class DatabaseQueue

class DatabaseQueue implements ReliableQueueInterface, QueueGarbageCollectionInterface {
    use DependencySerializationTrait;
    
    /**
     * The database table name.
     */
    const TABLE_NAME = 'queue';
    
    /**
     * The name of the queue this instance is working with.
     *
     * @var string
     */
    protected $name;
    
    /**
     * The database connection.
     *
     * @var \Drupal\Core\Database\Connection
     */
    protected $connection;
    
    /**
     * Constructs a \Drupal\Core\Queue\DatabaseQueue object.
     *
     * @param string $name
     *   The name of the queue.
     * @param \Drupal\Core\Database\Connection $connection
     *   The Connection object containing the key-value tables.
     */
    public function __construct($name, Connection $connection) {
        $this->name = $name;
        $this->connection = $connection;
    }
    
    /**
     * {@inheritdoc}
     */
    public function createItem($data) {
        $try_again = FALSE;
        try {
            $id = $this->doCreateItem($data);
        } catch (\Exception $e) {
            // If there was an exception, try to create the table.
            if (!($try_again = $this->ensureTableExists())) {
                // If the exception happened for other reason than the missing table,
                // propagate the exception.
                throw $e;
            }
        }
        // Now that the table has been created, try again if necessary.
        if ($try_again) {
            $id = $this->doCreateItem($data);
        }
        return $id;
    }
    
    /**
     * Adds a queue item and store it directly to the queue.
     *
     * @param $data
     *   Arbitrary data to be associated with the new task in the queue.
     *
     * @return
     *   A unique ID if the item was successfully created and was (best effort)
     *   added to the queue, otherwise FALSE. We don't guarantee the item was
     *   committed to disk etc, but as far as we know, the item is now in the
     *   queue.
     */
    protected function doCreateItem($data) {
        $query = $this->connection
            ->insert(static::TABLE_NAME)
            ->fields([
            'name' => $this->name,
            'data' => serialize($data),
            // We cannot rely on REQUEST_TIME because many items might be created
            // by a single request which takes longer than 1 second.
'created' => time(),
        ]);
        // Return the new serial ID, or FALSE on failure.
        return $query->execute();
    }
    
    /**
     * {@inheritdoc}
     */
    public function numberOfItems() {
        try {
            return (int) $this->connection
                ->query('SELECT COUNT(item_id) FROM {' . static::TABLE_NAME . '} WHERE name = :name', [
                ':name' => $this->name,
            ])
                ->fetchField();
        } catch (\Exception $e) {
            $this->catchException($e);
            // If there is no table there cannot be any items.
            return 0;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function claimItem($lease_time = 30) {
        // Claim an item by updating its expire fields. If claim is not successful
        // another thread may have claimed the item in the meantime. Therefore loop
        // until an item is successfully claimed or we are reasonably sure there
        // are no unclaimed items left.
        while (TRUE) {
            try {
                $item = $this->connection
                    ->queryRange('SELECT data, created, item_id FROM {' . static::TABLE_NAME . '} q WHERE expire = 0 AND name = :name ORDER BY created, item_id ASC', 0, 1, [
                    ':name' => $this->name,
                ])
                    ->fetchObject();
            } catch (\Exception $e) {
                $this->catchException($e);
            }
            // If the table does not exist there are no items currently available to
            // claim.
            if (empty($item)) {
                return FALSE;
            }
            // Try to update the item. Only one thread can succeed in UPDATEing the
            // same row. We cannot rely on REQUEST_TIME because items might be
            // claimed by a single consumer which runs longer than 1 second. If we
            // continue to use REQUEST_TIME instead of the current time(), we steal
            // time from the lease, and will tend to reset items before the lease
            // should really expire.
            $update = $this->connection
                ->update(static::TABLE_NAME)
                ->fields([
                'expire' => time() + $lease_time,
            ])
                ->condition('item_id', $item->item_id)
                ->condition('expire', 0);
            // If there are affected rows, this update succeeded.
            if ($update->execute()) {
                $item->data = unserialize($item->data);
                return $item;
            }
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function releaseItem($item) {
        try {
            $update = $this->connection
                ->update(static::TABLE_NAME)
                ->fields([
                'expire' => 0,
            ])
                ->condition('item_id', $item->item_id);
            return $update->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
            // If the table doesn't exist we should consider the item released.
            return TRUE;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteItem($item) {
        try {
            $this->connection
                ->delete(static::TABLE_NAME)
                ->condition('item_id', $item->item_id)
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function createQueue() {
        // All tasks are stored in a single database table (which is created on
        // demand) so there is nothing we need to do to create a new queue.
    }
    
    /**
     * {@inheritdoc}
     */
    public function deleteQueue() {
        try {
            $this->connection
                ->delete(static::TABLE_NAME)
                ->condition('name', $this->name)
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function garbageCollection() {
        try {
            // Clean up the queue for failed batches.
            $this->connection
                ->delete(static::TABLE_NAME)
                ->condition('created', REQUEST_TIME - 864000, '<')
                ->condition('name', 'drupal_batch:%', 'LIKE')
                ->execute();
            // Reset expired items in the default queue implementation table. If that's
            // not used, this will simply be a no-op.
            $this->connection
                ->update(static::TABLE_NAME)
                ->fields([
                'expire' => 0,
            ])
                ->condition('expire', 0, '<>')
                ->condition('expire', REQUEST_TIME, '<')
                ->execute();
        } catch (\Exception $e) {
            $this->catchException($e);
        }
    }
    
    /**
     * Check if the table exists and create it if not.
     */
    protected function ensureTableExists() {
        try {
            $database_schema = $this->connection
                ->schema();
            if (!$database_schema->tableExists(static::TABLE_NAME)) {
                $schema_definition = $this->schemaDefinition();
                $database_schema->createTable(static::TABLE_NAME, $schema_definition);
                return TRUE;
            }
        } catch (DatabaseException $e) {
            return TRUE;
        }
        return FALSE;
    }
    
    /**
     * Act on an exception when queue might be stale.
     *
     * If the table does not yet exist, that's fine, but if the table exists and
     * yet the query failed, then the queue is stale and the exception needs to
     * propagate.
     *
     * @param $e
     *   The exception.
     *
     * @throws \Exception
     *   If the table exists the exception passed in is rethrown.
     */
    protected function catchException(\Exception $e) {
        if ($this->connection
            ->schema()
            ->tableExists(static::TABLE_NAME)) {
            throw $e;
        }
    }
    
    /**
     * Defines the schema for the queue table.
     *
     * @internal
     */
    public function schemaDefinition() {
        return [
            'description' => 'Stores items in queues.',
            'fields' => [
                'item_id' => [
                    'type' => 'serial',
                    'unsigned' => TRUE,
                    'not null' => TRUE,
                    'description' => 'Primary Key: Unique item ID.',
                ],
                'name' => [
                    'type' => 'varchar_ascii',
                    'length' => 255,
                    'not null' => TRUE,
                    'default' => '',
                    'description' => 'The queue name.',
                ],
                'data' => [
                    'type' => 'blob',
                    'not null' => FALSE,
                    'size' => 'big',
                    'serialize' => TRUE,
                    'description' => 'The arbitrary data for the item.',
                ],
                'expire' => [
                    'type' => 'int',
                    'not null' => TRUE,
                    'default' => 0,
                    'description' => 'Timestamp when the claim lease expires on the item.',
                ],
                'created' => [
                    'type' => 'int',
                    'not null' => TRUE,
                    'default' => 0,
                    'description' => 'Timestamp when the item was created.',
                ],
            ],
            'primary key' => [
                'item_id',
            ],
            'indexes' => [
                'name_created' => [
                    'name',
                    'created',
                ],
                'expire' => [
                    'expire',
                ],
            ],
        ];
    }

}