render template against merged release template before rendering final values

base-1.yaml.gotmpl

# controls which charts are installed
environments:
    default:
        values:
        - fake:
            install: false
    ci_cd:
        values:
        - fake:
            install: true

base-2.yaml.gotmpl

# controls values in the release
# but elastic-exporter.values.gotmpl is evaluated too early
# want a method to apply the template after all other values are merged
templates:
    release_template:
        values:
            - chart:
                key: template-default-value
            - elastic-exporter.values.gotmpl
        installedTemplate: |
            {{`{{ eq .Values.fake.install true }}`}}

demo.sh

# passes because installedTemplate will be set to false
helmfile diff --environment=default && echo "environment=default: PASS"

# fails because elastic-exporter.values.gotmpl will be rendered
# but it tries to set a value whose key is loaded in the same render
helmfile diff --environment=ci_cd && echo "environment=ci_cd: PASS" || echo "environment=ci_cd: FAIL"

elastic-exporter.values.gotmpl

{{/* won't render because chart.key is created in the same base and doesn't "exist" yet */}}
{{- $chartVals := .Values }}
{{- $chartVals = $chartVals | setValueAtPath "chart.key" "chart-specific-val" }}
{{ toYaml $chartVals }}

helmfile.yaml

bases:
- base-1.yaml.gotmpl
---
bases:
- base-2.yaml.gotmpl
---
repositories:
- name: prometheus-community
  url: https://prometheus-community.github.io/helm-charts

releases:
- name: elastic-exporter
  namespace: monitoring
  chart: prometheus-community/prometheus-elasticsearch-exporter
  inherit:
  - template: release_template

output of bash demo.sh

Adding repo prometheus-community https://prometheus-community.github.io/helm-charts
"prometheus-community" has been added to your repositories

Listing releases matching ^elastic-exporter$
environment=default: PASS
Adding repo prometheus-community https://prometheus-community.github.io/helm-charts
"prometheus-community" has been added to your repositories

in ./helmfile.yaml: failed processing release elastic-exporter: failed to render values files "elastic-exporter.values.gotmpl": failed to render [elastic-exporter.values.gotmpl], because of template: stringTemplate:3:30: executing "stringTemplate" at <setValueAtPath "chart.key" "chart-specific-val">: error calling setValueAtPath: failed to set value at path "chart.key": value for key "chart" does not exist
environment=ci_cd: FAIL

This gist was created to showcase a dilemma in helmfile/helmfile#1261

Note that if you changed elastic-exporter.values.gotmpl to the following that it renders successfully.

chart:
  key: chart-specific-val

The point is that programmatic merging with custom template functions like setValueAtPath can't be used because it seems that .Values is not kept up to date as the values/files are loaded in the current phase.