[php-src] Issue #11498: SIGCHLD is not always returned from proc_open

Issue: https://github.com/php/php-src/issues/11498 Author: taka-oyama ### Description The following code: ```php <?php ini_set('display_errors', '1'); ini_set('display_startup_errors', '1'); error_reporting(E_ALL); // store processes in an array as [$pid => $process, ...] $processes = []; // catch signals from child processes and unset processes from the array pcntl_async_signals(true); pcntl_signal(SIGCHLD, function($sig, $info) use (&$processes) { unset($processes[$info['pid']]); }, false); // open 10 processes and store them in an array foreach (range(0, 10) as $i) { $process = proc_open('echo $$', [], $pipes); $pid = proc_get_status($process)['pid']; $processes[$pid] = $process; } // wait for all processes to exit while(true) { echo ' processes remaining:' . count($processes) . PHP_EOL; if (empty($processes)) { break; } sleep(1); } // Check that all processes have been removed. var_dump($processes); ``` Resulted in this output: ``` 921 922 processes remaining:9 925 926 924 processes remaining:8 923 processes remaining:6 931 929 927 processes remaining:5 928 930 processes remaining:2 processes remaining:2 processes remaining:2 processes remaining:2 processes remaining:2 processes remaining:2 processes remaining:2 ``` But I expected this output instead: ``` 837 838 processes remaining:10 842 840 processes remaining:8 processes remaining:7 846 841 processes remaining:6 847 processes remaining:5 processes remaining:4 839 843 processes remaining:3 844 processes remaining:1 845 processes remaining:0 array(0) { } ``` There were some rare cases where the script would exit with code 255 with no errors. ``` root@c09c0fa26c4d:/app# php test.php 933 934 936 processes remaining:8 937 935 processes remaining:7 938 939 941 processes remaining:5 940 942 943 root@c09c0fa26c4d:/app# echo $? 255 ``` ### PHP Version PHP 8.2.7 (cli) (built: Jun 13 2023 23:05:53) (NTS) ### Operating System Debian GNU/Linux 12