Helm v4 breaking change in `helm template --post-renderer`, Skaffold failing

[AndiDog]

You'll get an error like this with Skaffold + Helm v4 pre-release:

Error: invalid argument "/opt/homebrew/bin/skaffold" for "--post-renderer" flag: plugin: {Name:/opt/homebrew/bin/skaffold Type:postrenderer/v1} not found

I'm trying to find out a good implementation path in the upstream issue helm/helm#31340.

[scottrigby]

hi @AndiDog 👋

I'm a Helm maintainer who proposed the new plugin system in Helm 4, which changed the post-renderer behavior. Skaffold should still work as a post-renderer in Helm 4, with some minimal changes on the Skaffold side. I'd like to work with you on fixing this in Skaffold as users test out the Helm 4 pre-releases leading up to the full 4.0.0 release scheduled for November (full Hem 4 development timeline is here: https://github.com/helm/community/blob/main/hips/hip-0012.md#the-process--timelines).

I reached out to you in the #skaffold k8s slack channel too, so we can have more real-time collaboration on this if you'd like to, especially given the timeline.

I read https://skaffold.dev/docs/deployers/helm/ to understand how Skaffold handles post-rendering with Helm 3.

You hit the nail on the head with how to do this in Helm 4, in your comment here helm/helm#31340 (comment).

To explain for everyone else – Helm 4 post-renderers have been moved to a new plugin system. Docs for Helm 4 are underway, but the implementation is pretty faithful to the proposal, which you can read here: https://github.com/helm/community/blob/main/hips/hip-0026.md. So yes, you can still use the skaffold binary for post-rendering in Helm 4, using the new subprocess plugin runtime and postrenderer/v1 plugin type. I replied with general information about this for any binary (sed as an example) on the upstream issue too, because I anticipate questions from other people about using other binaries.

As a follow-up, you may also want to look at building a Skaffold plugin for the Wasm runtime too – which is a big reason why we upgraded the plugin system in Helm 4 – but I'd suggest starting with the subprocess runtime because it's essentially a drop-in replacement for the way you do post-rendering with Helm 3. This can help get it out the door for folks to test the Helm 4 pre-release and before the 4.0.0 release. But I'd be happy to look at a Wasm plugin with you too.