PHP Parallel Extension: Worker in pool exited unexpectedly

[dercoder]

Issue:

PHP show warnings when the pool size is not big enough. This happens only with the php parallel extension.

I create a small repository for testing:

https://github.com/dercoder/php-amphp-parallel-issue

Example:

<?php

require_once 'vendor/autoload.php';

use Amp\Loop;
use Amp\Parallel\Worker\DefaultPool;

use function Amp\ParallelFunctions\parallelMap;

Loop::run(
    function () {
        $pool = new DefaultPool(3);
        $names = ['Alex', 'Mike', 'John', 'Harry', 'Felix', 'Nina', 'Marie', 'Steven'];

        parallelMap(
            $names,
            function ($name) {
                sleep(rand(1, 3));
                $date = new DateTime();
                echo "{$date->format('H:i:s')} => {$name}\r\n";
            },
            $pool
        );
    }
);

Output:

PHP Warning:  Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250

Warning: Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250
PHP Warning:  Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250

Warning: Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250
PHP Warning:  Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250

Warning: Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250
PHP Warning:  Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250

Warning: Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250
PHP Warning:  Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250

Warning: Worker in pool exited unexpectedly with code -1 in /home/ec2-user/php-amphp-parallel-issue/vendor/amphp/parallel/lib/Worker/DefaultPool.php on line 250
14:20:23 => Felix
14:20:24 => Mike
14:20:24 => Alex
14:20:24 => Marie
14:20:24 => Steven
14:20:25 => John
14:20:25 => Harry
14:20:25 => Nina