'],
+      ['data' => $this->t('Replies'), 'field' => 'f.comment_count'],
+      ['data' => $this->t('Last reply'), 'field' => 'f.last_comment_timestamp'],
+    ];
+
+    $order = $this->getTopicOrder($sortby);
+    for ($i = 0; $i < count($header); $i++) {
+      if ($header[$i]['field'] == $order['field']) {
+        $header[$i]['sort'] = $order['sort'];
+      }
+    }
+
+    $query = $this->connection->select('forum_index', 'f')
+      ->extend(PagerSelectExtender::class)
+      ->extend(TableSortExtender::class);
+    $query->fields('f');
+    $query
+      ->condition('f.tid', $tid)
+      ->addTag('node_access')
+      ->addMetaData('base_table', 'forum_index')
+      ->orderBy('f.sticky', 'DESC')
+      ->orderByHeader($header)
+      ->limit($forum_per_page);
+
+    $count_query = $this->connection->select('forum_index', 'f');
+    $count_query->condition('f.tid', $tid);
+    $count_query->addExpression('COUNT(*)');
+    $count_query->addTag('node_access');
+    $count_query->addMetaData('base_table', 'forum_index');
+
+    $query->setCountQuery($count_query);
+    $result = $query->execute();
+    $nids = [];
+    foreach ($result as $record) {
+      $nids[] = $record->nid;
+    }
+    if ($nids) {
+      $nodes = $this->entityTypeManager->getStorage('node')->loadMultiple($nids);
+
+      $query = $this->connection->select('node_field_data', 'n')
+        ->extend(TableSortExtender::class);
+      $query->fields('n', ['nid']);
+
+      $query->join('comment_entity_statistics', 'ces', "[n].[nid] = [ces].[entity_id] AND [ces].[field_name] = 'comment_forum' AND [ces].[entity_type] = 'node'");
+      $query->fields('ces', [
+        'cid',
+        'last_comment_uid',
+        'last_comment_timestamp',
+        'comment_count',
+      ]);
+
+      $query->join('forum_index', 'f', '[f].[nid] = [n].[nid]');
+      $query->addField('f', 'tid', 'forum_tid');
+
+      $query->join('users_field_data', 'u', '[n].[uid] = [u].[uid] AND [u].[default_langcode] = 1');
+      $query->addField('u', 'name');
+
+      $query->join('users_field_data', 'u2', '[ces].[last_comment_uid] = [u2].[uid] AND [u].[default_langcode] = 1');
+
+      $query->addExpression('CASE [ces].[last_comment_uid] WHEN 0 THEN [ces].[last_comment_name] ELSE [u2].[name] END', 'last_comment_name');
+
+      $query
+        ->orderBy('f.sticky', 'DESC')
+        ->orderByHeader($header)
+        ->condition('n.nid', $nids, 'IN')
+        // @todo This should be actually filtering on the desired node language
+        //   and just fall back to the default language.
+        ->condition('n.default_langcode', 1);
+
+      $result = [];
+      foreach ($query->execute() as $row) {
+        $topic = $nodes[$row->nid];
+        $topic->comment_mode = $topic->comment_forum->status;
+
+        foreach ($row as $key => $value) {
+          $topic->{$key} = $value;
+        }
+        $result[] = $topic;
+      }
+    }
+    else {
+      $result = [];
+    }
+
+    $topics = [];
+    $first_new_found = FALSE;
+    foreach ($result as $topic) {
+      if ($account->isAuthenticated()) {
+        // A forum is new if the topic is new, or if there are new comments since
+        // the user's last visit.
+        if ($topic->forum_tid != $tid) {
+          $topic->new = 0;
+        }
+        else {
+          $history = $this->lastVisit($topic->id(), $account);
+          $topic->new_replies = $this->commentManager->getCountNewComments($topic, 'comment_forum', $history);
+          $topic->new = $topic->new_replies || ($topic->last_comment_timestamp > $history);
+        }
+      }
+      else {
+        // Do not track "new replies" status for topics if the user is anonymous.
+        $topic->new_replies = 0;
+        $topic->new = 0;
+      }
+
+      // Make sure only one topic is indicated as the first new topic.
+      $topic->first_new = FALSE;
+      if ($topic->new != 0 && !$first_new_found) {
+        $topic->first_new = TRUE;
+        $first_new_found = TRUE;
+      }
+
+      if ($topic->comment_count > 0) {
+        $last_reply = new \stdClass();
+        $last_reply->created = $topic->last_comment_timestamp;
+        $last_reply->name = $topic->last_comment_name;
+        $last_reply->uid = $topic->last_comment_uid;
+        $topic->last_reply = $last_reply;
+      }
+      $topics[$topic->id()] = $topic;
+    }
+
+    return ['topics' => $topics, 'header' => $header];
+
+  }
+
+  /**
+   * Gets topic sorting information based on an integer code.
+   *
+   * @param int $sortby
+   *   One of the following integers indicating the sort criteria:
+   *   - ForumManager::NEWEST_FIRST: Date - newest first.
+   *   - ForumManager::OLDEST_FIRST: Date - oldest first.
+   *   - ForumManager::MOST_POPULAR_FIRST: Posts with the most comments first.
+   *   - ForumManager::LEAST_POPULAR_FIRST: Posts with the least comments first.
+   *
+   * @return array
+   *   An array with the following values:
+   *   - field: A field for an SQL query.
+   *   - sort: 'asc' or 'desc'.
+   */
+  protected function getTopicOrder($sortby) {
+    switch ($sortby) {
+      case static::NEWEST_FIRST:
+        return ['field' => 'f.last_comment_timestamp', 'sort' => 'desc'];
+
+      case static::OLDEST_FIRST:
+        return ['field' => 'f.last_comment_timestamp', 'sort' => 'asc'];
+
+      case static::MOST_POPULAR_FIRST:
+        return ['field' => 'f.comment_count', 'sort' => 'desc'];
+
+      case static::LEAST_POPULAR_FIRST:
+        return ['field' => 'f.comment_count', 'sort' => 'asc'];
+
+    }
+  }
+
+  /**
+   * Gets the last time the user viewed a node.
+   *
+   * @param int $nid
+   *   The node ID.
+   * @param \Drupal\Core\Session\AccountInterface $account
+   *   Account to fetch last time for.
+   *
+   * @return int
+   *   The timestamp when the user last viewed this node, if the user has
+   *   previously viewed the node; otherwise HISTORY_READ_LIMIT.
+   */
+  protected function lastVisit($nid, AccountInterface $account) {
+    if (empty($this->history[$nid])) {
+      $result = $this->connection->select('history', 'h')
+        ->fields('h', ['nid', 'timestamp'])
+        ->condition('uid', $account->id())
+        ->execute();
+      foreach ($result as $t) {
+        $this->history[$t->nid] = $t->timestamp > HISTORY_READ_LIMIT ? $t->timestamp : HISTORY_READ_LIMIT;
+      }
+    }
+    return $this->history[$nid] ?? HISTORY_READ_LIMIT;
+  }
+
+  /**
+   * Provides the last post information for the given forum tid.
+   *
+   * @param int $tid
+   *   The forum tid.
+   *
+   * @return object
+   *   The last post for the given forum.
+   */
+  protected function getLastPost($tid) {
+    if (!empty($this->lastPostData[$tid])) {
+      return $this->lastPostData[$tid];
+    }
+    // Query "Last Post" information for this forum.
+    $query = $this->connection->select('node_field_data', 'n');
+    $query->join('forum', 'f', '[n].[vid] = [f].[vid] AND [f].[tid] = :tid', [':tid' => $tid]);
+    $query->join('comment_entity_statistics', 'ces', "[n].[nid] = [ces].[entity_id] AND [ces].[field_name] = 'comment_forum' AND [ces].[entity_type] = 'node'");
+    $query->join('users_field_data', 'u', '[ces].[last_comment_uid] = [u].[uid] AND [u].[default_langcode] = 1');
+    $query->addExpression('CASE [ces].[last_comment_uid] WHEN 0 THEN [ces].[last_comment_name] ELSE [u].[name] END', 'last_comment_name');
+
+    $topic = $query
+      ->fields('ces', ['last_comment_timestamp', 'last_comment_uid'])
+      ->condition('n.status', 1)
+      ->orderBy('last_comment_timestamp', 'DESC')
+      ->range(0, 1)
+      ->addTag('node_access')
+      ->execute()
+      ->fetchObject();
+
+    // Build the last post information.
+    $last_post = new \stdClass();
+    if (!empty($topic->last_comment_timestamp)) {
+      $last_post->created = $topic->last_comment_timestamp;
+      $last_post->name = $topic->last_comment_name;
+      $last_post->uid = $topic->last_comment_uid;
+    }
+
+    $this->lastPostData[$tid] = $last_post;
+    return $last_post;
+  }
+
+  /**
+   * Provides statistics for a forum.
+   *
+   * @param int $tid
+   *   The forum tid.
+   *
+   * @return object|null
+   *   Statistics for the given forum if statistics exist, else NULL.
+   */
+  protected function getForumStatistics($tid) {
+    if (empty($this->forumStatistics)) {
+      // Prime the statistics.
+      $query = $this->connection->select('node_field_data', 'n');
+      $query->join('comment_entity_statistics', 'ces', "[n].[nid] = [ces].[entity_id] AND [ces].[field_name] = 'comment_forum' AND [ces].[entity_type] = 'node'");
+      $query->join('forum', 'f', '[n].[vid] = [f].[vid]');
+      $query->addExpression('COUNT([n].[nid])', 'topic_count');
+      $query->addExpression('SUM([ces].[comment_count])', 'comment_count');
+      $this->forumStatistics = $query
+        ->fields('f', ['tid'])
+        ->condition('n.status', 1)
+        ->condition('n.default_langcode', 1)
+        ->groupBy('tid')
+        ->addTag('node_access')
+        ->execute()
+        ->fetchAllAssoc('tid');
+    }
+
+    if (!empty($this->forumStatistics[$tid])) {
+      return $this->forumStatistics[$tid];
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getChildren($vid, $tid) {
+    if (!empty($this->forumChildren[$tid])) {
+      return $this->forumChildren[$tid];
+    }
+    $forums = [];
+    $_forums = $this->entityTypeManager->getStorage('taxonomy_term')->loadTree($vid, $tid, NULL, TRUE);
+    foreach ($_forums as $forum) {
+      if (!$forum->access('view')) {
+        continue;
+      }
+      // Merge in the topic and post counters.
+      if (($count = $this->getForumStatistics($forum->id()))) {
+        $forum->num_topics = $count->topic_count;
+        $forum->num_posts = $count->topic_count + $count->comment_count;
+      }
+      else {
+        $forum->num_topics = 0;
+        $forum->num_posts = 0;
+      }
+
+      // Merge in last post details.
+      $forum->last_post = $this->getLastPost($forum->id());
+      $forums[$forum->id()] = $forum;
+    }
+
+    $this->forumChildren[$tid] = $forums;
+    return $forums;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getIndex() {
+    if ($this->index) {
+      return $this->index;
+    }
+
+    $vid = $this->configFactory->get('forum.settings')->get('vocabulary');
+    $index = $this->entityTypeManager->getStorage('taxonomy_term')->create([
+      'tid' => 0,
+      'container' => 1,
+      'parents' => [],
+      'isIndex' => TRUE,
+      'vid' => $vid,
+    ]);
+
+    // Load the tree below.
+    $index->forums = $this->getChildren($vid, 0);
+    $this->index = $index;
+    return $index;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function resetCache() {
+    // Reset the index.
+    $this->index = NULL;
+    // Reset history.
+    $this->history = [];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function checkNodeType(NodeInterface $node) {
+    // Fetch information about the forum field.
+    $field_definitions = $this->entityFieldManager->getFieldDefinitions('node', $node->bundle());
+    return !empty($field_definitions['taxonomy_forums']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function unreadTopics($term, $uid) {
+    $query = $this->connection->select('node_field_data', 'n');
+    $query->join('forum', 'f', '[n].[vid] = [f].[vid] AND [f].[tid] = :tid', [':tid' => $term]);
+    $query->leftJoin('history', 'h', '[n].[nid] = [h].[nid] AND [h].[uid] = :uid', [':uid' => $uid]);
+    $query->addExpression('COUNT([n].[nid])', 'count');
+    return $query
+      ->condition('status', 1)
+      // @todo This should be actually filtering on the desired node status
+      //   field language and just fall back to the default language.
+      ->condition('n.default_langcode', 1)
+      ->condition('n.created', HISTORY_READ_LIMIT, '>')
+      ->isNull('h.nid')
+      ->addTag('node_access')
+      ->execute()
+      ->fetchField();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __sleep() {
+    $vars = $this->defaultSleep();
+    // Do not serialize static cache.
+    unset($vars['history'], $vars['index'], $vars['lastPostData'], $vars['forumChildren'], $vars['forumStatistics']);
+    return $vars;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function __wakeup() {
+    $this->defaultWakeup();
+    // Initialize static cache.
+    $this->history = [];
+    $this->lastPostData = [];
+    $this->forumChildren = [];
+    $this->forumStatistics = [];
+    $this->index = NULL;
+  }
+
+}

web/core/modules/forum/src/ForumManagerInterface.php

@@ -0,0 +1,80 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\Core\Session\AccountInterface;
+use Drupal\node\NodeInterface;
+
+/**
+ * Provides forum manager interface.
+ */
+interface ForumManagerInterface {
+
+  /**
+   * Gets list of forum topics.
+   *
+   * @param int $tid
+   *   Term ID.
+   * @param \Drupal\Core\Session\AccountInterface $account
+   *   Account to fetch topics for.
+   *
+   * @return array
+   *   Array with keys 'topics' and 'header'.
+   */
+  public function getTopics($tid, AccountInterface $account);
+
+  /**
+   * Utility method to fetch the child forums for a given forum.
+   *
+   * @param int $vid
+   *   The forum vocabulary ID.
+   * @param int $tid
+   *   The forum ID to fetch the children for.
+   *
+   * @return array
+   *   Array of children.
+   */
+  public function getChildren($vid, $tid);
+
+  /**
+   * Generates and returns the forum index.
+   *
+   * The forum index is a pseudo term that provides an overview of all forums.
+   *
+   * @return \Drupal\taxonomy\TermInterface
+   *   A pseudo term representing the overview of all forums.
+   */
+  public function getIndex();
+
+  /**
+   * Resets the ForumManager index and history.
+   */
+  public function resetCache();
+
+  /**
+   * Checks whether a node can be used in a forum, based on its content type.
+   *
+   * @param \Drupal\node\NodeInterface $node
+   *   A node entity.
+   *
+   * @return bool
+   *   Boolean indicating if the node can be assigned to a forum.
+   */
+  public function checkNodeType(NodeInterface $node);
+
+  /**
+   * Calculates the number of new posts in a forum that the user has not yet read.
+   *
+   * Nodes are new if they are newer than HISTORY_READ_LIMIT.
+   *
+   * @param int $term
+   *   The term ID of the forum.
+   * @param int $uid
+   *   The user ID.
+   *
+   * @return int
+   *   The number of new posts in the forum that have not been read by the user.
+   */
+  public function unreadTopics($term, $uid);
+
+}

web/core/modules/forum/src/ForumSettingsForm.php

@@ -0,0 +1,81 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\Core\Form\ConfigFormBase;
+use Drupal\Core\Form\FormStateInterface;
+
+/**
+ * Configure forum settings for this site.
+ *
+ * @internal
+ */
+class ForumSettingsForm extends ConfigFormBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getFormId() {
+    return 'forum_admin_settings';
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEditableConfigNames() {
+    return ['forum.settings'];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function buildForm(array $form, FormStateInterface $form_state) {
+    $config = $this->config('forum.settings');
+
+    $options = [5, 10, 15, 20, 25, 30, 35, 40, 50, 60, 80, 100, 150, 200, 250, 300, 350, 400, 500];
+    $form['forum_hot_topic'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Hot topic threshold'),
+      '#default_value' => $config->get('topics.hot_threshold'),
+      '#options' => array_combine($options, $options),
+      '#description' => $this->t('The number of replies a topic must have to be considered "hot".'),
+    ];
+    $options = [10, 25, 50, 75, 100];
+    $form['forum_per_page'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Topics per page'),
+      '#default_value' => $config->get('topics.page_limit'),
+      '#options' => array_combine($options, $options),
+      '#description' => $this->t('Default number of forum topics displayed per page.'),
+    ];
+    $order = [
+      1 => $this->t('Date - newest first'),
+      2 => $this->t('Date - oldest first'),
+      3 => $this->t('Posts - most active first'),
+      4 => $this->t('Posts - least active first'),
+    ];
+    $form['forum_order'] = [
+      '#type' => 'radios',
+      '#title' => $this->t('Default order'),
+      '#default_value' => $config->get('topics.order'),
+      '#options' => $order,
+      '#description' => $this->t('Default display order for topics.'),
+    ];
+
+    return parent::buildForm($form, $form_state);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function submitForm(array &$form, FormStateInterface $form_state) {
+    $this->config('forum.settings')
+      ->set('topics.hot_threshold', $form_state->getValue('forum_hot_topic'))
+      ->set('topics.page_limit', $form_state->getValue('forum_per_page'))
+      ->set('topics.order', $form_state->getValue('forum_order'))
+      ->save();
+
+    parent::submitForm($form, $form_state);
+  }
+
+}

web/core/modules/forum/src/ForumUninstallValidator.php

@@ -0,0 +1,130 @@
+<?php
+
+namespace Drupal\forum;
+
+use Drupal\Core\Config\ConfigFactoryInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Extension\ModuleUninstallValidatorInterface;
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\Core\StringTranslation\TranslationInterface;
+use Drupal\taxonomy\VocabularyInterface;
+
+/**
+ * Prevents forum module from being uninstalled under certain conditions.
+ *
+ * These conditions are when any forum nodes exist or there are any terms in the
+ * forum vocabulary.
+ */
+class ForumUninstallValidator implements ModuleUninstallValidatorInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The config factory.
+   *
+   * @var \Drupal\Core\Config\ConfigFactoryInterface
+   */
+  protected $configFactory;
+
+  /**
+   * Constructs a new ForumUninstallValidator.
+   *
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
+   *   The config factory.
+   * @param \Drupal\Core\StringTranslation\TranslationInterface $string_translation
+   *   The string translation service.
+   */
+  public function __construct(EntityTypeManagerInterface $entity_type_manager, ConfigFactoryInterface $config_factory, TranslationInterface $string_translation) {
+    $this->entityTypeManager = $entity_type_manager;
+    $this->configFactory = $config_factory;
+    $this->stringTranslation = $string_translation;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($module) {
+    $reasons = [];
+    if ($module == 'forum') {
+      if ($this->hasForumNodes()) {
+        $reasons[] = $this->t('To uninstall Forum, first delete all <em>Forum</em> content');
+      }
+
+      $vocabulary = $this->getForumVocabulary();
+      if (!empty($vocabulary) && $this->hasTermsForVocabulary($vocabulary)) {
+        if ($vocabulary->access('view')) {
+          $reasons[] = $this->t('To uninstall Forum, first delete all <a href=":url">%vocabulary</a> terms', [
+            '%vocabulary' => $vocabulary->label(),
+            ':url' => $vocabulary->toUrl('overview-form')->toString(),
+          ]);
+        }
+        else {
+          $reasons[] = $this->t('To uninstall Forum, first delete all %vocabulary terms', [
+            '%vocabulary' => $vocabulary->label(),
+          ]);
+        }
+      }
+    }
+
+    return $reasons;
+  }
+
+  /**
+   * Determines if there are any forum nodes or not.
+   *
+   * @return bool
+   *   TRUE if there are forum nodes, FALSE otherwise.
+   */
+  protected function hasForumNodes() {
+    $nodes = $this->entityTypeManager->getStorage('node')->getQuery()
+      ->condition('type', 'forum')
+      ->accessCheck(FALSE)
+      ->range(0, 1)
+      ->execute();
+    return !empty($nodes);
+  }
+
+  /**
+   * Determines if there are any taxonomy terms for a specified vocabulary.
+   *
+   * @param \Drupal\taxonomy\VocabularyInterface $vocabulary
+   *   The vocabulary to check for terms.
+   *
+   * @return bool
+   *   TRUE if there are terms for this vocabulary, FALSE otherwise.
+   */
+  protected function hasTermsForVocabulary(VocabularyInterface $vocabulary) {
+    $terms = $this->entityTypeManager->getStorage('taxonomy_term')->getQuery()
+      ->condition('vid', $vocabulary->id())
+      ->accessCheck(FALSE)
+      ->range(0, 1)
+      ->execute();
+    return !empty($terms);
+  }
+
+  /**
+   * Returns the vocabulary configured for forums.
+   *
+   * @return \Drupal\taxonomy\VocabularyInterface
+   *   The vocabulary entity for forums.
+   */
+  protected function getForumVocabulary() {
+    $vid = $this->configFactory->get('forum.settings')->get('vocabulary');
+    if (!empty($vid)) {
+      return $this->entityTypeManager->getStorage('taxonomy_vocabulary')->load($vid);
+    }
+    else {
+      return NULL;
+    }
+  }
+
+}

web/core/modules/forum/src/Plugin/Block/ActiveTopicsBlock.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\forum\Plugin\Block;
+
+use Drupal\Core\Block\Attribute\Block;
+use Drupal\Core\Database\Database;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Provides an 'Active forum topics' block.
+ */
+#[Block(
+  id: "forum_active_block",
+  admin_label: new TranslatableMarkup("Active forum topics"),
+  category: new TranslatableMarkup("Lists (Views)")
+)]
+class ActiveTopicsBlock extends ForumBlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildForumQuery() {
+    return Database::getConnection()->select('forum_index', 'f')
+      ->fields('f')
+      ->addTag('node_access')
+      ->addMetaData('base_table', 'forum_index')
+      ->orderBy('f.last_comment_timestamp', 'DESC')
+      ->range(0, $this->configuration['block_count']);
+  }
+
+}

web/core/modules/forum/src/Plugin/Block/ForumBlockBase.php

@@ -0,0 +1,96 @@
+<?php
+
+namespace Drupal\forum\Plugin\Block;
+
+use Drupal\Core\Access\AccessResult;
+use Drupal\Core\Block\BlockBase;
+use Drupal\Core\Cache\Cache;
+use Drupal\Core\Form\FormStateInterface;
+use Drupal\Core\Session\AccountInterface;
+use Drupal\Core\Url;
+
+/**
+ * Provides a base class for Forum blocks.
+ */
+abstract class ForumBlockBase extends BlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function build() {
+    $result = $this->buildForumQuery()->execute();
+    $elements = [];
+    if ($node_title_list = node_title_list($result)) {
+      $elements['forum_list'] = $node_title_list;
+      $elements['forum_more'] = [
+        '#type' => 'more_link',
+        '#url' => Url::fromRoute('forum.index'),
+        '#attributes' => ['title' => $this->t('Read the latest forum topics.')],
+      ];
+    }
+    return $elements;
+  }
+
+  /**
+   * Builds the select query to use for this forum block.
+   *
+   * @return \Drupal\Core\Database\Query\Select
+   *   A Select object.
+   */
+  abstract protected function buildForumQuery();
+
+  /**
+   * {@inheritdoc}
+   */
+  public function defaultConfiguration() {
+    return [
+      'properties' => [
+        'administrative' => TRUE,
+      ],
+      'block_count' => 5,
+    ];
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function blockAccess(AccountInterface $account) {
+    return AccessResult::allowedIfHasPermission($account, 'access content');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockForm($form, FormStateInterface $form_state) {
+    $range = range(2, 20);
+    $form['block_count'] = [
+      '#type' => 'select',
+      '#title' => $this->t('Number of topics'),
+      '#default_value' => $this->configuration['block_count'],
+      '#options' => array_combine($range, $range),
+    ];
+    return $form;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function blockSubmit($form, FormStateInterface $form_state) {
+    $this->configuration['block_count'] = $form_state->getValue('block_count');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheContexts() {
+    return Cache::mergeContexts(parent::getCacheContexts(), ['user.node_grants:view']);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getCacheTags() {
+    return Cache::mergeTags(parent::getCacheTags(), ['node_list']);
+  }
+
+}

web/core/modules/forum/src/Plugin/Block/NewTopicsBlock.php

@@ -0,0 +1,31 @@
+<?php
+
+namespace Drupal\forum\Plugin\Block;
+
+use Drupal\Core\Block\Attribute\Block;
+use Drupal\Core\Database\Database;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+
+/**
+ * Provides a 'New forum topics' block.
+ */
+#[Block(
+  id: "forum_new_block",
+  admin_label: new TranslatableMarkup("New forum topics"),
+  category: new TranslatableMarkup("Lists (Views)")
+)]
+class NewTopicsBlock extends ForumBlockBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function buildForumQuery() {
+    return Database::getConnection()->select('forum_index', 'f')
+      ->fields('f')
+      ->addTag('node_access')
+      ->addMetaData('base_table', 'forum_index')
+      ->orderBy('f.created', 'DESC')
+      ->range(0, $this->configuration['block_count']);
+  }
+
+}

web/core/modules/forum/src/Plugin/Validation/Constraint/ForumLeafConstraint.php

@@ -0,0 +1,21 @@
+<?php
+
+namespace Drupal\forum\Plugin\Validation\Constraint;
+
+use Drupal\Core\StringTranslation\TranslatableMarkup;
+use Drupal\Core\Validation\Attribute\Constraint;
+use Symfony\Component\Validator\Constraint as SymfonyConstraint;
+
+/**
+ * Checks that the node is assigned only a "leaf" term in the forum taxonomy.
+ */
+#[Constraint(
+  id: 'ForumLeaf',
+  label: new TranslatableMarkup('Forum leaf', [], ['context' => 'Validation'])
+)]
+class ForumLeafConstraint extends SymfonyConstraint {
+
+  public $selectForum = 'Select a forum.';
+  public $noLeafMessage = 'The item %forum is a forum container, not a forum. Select one of the forums below instead.';
+
+}

web/core/modules/forum/src/Plugin/Validation/Constraint/ForumLeafConstraintValidator.php

@@ -0,0 +1,33 @@
+<?php
+
+namespace Drupal\forum\Plugin\Validation\Constraint;
+
+use Symfony\Component\Validator\Constraint;
+use Symfony\Component\Validator\ConstraintValidator;
+
+/**
+ * Validates the ForumLeaf constraint.
+ */
+class ForumLeafConstraintValidator extends ConstraintValidator {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validate($items, Constraint $constraint) {
+    $item = $items->first();
+    if (!isset($item)) {
+      return NULL;
+    }
+
+    // Verify that a term has been selected.
+    if (!$item->entity) {
+      $this->context->addViolation($constraint->selectForum);
+    }
+
+    // The forum_container flag must not be set.
+    if (!empty($item->entity->forum_container->value)) {
+      $this->context->addViolation($constraint->noLeafMessage, ['%forum' => $item->entity->getName()]);
+    }
+  }
+
+}

web/core/modules/forum/src/Plugin/migrate/process/ForumVocabulary.php

@@ -0,0 +1,44 @@
+<?php
+
+namespace Drupal\forum\Plugin\migrate\process;
+
+use Drupal\migrate\Attribute\MigrateProcess;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\Row;
+
+/**
+ * Checks if the vocabulary being migrated is the one used for forums.
+ *
+ * Drupal 8 Forum is expecting specific machine names for its field and
+ * vocabulary names. This process plugin forces a given machine name to the
+ * field or vocabulary that is being migrated.
+ *
+ * The 'forum_vocabulary' source property is evaluated in the
+ * d6_taxonomy_vocabulary or d7_taxonomy_vocabulary source plugins and is set to
+ * true if the vocabulary vid being migrated is the same as the one in the
+ * 'forum_nav_vocabulary' variable on the source site.
+ *
+ * Example:
+ *
+ * @code
+ * process:
+ *   field_name:
+ *     plugin: forum_vocabulary
+ *     machine_name: taxonomy_forums
+ * @endcode
+ */
+#[MigrateProcess('forum_vocabulary')]
+class ForumVocabulary extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    if ($row->getSourceProperty('forum_vocabulary') && !empty($this->configuration['machine_name'])) {
+      $value = $this->configuration['machine_name'];
+    }
+    return $value;
+  }
+
+}

web/core/modules/forum/src/ProxyClass/ForumUninstallValidator.php

@@ -0,0 +1,88 @@
+<?php
+// phpcs:ignoreFile
+
+/**
+ * This file was generated via php core/scripts/generate-proxy-class.php 'Drupal\forum\ForumUninstallValidator' "core/modules/forum/src".
+ */
+
+namespace Drupal\forum\ProxyClass {
+
+    /**
+     * Provides a proxy class for \Drupal\forum\ForumUninstallValidator.
+     *
+     * @see \Drupal\Component\ProxyBuilder
+     */
+    class ForumUninstallValidator implements \Drupal\Core\Extension\ModuleUninstallValidatorInterface
+    {
+
+        use \Drupal\Core\DependencyInjection\DependencySerializationTrait;
+
+        /**
+         * The id of the original proxied service.
+         *
+         * @var string
+         */
+        protected $drupalProxyOriginalServiceId;
+
+        /**
+         * The real proxied service, after it was lazy loaded.
+         *
+         * @var \Drupal\forum\ForumUninstallValidator
+         */
+        protected $service;
+
+        /**
+         * The service container.
+         *
+         * @var \Symfony\Component\DependencyInjection\ContainerInterface
+         */
+        protected $container;
+
+        /**
+         * Constructs a ProxyClass Drupal proxy object.
+         *
+         * @param \Symfony\Component\DependencyInjection\ContainerInterface $container
+         *   The container.
+         * @param string $drupal_proxy_original_service_id
+         *   The service ID of the original service.
+         */
+        public function __construct(\Symfony\Component\DependencyInjection\ContainerInterface $container, $drupal_proxy_original_service_id)
+        {
+            $this->container = $container;
+            $this->drupalProxyOriginalServiceId = $drupal_proxy_original_service_id;
+        }
+
+        /**
+         * Lazy loads the real service from the container.
+         *
+         * @return object
+         *   Returns the constructed real service.
+         */
+        protected function lazyLoadItself()
+        {
+            if (!isset($this->service)) {
+                $this->service = $this->container->get($this->drupalProxyOriginalServiceId);
+            }
+
+            return $this->service;
+        }
+
+        /**
+         * {@inheritdoc}
+         */
+        public function validate($module)
+        {
+            return $this->lazyLoadItself()->validate($module);
+        }
+
+        /**
+         * {@inheritdoc}
+         */
+        public function setStringTranslation(\Drupal\Core\StringTranslation\TranslationInterface $translation)
+        {
+            return $this->lazyLoadItself()->setStringTranslation($translation);
+        }
+
+    }
+
+}