CommentForm::save

public CommentForm::save(array $form, FormStateInterface $form_state)

public function save(array $form, FormStateInterface $form_state) {
  $comment = $this->entity;
  $entity = $comment->getCommentedEntity();
  $field_name = $comment->getFieldName();
  $uri = $entity->urlInfo();
  $logger = $this->logger('content');

  if ($this->currentUser->hasPermission('post comments') && ($this->currentUser->hasPermission('administer comments') || $entity->{$field_name}->status == CommentItemInterface::OPEN)) {
    $comment->save();
    $form_state->setValue('cid', $comment->id());

    // Add a log entry.
    $logger->notice('Comment posted: %subject.', array(
      '%subject' => $comment->getSubject(),
      'link' => $this->l(t('View'), $comment->urlInfo()->setOption('fragment', 'comment-' . $comment->id()))
    ));

    // Explain the approval queue if necessary.
    if (!$comment->isPublished()) {
      if (!$this->currentUser->hasPermission('administer comments')) {
        drupal_set_message($this->t('Your comment has been queued for review by site administrators and will be published after approval.'));
      }
    }
    else {
      drupal_set_message($this->t('Your comment has been posted.'));
    }
    $query = array();
    // Find the current display page for this comment.
    $field_definition = $this->entityManager->getFieldDefinitions($entity->getEntityTypeId(), $entity->bundle())[$field_name];
    $page = $this->entityManager->getStorage('comment')->getDisplayOrdinal($comment, $field_definition->getSetting('default_mode'), $field_definition->getSetting('per_page'));
    if ($page > 0) {
      $query['page'] = $page;
    }
    // Redirect to the newly posted comment.
    $uri->setOption('query', $query);
    $uri->setOption('fragment', 'comment-' . $comment->id());
  }
  else {
    $logger->warning('Comment: unauthorized comment submitted or comment submitted to a closed post %subject.', array('%subject' => $comment->getSubject()));
    drupal_set_message($this->t('Comment: unauthorized comment submitted or comment submitted to a closed post %subject.', array('%subject' => $comment->getSubject())), 'error');
    // Redirect the user to the entity they are commenting on.
  }
  $form_state->setRedirectUrl($uri);
}