Option to error on missing postBuild substitution variable

[sjdrc]

Describe the bug

I'm currently trying to convert some helmfiles into flux manifests.

We currently use environment values to allow us to configure an entire application stack, and allow users to install everything according to our configuration but with setting their own required inputs. As helmfile uses gotemplates for generating the final manifests we can do logic such as requiring environment values be set (with {{ required "You must provide a value for someValue!" .Values.someValue }}).

It looks like flux post build variable substitution will do the trick for us, and the bash string replacement functions are useful, but I'm quite surprised that by default an unset variable will just evaluate to an empty string. I want to set up flux manifests that require these values to be set and returns an error if you do not.

The kubernetes cluster-api project have an open issue discussing switching to an alternative implementation of envsubst, but there is not feature parity between the two projects.

Would it be possible to add an option to flux to error if any of the variables are not set? (Or evaluate to an empty string)

Steps to reproduce

N/A

Expected behavior

Able to enforce users to provide values for postBuild.substitute

Screenshots and recordings

No response

OS / Distro

N/A

Flux version

N/A

Flux check

N/A

Git provider

No response

Container Registry provider

No response

Additional context

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[souleb]

I looked at the linked issue.

If I'm not wrong with the proposed replacement we are going to lose a couple of the syntax supported by the current library (https://github.com/drone/envsubst vs https://github.com/a8m/envsubst#docs), this will be breaking for the users.

I agree with this, we don't want to break our users setup, this could result in major pain in order to replace whole setups.

I think the best would be for drone/envsubst#34 to land.

[sjdrc]

I think the best would be for drone/envsubst#34 to land.

That repo's last update is over 3 and a half years old and they've disabled their issue tracker (and it's that PR's birthday today!). Looks to be unmaintained.

I did find this fork though https://github.com/gomodules/envsubst that actually already has this functionality added to it!

Would you consider switching to use this package instead? If so I can open a PR.

[stefanprodan]

Would you consider switching to use this package instead?

Let's see if @tamalsaha is keen on maintaining it going forward.

[tamalsaha]

Yes, we maintain that project and use it internally.

[stefanprodan]

@sjdrc we could switch to gomodules/envsubst and add a feature flag to kustomize-controller that when set, would fail the reconciliation when vars are missing. We also need to look into what other changes are in gomodules/envsubst and make sure they don't change the behaviour. Given that kustomize-controller is GA, any behaviour changes to substitutions must be opt-in.

[sjdrc]

That would be excellent and would fulfill my use case perfectly!

[stefanprodan]

@sjdrc can you help implementing this?

[sjdrc]

@stefanprodan
I'm not too familiar with Go, but I'm happy to give it a try over the long weekend!

[stefanprodan]

@sjdrc I've read all the code changes in https://github.com/gomodules/envsubst and we can't use. The PR from upstream wasn't ported, instead this fork errors out for undefined vars which breaks Flux.