FPM idle or req timeout causes no child available if write is delayed

Description

This problem can happen in ondemand mode if the connection is established but there is a significant write delay and pm.process_idle_timeout is set even to shorter value. In such case process idle can remove all children. But probably because the connection is established, it is past creating new child so the request fails.

The following tests results in such issue:

--TEST--
FPM: Process manager config pm.process_idle_timeout with keep alive set
--SKIPIF--
<?php
include "skipif.inc";
if (!getenv("FPM_RUN_RESOURCE_HEAVY_TESTS")) die("skip resource heavy test");
if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
?>
--FILE--
<?php

require_once "tester.inc";

$cfg = <<<EOT
[global]
error_log = {{FILE:LOG}}
log_level = debug
[unconfined]
listen = {{ADDR}}
pm = ondemand
pm.max_children = 3
pm.min_spare_servers = 1
pm.process_idle_timeout = 1
pm.status_path = /status
;request_terminate_timeout = 2
EOT;

$code = <<<EOT
<?php
usleep(300000);
EOT;

$tester = new FPM\Tester($cfg, $code, clientTransport: 'stream');
$tester->start();
$tester->expectLogStartNotices();
//$tester->multiRequest(2, connKeepAlive: true);
$tester->multiRequest(2, connKeepAlive: true, writeDelay: 3000, readTimeout: 5000);
$tester->status([
    'total processes' => 2,
]);
// wait for process idle timeout
sleep(2);
$tester->status([
    'total processes' => 2,
]);
$tester->terminate();
$tester->expectLogTerminatingNotices();
$tester->close();

?>
Done
--EXPECT--
Done
--CLEAN--
<?php
require_once "tester.inc";
FPM\Tester::clean();
?>

Resulted in this output:

PHP Notice:  fwrite(): Send of 611 bytes failed with errno=104 Connection reset by peer

But I expected this output instead:

no error

PHP Version

PHP 8

Operating System

Linux

Start with the embedded FPM\Tester reproducer, especially the ondemand configuration, pm.process_idle_timeout, keep-alive connection, and delayed write. Run the resource-heavy test on Linux and inspect the FPM process-idle handling and related test output; done means the delayed request remains usable and produces no connection-reset notice.