feat(stackable-versioned): Add conversion tracking

[Techassi]

Part of stackabletech/issues#642

Tracking changes across sub structs

This is a rough sketch of what enabling this will look like. Support for this is essential so effectively track all changes values. Fields will be targeted by JSONPath like selectors.

use kube::CustomResource;
use schemars::JsonSchema;
use serde::{Deserialize, Serialize};
use stackable_versioned::{ChangedValues, versioned};

// A from trait which allows turning one type into another while also being able to track changes
// across upgrades and downgrades. This allows nested sub structs to bubble up their tracked changes.
pub trait TrackingFrom<T, S>
where
    Self: Sized,
    S: TrackingStatus,
{
    fn tracking_from(value: T, status: S) -> Self;
}

// This Into trait mirrors the standard Into trait, just with the ability to track changes. This
// trait is auto-implemented by all U: TrackingFrom.
pub trait TrackingInto<T, S>
where
    Self: Sized,
    S: TrackingStatus,
{
    fn tracking_into(self, status: S) -> T;
}

impl<T, U, S> TrackingInto<U, S> for T
where
    S: TrackingStatus,
    U: TrackingFrom<T, S>,
{
    fn tracking_into(self, status: S) -> U {
        U::tracking_from(self, status)
    }
}

// Generically access tracked changes in any status which implements this. The generated status
// struct will automatically implement this.
pub trait TrackingStatus {
    fn changes(&mut self) -> &mut ChangedValues;
}

#[versioned(
    version(name = "v1alpha1"),
    version(name = "v1beta1"),
    options(skip(from))
)]
mod versioned {
    #[versioned(
        k8s(
        group = "test.stackable.tech",
        status = FooStatus,
        // options(experimental_conversion_tracking)
    )
    )]
    #[derive(Clone, Debug, CustomResource, Serialize, Deserialize, JsonSchema)]
    pub struct FooSpec {
        // #[versioned(k8s(nested))]
        bar: Bar,
    }

    // #[versioned(k8s(nested))]
    #[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
    pub struct Bar {
        baz: usize,

        #[versioned(added(since = "v1beta1"))]
        quox: u16,
    }
}

#[derive(Clone, Debug, Deserialize, Serialize, JsonSchema)]
pub struct FooStatus {
    changes: ChangedValues,
}

// Those two impls are generated by default because FooSpec describes the root struct.
impl From<v1alpha1::Foo> for v1beta1::Foo {
    fn from(value: v1alpha1::Foo) -> Self {
        // Call spec.tracking_into(status), because at least one field was marked with k8s(nested)
        todo!()
    }
}

impl From<v1beta1::Foo> for v1alpha1::Foo {
    fn from(value: v1beta1::Foo) -> Self {
        todo!()
    }
}

// If k8s(nested) is applied to a field, we know that we instead need to generate the TrackingFrom
// impl instead of the standard From impl.
impl<S> TrackingFrom<v1alpha1::FooSpec, S> for v1beta1::FooSpec
where
    S: TrackingStatus,
{
    fn tracking_from(value: v1alpha1::FooSpec, status: S) -> Self {
        Self {
            // The following generation is already in place, just needs to be adjusted to be able
            // to generate tracking_into for fields which need it. Currently in always emits .into()
            bar: value.bar.tracking_into(status),
        }
    }
}

impl<S> TrackingFrom<v1beta1::FooSpec, S> for v1alpha1::FooSpec
where
    S: TrackingStatus,
{
    fn tracking_from(value: v1beta1::FooSpec, status: S) -> Self {
        Self {
            bar: value.bar.tracking_into(status),
        }
    }
}

// The following two impls are generated if k8s(nested) is set on a sub struct
impl<S> TrackingFrom<v1alpha1::Bar, S> for v1beta1::Bar
where
    S: TrackingStatus,
{
    fn tracking_from(value: v1alpha1::Bar, status: S) -> Self {
        // This is a nested sub struct and thus knows which fields change in what way. As such,
        // it can insert into and read from the provided status (which has access to a mutable
        // changes tracking section).

        Self {
            baz: value.baz,
            quox: Default::default(),
        }
    }
}

impl<S> TrackingFrom<v1beta1::Bar, S> for v1alpha1::Bar
where
    S: TrackingStatus,
{
    fn tracking_from(value: v1beta1::Bar, status: S) -> Self {
        Self { baz: value.baz }
    }
}

// This is to be generated automatically when conversion_tracking is enabled
impl TrackingStatus for FooStatus {
    fn changes(&mut self) -> &mut ChangedValues {
        &mut self.changes
    }
}