Support for `ExternalArtifact` resources using as `sourceRef` for Flux `Kustomization`

[ringods]

What problem are you trying to solve?

My flux setup uses ArtifactGenerator to create ExternalArtifact resources. Here is my setup:

---
apiVersion: source.extensions.fluxcd.io/v1beta1
kind: ArtifactGenerator
metadata:
  name: flux-system
  namespace: flux-system
spec:
  sources:
    - alias: monorepo
      kind: GitRepository
      name: flux-system
  artifacts:
    - name: infrastructure
      originRevision: "@monorepo"
      copy:
        - from: "@monorepo/kubernetes/infrastructure/base/**"
          to: "@artifact/base/"
        - from: "@monorepo/kubernetes/infrastructure/desk8s/**"
          to: "@artifact/desk8s/"

I created this to have much shorter paths in all of my config files. Flux reconciles this to an ExternalArtifact within my cluster, for instance:

apiVersion: source.toolkit.fluxcd.io/v1
kind: ExternalArtifact
metadata:
  creationTimestamp: "2026-07-20T10:10:52Z"
  generation: 1
  labels:
    app.kubernetes.io/managed-by: source-watcher
    source.extensions.fluxcd.io/generator: b5062abe-a949-4f1d-9bbb-7ff8cb83112a
  name: infrastructure
  namespace: flux-system
  resourceVersion: "250754237"
  uid: e79f0af9-9ee3-49a5-b86a-0d6d2f7eb799
spec:
  sourceRef:
    apiVersion: source.extensions.fluxcd.io/v1beta1
    kind: ArtifactGenerator
    name: flux-system
    namespace: flux-system
status:
  artifact:
    digest: sha256:2352314c5452d7343f3dd1dc4d78a16ea0e72beb1e31c739681f642c4ade607a
    lastUpdateTime: "2026-07-21T13:16:08Z"
    metadata:
      org.opencontainers.image.revision: refs/heads/main@sha1:875be6e9a35743ad2d20f090cf01da9ac7ff07c8
    path: externalartifact/flux-system/infrastructure/3214410609.tar.gz
    revision: latest@sha256:2352314c5452d7343f3dd1dc4d78a16ea0e72beb1e31c739681f642c4ade607a
    size: 1926
    url: http://source-watcher.flux-system.svc.cluster.local./externalartifact/flux-system/infrastructure/3214410609.tar.gz
  conditions:
  - lastTransitionTime: "2026-07-21T13:16:08Z"
    message: Artifact is ready
    observedGeneration: 1
    reason: Succeeded
    status: "True"
    type: Ready

When using flate test, this ExternalArtifact resource is not available in the Git repository, so the validation ends pretty fast in my setup:

$ flate test all --path ./kubernetes

  ✓  HelmRelease    flux-system/flux-operator
  ✓  OCIRepository  flux-system/flux-operator
  ⊘  Kustomization  flux-system/infrastructure  blocked by flux-system/infrastructure (not found)
  ⊘  Kustomization  system-upgrade/tuppr  blocked by flux-system/infrastructure (not found)
  ⊘  Kustomization  system-upgrade/tuppr-upgrades  blocked by flux-system/infrastructure (not found)

  ✗ 2 passed · 3 blocked   1.8s

The blocked by errors refer to the non-existant ExternalArtifact.

Proposed solution

It would be nice if flate could "evaluate" the ArtifactGenerator and retain in-memory ExternalArtifact resources with local paths so flate test works.

Alternatives considered

No response