#3476814: Handle nulled duplicate jobs

• /tests/modules/advancedqueue_test/src/Plugin/AdvancedQueue/JobType/AvoidDuplicates.php
• /src/Entity/Queue.php
• /tests/modules/advancedqueue_test/src/Plugin/AdvancedQueue/JobType/RejectDuplicates.php
• /tests/modules/advancedqueue_test/src/Plugin/AdvancedQueue/JobType/SkipDuplicates.php
• /tests/src/Kernel/DatabaseBackendTest.php

Closes #3476814

src/Entity/Queue.php

@@ -139,7 +139,9 @@ class Queue extends ConfigEntityBase implements QueueInterface {
    */
   public function enqueueJob(Job $job, $delay = 0) {
     $job = $this->prepareJob($job);
-    return $this->getBackend()->enqueueJob($job, $delay);
+    if ($job) {
+      $this->getBackend()->enqueueJob($job, $delay);
+    }
   }
 
   /**
@@ -148,24 +150,30 @@ class Queue extends ConfigEntityBase implements QueueInterface {
   public function enqueueJobs(array $jobs, $delay = 0) {
     $prepared_jobs = [];
     foreach ($jobs as $job) {
-      $prepared_jobs[] = $this->prepareJob($job);
+      if ($prepared_job = $this->prepareJob($job)) {
+        $prepared_jobs[] = $prepared_job;
+      }
+    }
+    if ($prepared_jobs) {
+      $this->getBackend()->enqueueJobs($prepared_jobs, $delay);
     }
-    return $this->getBackend()->enqueueJobs($prepared_jobs, $delay);
   }
 
   /**
    * Prepare a job for enqueueing.
+   * 
+   * If a null is returned, this is an instruction to discard the job.
    *
    * @param \Drupal\advancedqueue\Job $job
    *   The job to prepare.
    *
-   * @return \Drupal\advancedqueue\Job
+   * @return \Drupal\advancedqueue\Job|null
    *   A job to enqueue. This may or may not be the $job object supplied.
    *
    * @throws \Drupal\advancedqueue\Exception\InvalidBackendException
    *   Throws an exception if the queue's backend cannot handle the given job.
    */
-  protected function prepareJob(Job $job): Job {
+  protected function prepareJob(Job $job): ?Job {
     $job->setQueueId($this->id());
     $job_type_manager = \Drupal::service('plugin.manager.advancedqueue_job_type');
     if (!$job_type_manager->getDefinition($job->getType())['allow_duplicates']) {