Extract Cleanup Interface from Reconciler

[csviri]

The nice part would be that, this would server also as a switch for the finalizer. So the if the cleanup interface is implemented the finalizers are used otherwise not.
However then we should probably extract the name customization to a new annotation too. Like @Finalizer(name = "my-custom-name") in order to customize the finalizer name.

Original idea from @scrocquesel .

[csviri]

Not sure if we should have a separate finalizer annotation, probably would be good enough to have the finalizerName stay on ControllerConfiguration.

[csviri]

A question is what should happen in case of managed dependent resources. If the Cleaner interface not implemented by the Reconciler. Still the cleanup should be called for the managed dependent resources?

cc @metacosm

[metacosm]

Not sure if we should have a separate finalizer annotation, probably would be good enough to have the finalizerName stay on ControllerConfiguration.

Maybe the finalizer name should be part of the Cleaner interface, with a default implementation matching what we currently have? This way, everything related would be part of the same interface. I'm not sure that this needs to be part of the annotation anymore since, if you want the behavior, you have to implement the Cleaner interface and you might as well provide the finalizer name there if you don't like the default one?

[metacosm]

That's a good question… but I'm not sure what the answer should be :)

[csviri]

Not sure if we should have a separate finalizer annotation, probably would be good enough to have the finalizerName stay on ControllerConfiguration.

Maybe the finalizer name should be part of the Cleaner interface, with a default implementation matching what we currently have? This way, everything related would be part of the same interface. I'm not sure that this needs to be part of the annotation anymore since, if you want the behavior, you have to implement the Cleaner interface and you might as well provide the finalizer name there if you don't like the default one?

Yeah, not sure either, I see there three options:

1. Have a separate annotation to configure cleaner. Like @CleanerConfiguration, what would have an attribute finalizerName. Or just simply having an annotation @Finalizer(name=""). The downside of this is that this will be a separate annotation to handle. The config service would still manage this property?
2. Have it as now on on @ControllerConfiguration. The is very easy to implement (basically it is), just could be confusing that one can configure it when there will be no annotation added. (However this is also true for 1.)
3. Have it as a default method on the Cleaner interface. This would work, but would be nicer I think with annotations, since we already do it for other features.

So honestly I would just stay with variant 2. - document it, and change it in following versions based on feedback, especially if it would be confusing.

[csviri]

Regarding the managed dependent resources. What I would do that maybe separate there the cleanup method to new interface too. And change the behavior based on that. That means:
If the reconciler does not implement the Cleaner interface, but there is a dependent resource which does. We will still add the finalizer and call the cleanup on that dependent resource.