Switching Fiber contexts during autoload may causes Class "%s" not found error

[Appla]

Description

The following codes:

• BasicObject.php

<?php

class BasicObject
{
}

• Constant.php

<?php

class Constant extends BasicObject
{
    public const 
        CONST_1 = 'PHP * Swoole',
        CONST_2 = 'PHP * Fiber';
}

• SimpleConst.php

<?php

class SimpleConst
{
    public const GREETINGS = 'hello world!';
}

• test.php

<?php

\spl_autoload_register(static function ($clsName) {
    if ($clsName === 'BasicObject') {
        Fiber::suspend('before load BasicObject in autoload');
    }
    $path = __DIR__ . '/' . \str_replace('\\', '/', $clsName) . '.php';
    if (\is_file($path)) {
        require $path;
    } else {
        echo "class {$clsName} not found\n";
    }
});

function work_fn(...$args)
{
    echo SimpleConst::GREETINGS . PHP_EOL;
    echo Fiber::suspend("before load constant\n");
    return Fiber::suspend(Constant::CONST_1);
}

$fiber = new Fiber(work_fn(...));
echo $fiber->start();
echo $fiber->resume("continue\n");
echo $fiber->resume(Constant::CONST_2);

Resulted in this output:

hello world!
before load constant
continue
before load BasicObject in autoload

Fatal error: Uncaught Error: Class "Constant" not found in /path/to/test.php:25
Stack trace:
#0 {main}
  thrown in /path/to/test.php on line 25

But I expected this output instead:

Class "Constant" should load successfully

This is similar to the issue discussed in swoole/swoole-src#5770.

Related codes:
Load unlinked CE

PHP Version

PHP 8.3.21 (cli) (built: May  6 2025 13:58:10) (NTS gcc x86_64)
Copyright (c) The PHP Group
Zend Engine v4.3.21, Copyright (c) Zend Technologies
    with Zend OPcache v8.3.21, Copyright (c), by Zend Technologies

Operating System

Fedora 42

[iluuu1994]

Can you clarify why you think this is a bug? Essentially, the fiber makes the control flow similar to:

https://3v4l.org/tXO5e

function declareP() {
    class P {}
}

function declareC() {
    class C extends P {
        const TEST = 'test';
    }
}

spl_autoload_register(static function ($class) {
    if ($class === 'P') {
        // The original code suspends when loading BasicObject during
        // inheritance of SimpleConst, and then accesses SimpleConst
        // in the main context before it was registered.
        echo C::TEST;
        declareP();
    } elseif ($class === 'C') {
        declareC();
    }
});

var_dump(C::TEST);

This makes it quite obvious why that can't work. The inheritance step must finish before accessing Constant.

[Appla]

I'm not sure if this qualifies as a bug, but we've encountered similar scenarios in our live application.
In our Swoole based application, due to file operations within the autoload function, context switching similar to what was demonstrated in the provided fiber example can occur. This subsequently leads to the situation described.

[iluuu1994]

I can see how this issue would occur with a suspend on file inclusion, and then starting to handle the next request, assuming this class is already loaded or can be loaded. However, there's nothing we can do here. It's probably best to block when including PHP files, it's questionable whether a full context switch would even be profitable here.

[ndossche]

In my opinion, not a bug in PHP, this should be fixed at Swoole's side then.

[iluuu1994]

I agree. For PHP itself, this wouldn't happen unless Fiber::suspend() is manually called. If Swoole implicitly suspends fibers on disk IO, it may be more surprising, but that's not an issue on our side (and they likely should change that behavior).