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

从内置的 FPM\Tester reproducer 开始，重点关注 ondemand 配置、pm.process_idle_timeout、keep-alive 连接和延迟写入。在 Linux 上运行资源占用较高的测试，并检查 FPM 的进程空闲处理和相关测试输出；当延迟请求仍然可用且不会产生连接重置通知时，即表示完成。