Why helm template function is not resolveing $labels var? - kubernetes-helm

Question

I am defining a PrometheusRule as follow:
prometheusRule:
rules:
- alert: SSLCertExpiringSoon
expr: probe_ssl_earliest_cert_expiry - time() < 86400 * 10
for: 0m
labels:
severity: warning
annotations:
summary: Blackbox SSL certificate will expire soon (instance {{ $labels.instance }})
description: "SSL certificate expires in 30 days\n VALUE = {{ $value }}\n LABELS = {{ $labels }}"
And the template yml from helm chart:
{{- if .Values.prometheusRule.enabled }}
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
name: {{ template "prometheus-blackbox-exporter.fullname" . }}
{{- with .Values.prometheusRule.namespace }}
namespace: {{ . }}
{{- end }}
labels:
{{- include "prometheus-blackbox-exporter.labels" . | nindent 4 }}
{{- with .Values.prometheusRule.additionalLabels -}}
{{- toYaml . | nindent 4 -}}
{{- end }}
spec:
{{- with .Values.prometheusRule.rules }}
groups:
- name: {{ template "prometheus-blackbox-exporter.name" $ }}
rules: {{ tpl (toYaml .) $ | nindent 8 }}
{{- end }}
{{- end }}
when I run helm template, tpl func it is not resolving the $labels and $values vars. When I remove annotations then the helm template is not complaining anymore. Where do fail?
error:
Error: template: prometheus-blackbox-exporter/templates/prometheusrule.yaml:18:16: executing "prometheus-blackbox-exporter/templates/prometheusrule.yaml" at <tpl (toYaml .) $>: error calling tpl: error during tpl function execution for "- alert: SSLCertExpiringSoon\n annotations:\n summary: Blackbox SSL certificate will expire soon (instance {{ $labels.instance\n }})\n expr: probe_ssl_earliest_cert_expiry - time() < 86400 * 10\n for: 0m\n labels:\n release: prometheus\n severity: warning\n- alert: SSLCertExpiringSoon\n annotations: null\n expr: probe_ssl_earliest_cert_expiry - time() < 86400 * 3\n for: 0m\n labels:\n severity: critical": parse error at (prometheus-blackbox-exporter/templates/prometheusrule.yaml:3): undefined variable "$labels"

Answer 1

Prometheus's alerting rules also use {{ ... $variable ... }} syntax, similar to Helm but with a different variant on the Go text/template syntax. When you pass this file through tpl, Helm tries to evaluate the embedded {{ ... }} template and evaluate any blocks there. Since $labels and $value aren't local variables defined at the Helm level, you get this error.
If you just want Prometheus to see this file as-is, and you don't need to replace anything at the Helm level (the file doesn't include references to .Values) then you don't need tpl
rules: {{ toYaml . | nindent 8 }}
If you do need tpl, then inside the included file you need to cause {{ to be emitted as a string and not processed as a template. One syntactic approach to it is to create a template block that prints out {{:
description: "VALUE = {{ "{{" }} $value }}"
# ^^^^^^^^^^ a {{ ... }} block that prints "{{"

Answer 2

The working version of syntax is as follow:
{{ `{{` }} $value }}