bug #36332 [Serializer] Fix unitialized properties (from PHP 7.4.2) when serializing context for the cache key (alanpoulain)

This PR was squashed before being merged into the 3.4 branch.

Discussion
----------

[Serializer] Fix unitialized properties (from PHP 7.4.2) when serializing context for the cache key

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix https://github.com/symfony/symfony/issues/35574 https://github.com/doctrine/orm/issues/8030
| License       | MIT
| Doc PR        | N/A

This bug only happens on the following conditions:
- A Doctrine entity (`Book`) having a relation with another entity (`Author`) is used;
- The `Author` entity uses typed properties (PHP 7.4) not initialized;
- The `Serializer` is used with the `Book` in the `OBJECT_TO_POPULATE` key in the context.

For instance:
```php
<?php

declare(strict_types=1);

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Entity */
class Book
{
    /**
     * @ORM\ManyToOne(targetEntity="Author")
     */
	public Author $author;

	public ?string $isbn;
}
```

```php
<?php

declare(strict_types=1);

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Entity */
class Author
{
    public ?string $name;
}
```

Or even:

```php
<?php

declare(strict_types=1);

namespace App\Entity;

use Doctrine\ORM\Mapping as ORM;

/** @ORM\Entity */
class Author
{
    private string $name;

    public function __construct()
    {
        $this->name = 'Leo';
    }
}
```

If the following is done (it's the case for instance in API Platform when a `PUT` is made):
```php
$serializer->deserialize('{"isbn":"2038717141"}', Book::class, 'json', ['object_to_populate' => $book]);
```

Then there will be the following error:
> Fatal error: Typed property Proxies\__CG__\App\Entity\Author::$ must not be accessed before initialization (in __sleep)

It's because of these lines in the `getCacheKey` method of the `AbstractObjectNormalizer`:
5da141b8d0/src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php (L405-L409)

Since the lazy proxyfied relation has a `__sleep` with unitialized properties, the `serialize` method will throw (since https://bugs.php.net/bug.php?id=79002: 846b647953).

I propose to fix this issue by unsetting the `OBJECT_TO_POPULATE` key in the context because I don't think it's useful for determining the attributes of the object.

For the next versions of Symfony, the fix should probably be elsewhere, in the default context.
For instance in Symfony 4.4, instead of:
15edfd39d4/src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php (L118)
It should be:
```php
$this->defaultContext[self::EXCLUDE_FROM_CACHE_KEY] = [self::CIRCULAR_REFERENCE_LIMIT_COUNTERS, self::OBJECT_TO_POPULATE];
```
But I'm not sure how it should be merged (another PR maybe?).

Commits
-------

1fafff7c10 [Serializer] Fix unitialized properties (from PHP 7.4.2) when serializing context for the cache key

src/Symfony/Component/Serializer/Normalizer/AbstractObjectNormalizer.php

@@ -400,6 +400,7 @@ abstract class AbstractObjectNormalizer extends AbstractNormalizer
      */
     private function getCacheKey($format, array $context)
     {
+        unset($context[self::OBJECT_TO_POPULATE]);
         unset($context['cache_key']); // avoid artificially different keys
         try {
             return md5($format.serialize([

src/Symfony/Component/Serializer/Tests/Normalizer/AbstractObjectNormalizerTest.php

@@ -163,6 +163,14 @@ class AbstractObjectNormalizerTest extends TestCase
         $this->assertEquals('bar', $stringCollection->children[1]);
     }
 
+    public function testDenormalizeNotSerializableObjectToPopulate()
+    {
+        $normalizer = new AbstractObjectNormalizerDummy();
+        $normalizedData = $normalizer->denormalize(['foo' => 'foo'], Dummy::class, null, [AbstractObjectNormalizer::OBJECT_TO_POPULATE => new NotSerializable()]);
+
+        $this->assertSame('foo', $normalizedData->foo);
+    }
+
     private function getDenormalizerForStringCollection()
     {
         $extractor = $this->getMockBuilder(PhpDocExtractor::class)->getMock();
@@ -379,3 +387,15 @@ class ArrayDenormalizerDummy implements DenormalizerInterface, SerializerAwareIn
         $this->serializer = $serializer;
     }
 }
+
+class NotSerializable
+{
+    public function __sleep()
+    {
+        if (class_exists(\Error::class)) {
+            throw new \Error('not serializable');
+        }
+
+        throw new \Exception('not serializable');
+    }
+}