[Serializer] Handle circular references

[dunglas]

Q	A
Bug fix?	Yes: avoid infinite loops. Allows to improve #5347
New feature?	yes (circular reference handler)
BC breaks?	no
Deprecations?	no
Tests pass?	yes
License	MIT
Doc PR	symfony/symfony-docs#4299

This PR adds handling of circular references in the Serializer component.
The number of allowed iterations is configurable (one by default).
The behavior when a circular reference is detected is configurable. By default an exception is thrown. Instead of throwing an exception, it's possible to register a custom handler (e.g.: a Doctrine Handler returning the object ID).

Usage:

use Symfony\Component\Serializer\Normalizer\GetSetMethodNormalizer;
use Symfony\Component\Serializer\Serializer;

class MyObj
{
    private $id = 1312;

    public function getId()
    {
        return $this->getId();
    }

    public function getMe()
    {
        return $this;
    }
}

$normalizer = new GetSetMethodNormalizer();
$normalizer->setCircularReferenceLimit(3);
$normalizer->setCircularReferenceHandler(function ($obj) {
    return $obj->getId();
});

$serializer = new Serializer([$normalizer]);
$serializer->normalize(new MyObj());

[nicolas-grekas]

Replacing the $context['circular_reference_limit'] array by an SplObjectStorage should prevent the many copies that currently occurs along the recursive walk.

[dunglas]

👍

[nicolas-grekas]

Hum, sorry, what I just said is wrong: an SplObjectStorage object would share the state between different branches of the recursive tree, which is not what you want I think.

[dunglas]

What about the commit I just pushed? It should work and reduce the memory usage?

[nicolas-grekas]

You should add a test that checks what happens when you serialize a structure that has many times the same object at sibling positions (more than circularReferenceLimit times)

[dunglas]

@nicolas-grekas Good catch.
I've switched back to an array implementation and added a test for siblings.

[nicolas-grekas]

Great, sorry for the misleading comment, at least we gained a test :-)

[dunglas]

You're welcome. It could have been a good improvement!

[fabpot]

Looks good to me. Can you submit a PR for the documentation? Thanks.

[dunglas]

@fabpot symfony/symfony-docs#4299

[fabpot]

👍

[jakzal]

@dunglas can you rebase? I think the failing tests were fixed in master.

[jakzal]

👍

[dunglas]

@jakzal rebased.

[jakzal]

And Travis is happy again :)

[fabpot]

Thank you @dunglas.