helm: "Unexpected unclosed action in template clause" when trying to render templated string from values.yaml

Many charts currently support Prometheus Operator by optionally adding “prometheusRule” objects to the cluster. A user can then just add the rules in his “values.yaml”. The supplied values can be templated strings and will be used by a “prometheusrule.yaml” template. This works for simple rules, but in most cases the chart will just refuse to render properly. The funny thing is, that even the example alerts that can be found in the values.yaml files of many charts are not working.

I don’t know where the problem exactly is, but it looks like something is trying to pretty print the values by inserting a “\n” at a position where it should not be and this breaks the template in most cases (see "{{ template \"test.fullname\"\n . }} in the complete error message below). I already tried all kinds of workarounds like removing whitespaces, playing with single and double quotes, yaml multiline … nothing seems to work.

values.yaml:

prometheusRule:
  enabled: true
  additionalLabels: {}
  namespace: "monitoring"
  rules:
    - alert: MyTestAlert
      expr: my_test_alert_with_a_long_name_kjlgnkjfdg{service="{{ template "test.fullname" . }}-mymetricsservice"} / 3600 > 1
      for: 1m
      labels:
        severity: critical
      annotations:
        description: |-
          The {{ template "test.fullname" . }} has a problem. The value is {{ "{{ $value }}" }}
        summary: Something {{ "{{ $value }}" }}

prometheusrule.yaml:

{{- if .Values.prometheusRule.enabled }}
apiVersion: monitoring.coreos.com/v1
kind: PrometheusRule
metadata:
  name: {{ template "test.fullname" . }}
{{- with .Values.prometheusRule.namespace }}
  namespace: {{ . }}
{{- end }}
  labels:
    app: {{ template "test.name" . }}
    chart: {{ template "test.chart" . }}
    release: {{ .Release.Name | quote }}
    heritage: {{ .Release.Service | quote }}
{{- with .Values.prometheusRule.additionalLabels }}
{{ toYaml . | indent 4 }}
{{- end }}
spec:
{{- with .Values.prometheusRule.rules }}
  groups:
    - name: {{ template "test.name" $ }}
      rules: {{ tpl (toYaml .) $ | nindent 8 }}
{{- end }}
{{- end }}

Output of helm template .:

Error: template: test/templates/prometheusrule.yaml:21:16: executing "test/templates/prometheusrule.yaml" at <tpl (toYaml .) $>: error calling tpl: error during tpl function execution for "- alert: MyTestAlert\n  annotations:\n    description: The {{ template \"test.fullname\" . }} has a problem. The value is\n      {{ \"{{ $value }}\" }}\n    summary: Something {{ \"{{ $value }}\" }}\n  expr: my_test_alert_with_a_long_name_kjlgnkjfdg{service=\"{{ template \"test.fullname\"\n    . }}-mymetricsservice\"} / 3600 > 1\n  for: 1m\n  labels:\n    severity: critical": parse error at (test/templates/prometheusrule.yaml:6): unexpected unclosed action in template clause

Output of helm version:

version.BuildInfo{Version:"v3.1.1",` GitCommit:"afe70585407b420d0097d07b21c47dc511525ac8", GitTreeState:"clean", GoVersion:"go1.13.8"}

Client: &version.Version{SemVer:"v2.14.1", GitCommit:"5270352a09c7e8b6e8c9593002a73535276507c0", GitTreeState:"clean"}
Server: &version.Version{SemVer:"v2.14.1", GitCommit:"5270352a09c7e8b6e8c9593002a73535276507c0", GitTreeState:"clean"}