Run ENV Substitution if empty substituted

[oliverbaehler]

Hi

I was playing around with variable substitutions. I made heavy use of defaulting mechanisms. Till I noticed, that defaults are not evaluated, when the owning Kustomization does not have any variables substituted. In fact no expression is evaluated. Looking at the code, this condition causes this:

kustomize-controller/controllers/kustomization_varsub.go

Line 75 in 80ebb79

if len(vars) > 0 {

I am not quiet sure if this is on purpose but the expected behavior for me would be if i have a default expression, it should default if the variable isn't set. But before trying to change the code I wanted to check with you guys, what you think about it.

[stefanprodan]

We can't enable var substitution by default, it needs to be opt-in (defined now as having at least one var). The reason for this is that envsub replaces all vars without a default with string empty and it has the potential to break scripts embedded in configmap and other objects.

[oliverbaehler]

Yeah i get that point, but shouldn't such resources, which might have such expressions in them disabled substitution (aka kustomize.toolkit.fluxcd.io/substitute: disabled). So I guess the way to work around this, is having something like this:

apiVersion: kustomize.toolkit.fluxcd.io/v1beta1
kind: Kustomization
metadata:
  name: shared
spec:
  ...
  postBuild:
    substitute:
      enabled: "true"

I guess that works for me, if there's no interest in this change.

[stefanprodan]

@oliverbaehler I would welcome a PR to the docs here that explains the current behaviour with your example on how to enable substitutions for default values.