Advanced templating for ApplicationSet

[speedfl]

Summary

Currently ApplicationSet template is using https://github.com/valyala/fasttemplate. But it does not provide advanced templating feature which can be useful for inline helm values

Motivation

When using inline helm values it could be nice to have templating capabilities such as loop, if etc

apiVersion: argoproj.io/v1alpha1
kind: ApplicationSet
metadata:
  name: guestbook
  namespace: argocd
spec:
  generators:
  - git:
      repoURL: https://github.com/argoproj/argo-cd.git
      revision: HEAD
      files:
      - path: "applicationset/examples/git-generator-files-discovery/cluster-config/**/config.json"
  template:
    metadata:
      name: '{{cluster.name}}-guestbook'
    spec:
      project: default
      source:
        repoURL: https://github.com/argoproj/argo-cd.git
        targetRevision: HEAD
        path: "applicationset/examples/git-generator-files-discovery/apps/guestbook"
      destination:
        server: '{{cluster.address}}'
        namespace: guestbook
      helm:
        values: |
          fieldWithIf: {{#isOk}}{{myOkValue}}{{/isOk}}{{^isOk}}{{myNonOkValue}}{{/isOk}}
          fieldWithLoop
          {{#elements}}
            - name: {{elements.name}}
              value: {{elements.value}}
          {{/elements}}

Proposal

Use Advanced templating such as Mustache. I am opened to work on a PR using https://github.com/hoisie/mustache

[todaywasawesome]

Would this allow setting default values? That would be HUGE.

[crenshaw-dev]

There's a PR to do something similar: #9873

I would like to see that PR drop fasttemplate support, since it's no longer being developed.

I'd rather use Go text templating with Sprig functions rather than Mustache. The Argo suite already makes use of a lot of different template languages/styles. I think keeping it close to the Go ecosystem is advisable.

[crenshaw-dev]

Oh! That PR is for full-template templating. If you only want it per-field, it should be much easier. I'd still advise using Go text templates rather than Mustache.