[9.x] Avoid memory leak in HandleExceptions during repeated Application startup

[olsavmic]

Partially resolves #44214.

Repeatedly running following script (which happens during PHPUnit test setup) slowly but steadily leaks memory since the register_shutdown_function is never released:

<?php

namespace Tests\Feature;

require_once __DIR__. '/../../vendor/autoload.php';

use App\Exceptions\Handler;
use Illuminate\Contracts\Console\Kernel;
use Illuminate\Contracts\Debug\ExceptionHandler;
use Illuminate\Foundation\Application;

function createApplication()
{

    $app = new Application(
        $_ENV['APP_BASE_PATH'] ?? dirname(dirname(__DIR__))
    );

    $app->singleton(
        \Illuminate\Contracts\Http\Kernel::class,
        \App\Http\Kernel::class
    );

    $app->singleton(
        Kernel::class,
        \App\Console\Kernel::class
    );

    $app->singleton(
        ExceptionHandler::class,
        Handler::class
    );

    $app->make(Kernel::class)->bootstrap();

    return $app;
}

for ($i = 1; $i < 10000; ++$i) {
    createApplication()->flush();
    Artisan::forgetBootstrappers();
    Queue::createPayloadUsing(null);
    HandleExceptions::forgetApp();
    gc_collect_cycles();

    dump('Using ' . ((int) (memory_get_usage(true) / (1024 * 1024))) . 'MB in ' . $i . ' iterations.');
}

[olsavmic]

Not ideal as it keeps reference to original $this but there is no way to unregister a shutdown function. To solve it properly, yet another layer of indirection with reference to $this must be kept static (which pretty much makes the HandleExceptions class a singleton, which it already pretty much is 🤷‍♂️).

Needs a bit more thought.

[olsavmic]

Not a bullet-proof solution but it keeps the API, the behavior with PHPUnit should remain the same while the leak seems to be gone.

The only issue is that the handleShutdown is potentially broken in tests as it holds a reference to the old $this. it could be solved by keeping a reference to $this in a static variable (singleton) but I don't think it's actually needed.

I understand that since the leak is pretty small, the changes here may not be worth it. The current behaviour leads people to look for bugs in the framework though and not their apps so perhaps that could be prevented.

[nunomaduro]

Closing this pull request for the reasons already mentioned here: #40656.

TL DR: You can't safely restore any of those handlers (at least on the way you are), as they may be used on vendors (such as PHPUnit, and error handler drivers), and by calling any of those restores, you don't know exactly what you are restoring. In addition, leaking a small closure is not what causing the memory exhausted cases we've seen in past issues. The $app instance on the HandleExceptions is already static, and gets destroyed between tests - assuming that does not get leaked anywhere else in people's applications.