proc_get_status() loses exit code when SIGCHLD interrupts waitpid()

[NinaPeng-developer]

Description

The following code:

while ($await_processes) {
	$await_processes_copy = $await_processes;

	foreach ($await_processes_copy as $worker_id => $process) {
		$proc_status = proc_get_status($process['process']);

		if ($proc_status === false) {
			throw new MultiProcessHydraException("Failed to get proc status of worker $worker_id");
		}

		if ($proc_status['running'] === false) {
			unset($await_processes[$worker_id]);
			$stdout = stream_get_contents(fopen($process['files']['stdout'], 'r'));
			$stderr = stream_get_contents(fopen($process['files']['stderr'], 'r'));
			$exit_code = $proc_status['exitcode'];

			$log_message = [
				'message' => "Awaited process",
				'job_class' => $this->worker_class,
				'stdout' => $stdout,
				'stderr' => $stderr,
				'worker_id' => $worker_id,
				'exit_code' => $exit_code,
			];
			$logger->log($log_message);

			// If any child-process exits with a non-zero code (such as a fatal, uncatchable error)
			//  process this individual worker as a failure, but continue to process the responses from
			//  each other process.
			if ($exit_code !== 0) {
				$worker_responses[] = [[
					'error' => "Worker process $worker_id exited with non-zero exit code: $exit_code. "
						. "Stderr: $stderr",
				]];
			} else {
				$worker_responses[] = $cache->getCacheValue($process['files']['cache_file'])->asArray();
			}

			unset($process['files']['cache_file']);
			foreach ($process['files'] as $_ => $file) {
				unlink($file);
			}
		}
	}
}
<?php

Resulted in this output:

Uncaught Exception: [MultiProcessHydraException] Worker process mp-hydra-6b0a280c-2428-41f5-b393-11e4477b4e9d-7 exited with non-zero exit code: -1.
...(multiple process exited with -1)

But I expected this output instead:

The parent process checked the exitcode of child process to be 0 and finish the job, because from our log, all the subtasks were successfully finished.

Investigation

PHP 8.3 installs an internal SIGCHLD signal handler (not present in PHP 8.1) as part of the pcntl extension. When a child process exits, this handler intercepts the SIGCHLD signal and interrupts the waitpid() syscall inside proc_get_status().

Verified by:

Test	PHP 8.1	PHP 8.3
SIGCHLD disposition	SIG_DFL	CUSTOM_HANDLER
proc_open("exit(42)") + proc_get_status()	exitcode: 42	exitcode: -1
Same with php -n (no extensions)	exitcode: 42	[Still broken — in PHP core](exitcode: -1)
With pcntl_signal(SIGCHLD, SIG_DFL) first	exitcode: 42	exitcode: 42

Temporary Fix

Added pcntl_signal(SIGCHLD, SIG_DFL) before spawning workers in MultiProcessHydra::spawn(), resetting SIGCHLD to default behavior so proc_get_status() can reap children without interruption.

References:

• php.net/proc_get_status — PHP 8.3 cached field documentation
• php/php-src#10250
• php/php-src#10239

PHP Version

PHP 8.3.25 (cli) (built: Jan  5 2026 18:10:06) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.25, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.25, Copyright (c), by Zend Technologies

Operating System

Amazon Linux 2 (AL2)

[jordikroon]

Hi @NinaPeng-developer

Would it be possible for you to provide a minimal self contained script that does not rely on any project specific classes or infrastructure? The example code you shared references classes and components like MultiProcessHydraException, which makes it difficult to reproduce the issue directly.

A small standalone script that only uses built in PHP functions would help others reproduce and verify the behaviour more easily across PHP versions.

[NinaPeng-developer]

The simplest way to reproduce it:

php -r '
$p = proc_open(["php","-n","-r","exit(42);"], [], $pipes);
while (($s = proc_get_status($p))["running"]) {
    usleep(10000); // 10ms
}
echo "exitcode: " . $s["exitcode"] . "\n";
echo "cached: " . var_export($s["cached"], true) . "\n";
'

From our ECS task:

exitcode: -1
cached: false

PHP version in this ECS task:

PHP 8.3.25 (cli) (built: Feb 25 2026 02:15:44) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.3.25, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.25, Copyright (c), by Zend Technologies

The same test in PHP8.1 returned 42 exitcode(We don't have PHP8.1 anymore since we published the quick workaround fix).

[iluuu1994]

Your example doesn't reproduce for me. The patch you have linked also doesn't install any new signal handlers, it only caches the result of waitpid() for subsequent calls. Can you make sure you don't have any 3rd party extensions enabled? I.e. also add the -n to the outer php -r '.

[NinaPeng-developer]

Hey, thanks for your help. I tried debugging with -n, but I was still seeing the same result, which is why I initially reported this as a bug.

After digging in a bit more, I think this is likely due to a PHP build setting(customThirdPartyBuild) that reaps the child process status without passing it back to the parent process. Given that, it does not seem like an issue on your side, so I am going to go ahead and close this issue.

Thanks for your time again!