[ApplicationSet] Support Go Templating in spec.generators

[llavaud]

Summary

Support Go Templating in ApplicationSet spec.generators

Motivation

I would like to be able to do some Go Templating on the parameters generated by another generator like in the following example:

spec:
  goTemplate: true
  generators:
    - matrix:
        generators:
          - git:
              repoURL: https://github.com/argoproj/applicationset.git
              revision: HEAD
              files:
                - path: "examples/git-generator-files-discovery/cluster-config/**/config.json"
          - clusters:
              selector:
                matchLabels:
                  argocd.argoproj.io/secret-type: cluster
                  kubernetes.io/environment: '{{default "foo" .my_label}}'

Proposal

Implemented in the same way as the support of Go Templating in spec.template ?

[llavaud]

after digging a little into the code, it seems that goTemplate must be supported in spec.generators.

with the following spec, I got a parsing error:

spec:
  goTemplate: true
  generators:
    - matrix:
        generators:
          - git:
              repoURL: https://github.com/myrepo.git
              revision: master
              files:
                - path: instances/*/argocd.yaml
          - clusters:
              selector:
                matchLabels:
                  argocd.argoproj.io/secret-type: cluster
                  cloud_provider: '{{default "aws" .cloud_provider}}'

It fails with the following error:

failed to get params for second generator in the matrix generator: child generator returned an error on parameter generation: failed to parse template {"clusters":{"selector":{"matchLabels":{"argocd.argoproj.io/secret-type":"cluster","cloud_provider":"{{default \"aws\" .cloud_provider}}"}},"template":{"metadata":{},"spec":{"source":{"repoURL":""},"destination":{} ,"project":""}}}}: template: :1: unexpected "\\" in operand

It seems to failed here when parsing the manifest.

[crenshaw-dev]

iirc the problem is that the "second generator" templating is using the old fashioned method of "marshal everything to JSON, template over it, then unmarshal back." It's janky and breaks easily. Instead we should use the same field-wise templating that we use in the template field.