Issue #3202211 by joachim: Added null queue backend

<?php

namespace Drupal\advancedqueue\Plugin\AdvancedQueue\Backend;

use Drupal\advancedqueue\Job;

use Drupal\Component\Datetime\TimeInterface;

use Drupal\Core\Database\Connection;

use Symfony\Component\DependencyInjection\ContainerInterface;

/**

 * Provides the null queue backend.

 *

 * This does not store jobs, and no jobs are ever available to be claimed.

 *

 * @AdvancedQueueBackend(

 *   id = "null_backend",

 *   label = @Translation("Null"),

 * )

 */

class NullBackend extends BackendBase implements SupportsDeletingJobsInterface, SupportsListingJobsInterface, SupportsReleasingJobsInterface {

  /**

   * {@inheritdoc}

   */

  public function createQueue() { }

  /**

   * {@inheritdoc}

   */

  public function deleteQueue() { }

  /**

   * {@inheritdoc}

   */

  public function cleanupQueue() {}

  /**

   * {@inheritdoc}

   */

  public function countJobs() {

    return [

      Job::STATE_QUEUED => 0,

      Job::STATE_PROCESSING => 0,

      Job::STATE_SUCCESS => 0,

      Job::STATE_FAILURE => 0,

    ];

  }

  /**

   * {@inheritdoc}

   */

  public function enqueueJob(Job $job, $delay = 0) { }

  /**

   * {@inheritdoc}

   */

  public function enqueueJobs(array $jobs, $delay = 0) { }

  /**

   * {@inheritdoc}

   */

  public function retryJob(Job $job, $delay = 0) { }

  /**

   * {@inheritdoc}

   */

  public function claimJob() {

    // Jobs disappear into a void: there are no jobs to queue.

  }

  /**

   * {@inheritdoc}

   */

  public function onSuccess(Job $job) { }

  /**

   * {@inheritdoc}

   */

  public function onFailure(Job $job) { }

  /**

   * {@inheritdoc}

   */

  public function releaseJob($job_id) { }

  /**

   * {@inheritdoc}

   */

  public function deleteJob($job_id) { }

}