Support for Kustomize based PostRenderer

[alex-berger]

This PR adds support for Kustomize based Post Renderer to the HelmRelease resource. This PR is a first draft, to gather feedback from the project maintainers and while it compiles is has not yet been well tested.

Goals:

• Add simple built-in Post Renderer capability which supports as subset of Kustomize (patchesStrategicMerge, patchesJson6902 and images) and which will work with the default helm-controller image.
• Add minimal framework to support more Post Renderers in future.

Non Goals:

• Supporting arbitrary non-built-in Post Renderers which might require special image with external tools included.

Example Usage

---
apiVersion: helm.toolkit.fluxcd.io/v2beta1
kind: HelmRelease
metadata:
  name: metrics-server
  namespace: kube-system
spec:
  interval: 1m
  chart:
    spec:
      chart: metrics-server
      version: "5.3.4"
      sourceRef:
        kind: HelmRepository
        name: bitnami
        namespace: kube-system
      interval: 1m
  postRenderers:
  - kustomize: 
       patchesStrategicMerge:
       -  kind: Deployment
          apiVersion: apps/v1
          metadata:
            name: metrics-server
            namespace: kube-system
          spec:
            template:
              spec:
                tolerations:
                - key: "workload-type"
                  operator: "Equal"
                  value: "cluster-services"
                  effect: "NoSchedule"

Motivation

As SRE / DevOps engineer I often have the need to patch Helm Charts from third-parties, either by forking them or by contributing to the upstream via pull requests. Both approaches are cumbersome and time intensive and often cause project delays and errors. Having simple built-in Helm Post Renderer support in helm-controller will allows me to patch Helm Releases immediately without having to fork a Helm Chart or waiting for upstream contributions to be integrated.

This PR also (partially) addresses #104

[hiddeco]

Thank you for putting time into this, as it has been sought after 🌷

To not directly import a lot of new functionalities (and half of Kustomize), and to encourage people to continue to make changes to charts upstream, I think it would be best if we limit the set of functionalities to what was discussed in the issue.

This would cover most people their base needs, and we can then start a discussion around any further expansions.

[hiddeco]

Couple of tiny nitpicks, otherwise looking OK to me. 👍

It would also be awesome if you could reflect to changes made to the in-code API in the documentation in docs/spec/v2beta1/helmreleases.md.

NB: you accidentally checked the full helm-controller binary into your commit, you probably want to remove this from the commit and push --force-with-lease :-)

[hiddeco]

Some additional end-to-end tests with base coverage would be very welcome too, to ensure the full picture is wired correctly.

[hiddeco]

Awesome work, and if you smash your commits together I would be happy to merge them! 🎉

The helm-controller release that is going to include these changes is going to take a little longer, as we want to move some of the API elements introduced here to fluxcd/pkg so they can be shared with the kustomize-controller. Work on this should however not take too long. 🌻