How can you call a helm 'helper' template from a subchart with the correct context?

Question

Helm charts define helper templates in _helpers.tpl which are used to create normalized names for the services. The standard form of the template for a service (DNS) name is:

{{- define "postgresql.fullname" -}}
{{- $name := default .Chart.Name .Values.nameOverride -}}
{{- printf "%s-%s" .Release.Name $name | trunc 63 | trimSuffix "-" -}}
{{- end -}}

If using postgresql as a subchart, how are you supposed to use DNS-based service discovery to refer to it? A common pattern seems to be to copy the subchart helpers into the parent chart.

{{- define "keycloak.postgresql.fullname" -}}
{{- $name := default "postgresql" .Values.postgresql.nameOverride -}}
{{- printf "%s-%s" .Release.Name $name | trunc 63 | trimSuffix "-" -}}
{{- end -}}

This is total madness for obvious reasons!!!

Surely there's a better way to use the subchart helper? If you could create a context object then it would be possible to invoke it, something like:

value: {{ template "postgresql.fullname" ({Chart: {Name: 'not-used'}, Release: {Name: .Release.Name}, Values: { nameOverride: .Values.postgresql.nameOverride}}) }}

Sadly I have no idea how to actually create such a context dynamically. This would still break if the helper function changed to reference new properties, but in an obvious way.

Alternatively, a different way to make available the service name from a subchart?

Answer 1

I wrote an issue helm/helm#4535 that summarizes the status-quo and proposes an enhancement to Helm which will solve this case.

For anyone looking for an interim solution, I wrote (see my follow-up comment for details) a meta-template that calls any given template in an "ersatz" subchart's scope. It works by synthesizing a dot-object. It is not perfect (not all fields are synthesized), but it will do:

{{- define "call-nested" }}
{{- $dot := index . 0 }}
{{- $subchart := index . 1 }}
{{- $template := index . 2 }}
{{- include $template (dict "Chart" (dict "Name" $subchart) "Values" (index $dot.Values $subchart) "Release" $dot.Release "Capabilities" $dot.Capabilities) }}
{{- end }}

Usage (to call a redis.fullname template of a redis subchart):

{{ include "call-nested" (list . "redis" "redis.fullname") }}