* Save some memory by moving things one by one to the array of items being returned, instead of
			 * unserializing all and then merging them with other items.
			 *
			 * PHPCS ignore is because this is the expected behavior - we're assigning a variable in the condition part of the loop.
			 */
			// phpcs:ignore Generic.CodeAnalysis.AssignmentInCondition.FoundInWhileCondition
			while ( ( $current_item = array_shift( $current_items ) ) !== null ) {
				// @codingStandardsIgnoreStart
				$current_item->value = unserialize( $current_item->value );
				// @codingStandardsIgnoreEnd

				$items[] = $current_item;
			}
		}

		if ( count( $items ) === 0 ) {
			$this->delete_checkout_id();

			return false;
		}

		return new Queue_Buffer( $buffer_id, $items );
	}

	/**
	 * Check in the queue.
	 *
	 * @param Automattic\Jetpack\Sync\Queue_Buffer $buffer Queue_Buffer object.
	 *
	 * @return bool|\WP_Error
	 */
	public function checkin( $buffer ) {
		$is_valid = $this->validate_checkout( $buffer );

		if ( is_wp_error( $is_valid ) ) {
			return $is_valid;
		}

		$this->delete_checkout_id();

		return true;
	}

	/**
	 * Close the buffer.
	 *
	 * @param Automattic\Jetpack\Sync\Queue_Buffer $buffer Queue_Buffer object.
	 * @param null|array                           $ids_to_remove Ids to remove from the queue.
	 *
	 * @return bool|\WP_Error
	 */
	public function close( $buffer, $ids_to_remove = null ) {
		$is_valid = $this->validate_checkout( $buffer );

		if ( is_wp_error( $is_valid ) ) {
			// Always delete ids_to_remove even when buffer is no longer checked-out.
			// They were processed by WP.com so safe to remove from queue.
			if ( $ids_to_remove !== null ) {
				$this->delete( $ids_to_remove );
			}
			return $is_valid;
		}

		$this->delete_checkout_id();

		// By default clear all items in the buffer.
		if ( $ids_to_remove === null ) {
			$ids_to_remove = $buffer->get_item_ids();
		}

		$this->delete( $ids_to_remove );

		return true;
	}

	/**
	 * Delete elements from the queue.
	 *
	 * @param array $ids Ids to delete.
	 *
	 * @return bool|int
	 */
	private function delete( $ids ) {
		if ( array() === $ids ) {
			return 0;
		}

		$this->queue_storage->delete_items_by_ids( $ids );

		return true;
	}

	/**
	 * Flushes all items from the queue.
	 *
	 * @return array
	 */
	public function flush_all() {
		$items = Utils::get_item_values( $this->fetch_items() );
		$this->reset();

		return $items;
	}

	/**
	 * Get all the items from the queue.
	 *
	 * @return array|object|null
	 */
	public function get_all() {
		return $this->fetch_items();
	}

	/**
	 * Forces Checkin of the queue.
	 * Use with caution, this could allow multiple processes to delete
	 * and send from the queue at the same time
	 */
	public function force_checkin() {
		$this->delete_checkout_id();
	}

	/**
	 * Checks if the queue is locked.
	 *
	 * @return bool
	 */
	public function is_locked() {
		return (bool) $this->get_checkout_id();
	}

	/**
	 * Locks checkouts from the queue
	 * tries to wait up to $timeout seconds for the queue to be empty.
	 *
	 * @param int $timeout The wait time in seconds for the queue to be empty.
	 *
	 * @return bool|int|\WP_Error
	 */
	public function lock( $timeout = 30 ) {
		$tries = 0;

		while ( $this->has_any_items() && $tries < $timeout ) {
			sleep( 1 );
			++$tries;
		}

		if ( 30 === $tries ) {
			return new WP_Error( 'lock_timeout', 'Timeout waiting for sync queue to empty' );
		}

		if ( $this->get_checkout_id() ) {
			return new WP_Error( 'unclosed_buffer', 'There is an unclosed buffer' );
		}

		// Hopefully this means we can acquire a checkout?
		$result = $this->set_checkout_id( 'lock' );

		if ( ! $result || is_wp_error( $result ) ) {
			return $result;
		}

		return true;
	}

	/**
	 * Unlocks the queue.
	 *
	 * @return bool|int
	 */
	public function unlock() {
		return $this->delete_checkout_id();
	}

	/**
	 * This option is specifically chosen to, as much as possible, preserve time order
	 * and minimise the possibility of collisions between multiple processes working
	 * at the same time.
	 *
	 * @return string
	 */
	protected function generate_option_name_timestamp() {
		return sprintf( '%.6f', microtime( true ) );
	}

	/**
	 * Gets the checkout ID.
	 *
	 * @return bool|string
	 */
	private function get_checkout_id() {
		global $wpdb;
		$checkout_value = $wpdb->get_var(
			$wpdb->prepare(
				"SELECT option_value FROM $wpdb->options WHERE option_name = %s",
				$this->get_lock_option_name()
			)
		);

		if ( $checkout_value ) {
			list( $checkout_id, $timestamp ) = explode( ':', $checkout_value );
			if ( (int) $timestamp > time() ) {
				return $checkout_id;
			}
		}

		return false;
	}

	/**
	 * Sets the checkout id.
	 *
	 * @param string $checkout_id The ID of the checkout.
	 *
	 * @return bool|int
	 */
	private function set_checkout_id( $checkout_id ) {
		global $wpdb;

		$expires     = time() + Defaults::$default_sync_queue_lock_timeout;
		$updated_num = $wpdb->query(
			$wpdb->prepare(
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s",
				"$checkout_id:$expires",
				$this->get_lock_option_name()
			)
		);

		if ( ! $updated_num ) {
			$updated_num = $wpdb->query(
				$wpdb->prepare(
					"INSERT INTO $wpdb->options ( option_name, option_value, autoload ) VALUES ( %s, %s, 'no' )",
					$this->get_lock_option_name(),
					"$checkout_id:$expires"
				)
			);
		}

		return $updated_num;
	}

	/**
	 * Deletes the checkout ID.
	 *
	 * @return bool|int
	 */
	private function delete_checkout_id() {
		global $wpdb;
		// Rather than delete, which causes fragmentation, we update in place.
		return $wpdb->query(
			$wpdb->prepare(
				"UPDATE $wpdb->options SET option_value = %s WHERE option_name = %s",
				'0:0',
				$this->get_lock_option_name()
			)
		);
	}

	/**
	 * Return the lock option name.
	 *
	 * @return string
	 */
	private function get_lock_option_name() {
		return "jpsq_{$this->id}_checkout";
	}

	/**
	 * Return the next data row option name.
	 *
	 * @return string
	 */
	private function get_next_data_row_option_name() {
		$timestamp = $this->generate_option_name_timestamp();

		// Row iterator is used to avoid collisions where we're writing data waaay fast in a single process.
		if ( PHP_INT_MAX === $this->row_iterator ) {
			$this->row_iterator = 0;
		} else {
			$this->row_iterator += 1;
		}

		return 'jpsq_' . $this->id . '-' . $timestamp . '-' . $this->random_int . '-' . $this->row_iterator;
	}

	/**
	 * Return the items in the queue.
	 *
	 * @param null|int $limit Limit to the number of items we fetch at once.
	 *
	 * @return array|object|null
	 */
	private function fetch_items( $limit = null ) {
		$items = $this->queue_storage->fetch_items( $limit );

		return $this->unserialize_values( $items );
	}

	/**
	 * Return items with specific ids.
	 *
	 * @param array $items_ids Array of event ids.
	 *
	 * @return array|object|null
	 */
	private function fetch_items_by_id( $items_ids ) {
		return $this->unserialize_values( $this->queue_storage->fetch_items_by_ids( $items_ids ) );
	}

	/**
	 * Unserialize item values.
	 *
	 * @param array $items Events from the Queue to be unserialized.
	 *
	 * @return mixed
	 */
	private function unserialize_values( $items ) {
		array_walk(
			$items,
			function ( $item ) {
				// @codingStandardsIgnoreStart
				$item->value = @unserialize( $item->value );
				// @codingStandardsIgnoreEnd
			}
		);

		return $items;
	}

	/**
	 * Return true if the buffer is still valid or an Error other wise.
	 *
	 * @param Automattic\Jetpack\Sync\Queue_Buffer $buffer The Queue_Buffer.
	 *
	 * @return bool|WP_Error
	 */
	private function validate_checkout( $buffer ) {
		if ( ! $buffer instanceof Queue_Buffer ) {
			return new WP_Error( 'not_a_buffer', 'You must checkin an instance of Automattic\\Jetpack\\Sync\\Queue_Buffer' );
		}

		$checkout_id = $this->get_checkout_id();

		if ( ! $checkout_id ) {
			return new WP_Error( 'buffer_not_checked_out', 'There are no checked out buffers' );
		}

		// TODO: change to strict comparison.
		if ( $checkout_id != $buffer->id ) { // phpcs:ignore Universal.Operators.StrictComparisons.LooseNotEqual
			return new WP_Error( 'buffer_mismatch', 'The buffer you checked in was not checked out' );
		}

		return true;
	}
}