fix(helm): support omitting image.registry in grafana-mcp chart

[michaelspinks]

What

Make the grafana-mcp chart render a valid image reference when image.registry is left empty, so the registry can be omitted (e.g. to pull from a mirror or let the runtime resolve the default registry) without producing a leading-slash, unpullable reference.

Why

The image is built inline in templates/deployment.yaml:

image: "{{ .Values.image.registry }}/{{ .Values.image.repository }}:{{ .Values.image.tag }}"

The / between registry and repository is written unconditionally. If image.registry is set to "", the result has a leading slash and is not a valid image reference, so the pod enters ImagePullBackOff:

/grafana:latest     # invalid reference

There is currently no way to omit the registry and let the container runtime resolve the repository against its default registry / a configured mirror.

Omitting the registry is a configuration users legitimately need:

• Air-gapped / mirrored environments. Registry mirrors are configured at the container runtime, which rewrites the host — so chart values should carry only repository:tag.
• Registry rewriting. Admission controllers (e.g. Kyverno) prepend an internal registry, so users leave registry: "".
• Consistency. Other charts let you omit the registry; grafana-mcp silently breaks with ImagePullBackOff, which reads like a bug.

Note: the default registry: mcp is intentional and unchanged — mcp/grafana resolves to docker.io/mcp/grafana (Docker Hub's mcp namespace). This PR only makes an empty registry valid; the default path still renders mcp/grafana:latest.
This is the same 2-segment registry/repository:tag leading-slash shape as the querydoc chart (#2124), and distinct from the bundled-PostgreSQL registry//name:tag double-slash bug (#2120), which uses a 3-segment shape. All fixed independently.

Fix

Introduce a grafana-mcp.image helper (the chart currently has none) that builds the path from non-empty segments via compact + join, and call it from the deployment:

{{- define "grafana-mcp.image" -}}
{{- $img := .Values.image -}}
{{- $parts := compact (list $img.registry $img.repository) -}}
{{- printf "%s:%s" (join "/" $parts) $img.tag -}}
{{- end -}}

# templates/deployment.yaml
image: "{{ include "grafana-mcp.image" . }}"

Rendered results:

registry	repository	tag	result
mcp	grafana	latest	mcp/grafana:latest (unchanged)
""	grafana	latest	grafana:latest (fixed)

Tests

Added two cases to helm/tools/grafana-mcp/tests/deployment_test.yaml (developed test-first): a default-image regression guard (the suite had no image assertion), and the empty-registry path.

  - it: should render the default container image
    template: deployment.yaml
    asserts:
      - equal:
          path: spec.template.spec.containers[0].image
          value: mcp/grafana:latest

  - it: should omit empty registry segment in image
    template: deployment.yaml
    set:
      image:
        registry: ""
    asserts:
      - equal:
          path: spec.template.spec.containers[0].image
          value: grafana:latest
      - notMatchRegex:
          path: spec.template.spec.containers[0].image
          pattern: "^/"

The empty-registry test fails without this change (/grafana:latest) and passes with it — revert the deployment.yaml/_helpers.tpl hunks to reproduce.

How to verify

make helm-tools                                       # generate grafana-mcp Chart.yaml
helm unittest helm/tools/grafana-mcp                  # full suite green (incl. new cases)
helm lint helm/tools/grafana-mcp

# test-independent reproduction:
helm template t helm/tools/grafana-mcp --set image.registry="" | grep image:
# before: /grafana:latest
# after:  grafana:latest

Scope

Limited to the grafana-mcp chart's image construction. Empty repository is not made valid here — omitting it leaves no image to pull. This PR only makes image.registry optional.

Checklist

• Commit is DCO signed-off (git commit -s).
• helm unittest helm/tools/grafana-mcp green; helm lint helm/tools/grafana-mcp clean.
• Default image still renders mcp/grafana:latest (regression guard).
• Diff limited to templates/_helpers.tpl, templates/deployment.yaml, and tests/deployment_test.yaml (generated chart files excluded).
• Test fails without the fix, passes with it.

[Copilot]

Pull request overview

Updates the helm/tools/grafana-mcp Helm chart to render a valid container image reference when image.registry is explicitly set to an empty string, preventing invalid leading-slash image values that cause ImagePullBackOff.

Changes:

• Introduced a grafana-mcp.image helper that builds the image path by joining only non-empty segments and appending the tag.
• Updated the Deployment template to use the new helper for the container image field.
• Added Helm unittest cases to guard the default image value and verify the empty-registry behavior (including a no-leading-slash check).

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
helm/tools/grafana-mcp/templates/_helpers.tpl	Adds grafana-mcp.image helper to compose image references without empty path segments.
helm/tools/grafana-mcp/templates/deployment.yaml	Switches the Deployment container image field to use the new helper.
helm/tools/grafana-mcp/tests/deployment_test.yaml	Adds regression + empty-registry Helm unittest assertions for the rendered image field.

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