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

組み込みの FPM\Tester reproducer から始め、特に ondemand 設定、pm.process_idle_timeout、keep-alive 接続、遅延書き込みを確認します。Linux 上でリソース負荷の高いテストを実行し、FPM のプロセスアイドル処理と関連するテスト出力を調査します。遅延したリクエストが引き続き使用可能で、connection-reset notice が生成されなければ完了です。