[Core] classmethod support for actors

[Darkdragon84]

Description

Please support calls to classmethods of actors. Currently this

import ray


class Foo:
    def __init__(self, x):
        self.x = x

    def run(self):
        pass

    @classmethod
    def main(cls, x):
        main_instance = cls(x)
        return main_instance.run()


@ray.remote
class Bar(Foo):
    def run(self):
        return self.x

ray.init()
x = ray.get(Bar.main.remote(1))
print(x)

fails with

Traceback (most recent call last):
  File "...", line 23, in <module>
    x = ray.get(Bar.main.remote(1))
AttributeError: 'function' object has no attribute 'remote'

I would expect to see the output

1

Use case

classmethods are useful concepts and are used all over the place. For example, they are a great way to overload the __init__ by implementing several from_<whatever>(data: whatever) classmethods.

[PhysicsACE]

@rkooo567 Can I make a PR for this?

[rkooo567]

@PhysicsACE can you create a simple proposal here? I think we need a bit of design (and clear specification of how the API looks like)

[PhysicsACE]

@rkooo567 Sure. Since classmethods are not bound to a specific instance of the class, we can execute classmethods as normal ray tasks using submit_task. The first requirement for classmethods is that the first argument of the function is the class that the method belongs to. We can do this inside ray by accessing the underlying function using the func magic method. Then, when executing, we can insert the respective ActorClass as the first argument into list_args to be executed. To enable this, we need to add serialization and deserialization helpers for the ActorClass. The second requirement for classmethods is that they can mutate class attributes. To support them, we keep a centralized copy of the class attributes in the kv store. Each ActorClass has a respective class_id and when we create new copies of the ActorClass through cloudpickle, we pass along the same class_id. Hence, all copies of the ActorClass have the same class_id and they can access and manipulate the contents of the class attributes in the kv store though custom get_attribute and setattr methods using the class_id.

This is what the API would look like based on the implementation. The screenshot is from a pytest case that passes.

[PhysicsACE]

@rkooo567 Is there feedback for this?