[Config] Don't add object-value of static properties in the signature of container metadata-cache

[arjenm]

Q	A
Branch?	3.4
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	No ticket created, see below
License	MIT

Running bin/console cache:clear can take a very long time and/or uses explosive amounts of memory if large protected/public static objects are available in classes for which metadata will be generated during the 'container dump'.

I.e. in \Symfony\Component\HttpKernel\Kernel::dumpContainer eventually a call to \Symfony\Component\Config\ResourceCheckerConfigCache::write is done. That writes a serialized version of the $metadata.
In that serialize a call to \Symfony\Component\Config\Resource\ReflectionClassResource::generateSignature for each service will eventually do a print_r on the class's (default) properties.

Class properties can also be (public/protected) static. PHP Reflection doesn't differentiate between those in that context. And those static objects can also have non-scalar values, including objects.

If such an object is very large, for instance a service that references the Container (with many services, some also referencing the container, etc), the print_r will run into very deep recursions. I.e. it will either take "forever" or use so much memory the OOM killer is activated (it got to 25GB...).

This scenario only triggers when a cache existed prior to running cache:clear, i.e. when 'isFresh' is called. When first removing the cache dir (or running cache:clear with --no-warmup) and than running cache:clear does not trigger the behavior, apparently the hash is created in a saver time in that scenario.

This sample project offers code that will prime a static property into a service (the trick is having a command with such a class-reference/service):
https://github.com/arjenm/symfony-service-recursion
Add a var_dump to the $defaults = $class->getDefaultProperties(); in \Symfony\Component\Config\Resource\ReflectionClassResource::generateSignature to see it in effect.
The first cache:clear will show a few null's; the second run - without removing the cache - will show a container instead of null.
Its still a tiny container, so it'll not trigger serious issues.

This PR prevents an object "default value" for a property from being fed to 'print_r'. Since normally an object-property should be null on a "fresh" look at a non-instantiated class it should be save to use (i.e. no BC).

[nicolas-grekas]

TIL that ReflectionClass:getDefaultProperties() doesn't return the class' default value for static properties...
The patch in this PR doesn't account for objects nested in arrays, but detecting them would be costly I believe, so let's go with this simple check.

[nicolas-grekas]

Thank you @arjenm.