Error: event_base_loop reentrant invocation

[Provoker]

Hello.
The following code is causing an error:
EventBase::loop(): event_base_loop: reentrant invocation. Only one event_base_loop can run on each event_base at once.

<?php

use React\EventLoop\Loop;
use React\Promise\Deferred;
use function React\Async\await;

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

Loop::addTimer(0, function () {
	$deferred = new Deferred();
	Loop::addTimer(0, static fn() => $deferred->resolve());
	await($deferred->promise());
});

Tested on ExtEventLoop.

[WyriHaximus]

Which version of this package are you using?

[Provoker]

@WyriHaximus,
react/async: dev-main (ff11a7a)
react/event-loop: v1.2.0

Tested on PHP 8.1.0 and PHP 8.1.1

[WyriHaximus]

Here you tried:

<?php

use React\EventLoop\Loop;
use React\Promise\Deferred;
use function React\Async\async;
use function React\Async\await;

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

Loop::addTimer(0, async(function () {
	$deferred = new Deferred();
	Loop::addTimer(0, static fn() => $deferred->resolve());
	await($deferred->promise());
}));

Or using https://github.com/reactphp/promise-timer#sleep

<?php

use React\EventLoop\Loop;
use React\Promise\Deferred;
use function React\Async\async;
use function React\Async\await;
use React\Promise\Timer\sleep;

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

Loop::addTimer(0, async(function () {
	await(sleep(0));
}));

[Provoker]

Thanks, there is no problem with async().
Does this mean that any function that contains await() should be wrapped with async() ?

[Provoker]

Forgotten async() call forces SimpleFiber::suspend() to call Loop::run()

async/src/SimpleFiber.php

Line 44 in ff11a7a

self::$scheduler = new \Fiber(static fn() => Loop::run());

But if Loop::run() is already called before await() is called, then this causes various problems.

Another example

<?php

use React\EventLoop\Loop;
use React\Promise\Deferred;

use function React\Async\await;

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

Loop::futureTick(/* missed await */ function () {
	$deferred = new Deferred();
	Loop::addTimer(0, static fn() => $deferred->resolve());
	await($deferred->promise());
});

Loop::futureTick((static fn() => 0));

PHP Fatal error:  Uncaught RuntimeException: Can't shift from an empty datastructure in /vendor/react/event-loop/src/Tick/FutureTickQueue.php:46
Stack trace:
#0 /vendor/react/event-loop/src/Tick/FutureTickQueue.php(46): SplQueue->dequeue()
#1 /vendor/react/event-loop/src/ExtEventLoop.php(203): React\EventLoop\Tick\FutureTickQueue->tick()
#2 /vendor/react/event-loop/src/Loop.php(55): React\EventLoop\ExtEventLoop->run()
#3 [internal function]: React\EventLoop\Loop::React\EventLoop\{closure}()
#4 {main}
  thrown in /vendor/react/event-loop/src/Tick/FutureTickQueue.php on line 46

As a result, we do not know the problem file and line. It's very easy to miss async() in future code.

Maybe just throw an exception instead of calling Loop::run() ?

[WyriHaximus]

Forgotten async() call forces SimpleFiber::suspend() to call Loop::run()

async/src/SimpleFiber.php

Line 44 in ff11a7a

self::$scheduler = new \Fiber(static fn() => Loop::run());

But if Loop::run() is already called before await() is called, then this causes various problems.
Another example

<?php

use React\EventLoop\Loop;
use React\Promise\Deferred;

use function React\Async\await;

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

Loop::futureTick(/* missed await */ function () {
	$deferred = new Deferred();
	Loop::addTimer(0, static fn() => $deferred->resolve());
	await($deferred->promise());
});

Loop::futureTick((static fn() => 0));

PHP Fatal error:  Uncaught RuntimeException: Can't shift from an empty datastructure in /vendor/react/event-loop/src/Tick/FutureTickQueue.php:46
Stack trace:
#0 /vendor/react/event-loop/src/Tick/FutureTickQueue.php(46): SplQueue->dequeue()
#1 /vendor/react/event-loop/src/ExtEventLoop.php(203): React\EventLoop\Tick\FutureTickQueue->tick()
#2 /vendor/react/event-loop/src/Loop.php(55): React\EventLoop\ExtEventLoop->run()
#3 [internal function]: React\EventLoop\Loop::React\EventLoop\{closure}()
#4 {main}
  thrown in /vendor/react/event-loop/src/Tick/FutureTickQueue.php on line 46

As a result, we do not know the problem file and line. It's very easy to miss async() in future code.

We MIGHT always wrap loop handers in an async() call in the future. For now, you have to do this yourself, and yes that means you might miss one.

But I'll also open a PR to test this package against all event loops we support because honestly I only develop and tested it against ext-uv. Also @clue has looked into if we still perse need the future ticks and we might drop them from the implementation. I'll also PR your example as a test, if it's not already in there in a maybe slightly different way.

Maybe just throw an exception instead of calling Loop::run() ?

Because that Fiber constructor argument triggers when the main Fiber starts.

[clue]

Thanks, there is no problem with async(). Does this mean that any function that contains await() should be wrapped with async() ?

Excellent input! The await() function can indeed be considered "blocking", so if you're using it from an event loop timer, event listener, or promise callback, you should always wrap it in an async() function. I've just filed #26 to add documentation for this behavior.

Technically, we could also wrap this automatically for each callback, but this incurs a significant overhead. This is especially true as we would have to do this for each callback even if it does not use await() internally, because there's no way to tell from the outside.

At the moment, explicitly using async() in these cases instead seems like a reasonable compromise between ease of use and performance. This is still an early development version and I'm curious to see how the ecosystem and performance considerations evolve over time! 👍

[WyriHaximus]

@Provoker Which version of ext-event are you running?

[Provoker]

@WyriHaximus, Latest stable from pecl

Sockets support => enabled
Debug support => disabled
Extra functionality support including HTTP, DNS, and RPC => enabled
OpenSSL support => enabled
Thread safety support => disabled
Extension version => 3.0.6
libevent2 headers version => 2.0.21-stable