Infinite loop when handling an exception (critical bug in my opinion)

[enumag]

I'm using the Symfony bundle. I start my consumer using bin/console enqueue/consume and sure enough it works just fine.

However if one of the consumed messages causes an exception, the consumer freezes without even writing anything to console or log - the exception is nowhere to be seen.

I used xdebug to find out what caused it and I believe the reason is that Symfony/Console runs into an infinite loop while trying to print the exception.

The reason why that happens is this code in your bundle:

enqueue-dev/pkg/enqueue/Consumption/QueueConsumer.php

Lines 320 to 331 in 499360e

	$wrapper = $e;
	while ($prev = $wrapper->getPrevious()) {
	if ($exception === $wrapper = $prev) {
	throw $e;
	}
	}
	if ($exception !== $wrapper) {
	$prev = new \ReflectionProperty('Exception', 'previous');
	$prev->setAccessible(true);
	$prev->setValue($wrapper, $exception);
	}

It effectively creates a pair of exceptions where each has set the other one as the previous:

$e->getPrevious()->getPrevious() === $e // true

(or a longer chain of exceptions where the last one has the first one as previous - still effectively a cycle of exceptions)

I don't quite understand what is the idea behind this but obviously when an error handler tries to dump the exception chain this causes an infinite loop because ->getPrevious() never returns null.

Can you explain what the code in QueueConsumer::onProcessorException() is supposed to do? I'm quite sure this isn't intentional. When I remove the code I linked above, everything works as expected.

[enumag]

Ping @nicolas-grekas. You're the original author of this code in Symfony. What on earth is the reason to create an infinite loop of exceptions?

[makasim]

As far as I understand this #725 fixes the issue. Could you check?

[enumag]

@makasim No it doesn't. I'm using v0.9.7 which already has this fix.

Maybe the condition there is supposed to be if ($exception !== $e) {? Because that's what happened in my case - $exception and $e are one and the same ($exception was rethrown here and then caught as $e) while $wrapper is different.

Also ping @DamienHarper - as the author of #725 you might have an idea.

[DamienHarper]

@enumag Sorry I couldn't reply earlier, #725 helped me fix some issues I was experiencing at that time. Recently, I faced again some infinite loops, caused by those self-referencing exceptions, making some of my commands crash again. Though I hadn't enough time to investigate more.
So thanks for the fix.