Issue #3128763 :Unique queue.

Closes #3128763

src/Queue/UniquePhpRedis.php

+<?php
+
+namespace Drupal\redis\Queue;
+
+/**
+ * Unique Redis queue implementation using PhpRedis extension backend.
+ *
+ * @ingroup queue
+ */
+class UniquePhpRedis extends PhpRedis {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function createItem($data) {
+    $record = new \stdClass();
+    $record->data = $data;
+    $record->qid = md5(serialize($data));
+
+    // We cannot rely on REQUEST_TIME because many items might be created
+    // by a single request which takes longer than 1 second.
+    $record->timestamp = time();
+
+    if (!$this->client->hsetnx($this->availableItems, $record->qid, serialize($record))) {
+      return FALSE;
+    }
+
+    $start_len = $this->client->lLen($this->availableListKey);
+    if ($start_len < $this->client->lpush($this->availableListKey, $record->qid)) {
+      return $record->qid;
+    }
+
+    return FALSE;
+  }
+
+}