parallel 多线程函数中返回，出现错误 caller must retain reference to Future

有点不懂这是怎么回事

<?php
use parallel\Runtime;
use parallel\Channel;

$thread_function = function (Channel $ch) {
	echo "test1\n";

	return;

	$ch->send("test2\n");
};

$r1 = new Runtime();
$ch1 = new Channel();

$args = array();
$args[0] = $ch1;
$r1->run($thread_function, $args);

echo $ch1->recv();

$ch1->close();

PHP Fatal error: Uncaught parallel\Runtime\Error\IllegalReturn: return on line 3 of task ignored by caller, caller must retain reference to Future in D:\Desktop\test.php:18

请问如何在该函数中中断执行？