Odd behavior with empty lists, concat, and toJson

[mcg1969]

Output of helm version:

version.BuildInfo{Version:"v3.7.2", GitCommit:"663a896f4a815053445eec4153677ddc24a0a361", GitTreeState:"clean", GoVersion:"go1.16.10"}

Output of kubectl version:

Client Version: version.Info{Major:"1", Minor:"23", GitVersion:"v1.23.3", GitCommit:"816c97ab8cff8a1c72eccca1026f7820e93e0d25", GitTreeState:"clean", BuildDate:"2022-01-25T21:25:17Z", GoVersion:"go1.17.6", Compiler:"gc", Platform:"darwin/amd64"}
Server Version: version.Info{Major:"1", Minor:"23", GitVersion:"v1.23.3", GitCommit:"816c97ab8cff8a1c72eccca1026f7820e93e0d25", GitTreeState:"clean", BuildDate:"2022-01-25T21:19:12Z", GoVersion:"go1.17.6", Compiler:"gc", Platform:"linux/amd64"}

Cloud Provider/Platform (AKS, GKE, Minikube etc.): Minikube 1.25.1

Odd behavior of the Helm/Spring concat function with regards to empty lists:

Input:

# {{ list | toJson }}
# {{ concat (list) (list) | toString }}
# {{ concat (list) (list) | toJson }}
# {{ dict | toJson }}
# {{ merge (dict) (dict) | toString }}
# {{ merge (dict) (dict) | toJson }}

What I would expect:

# []
# []
# []
# {}
# map[]
# {}

What I actually get: in the concat case, a null instead of the empty list.

# []
# []
# null
# {}
# map[]
# {}

I've already worked around this, but I wanted to report it and see if this is intended behavior—because if it is, the docs do not indicate this would be the behavior. Perhaps there is a subtlety of the JSON standard I'm missing!

[bacongobbler]

hmm... Functions like concat are provided by third-party libraries. Specifically sprig. I would file issues related to function output there. When a fix has been made upstream, we can bump to the latest version of sprig and receive the fix.

Thanks for the report.

[bacongobbler]

however, if it's toJson that may be the culprit, here's the source for that:

helm/pkg/engine/funcs.go

Lines 140 to 147 in a499b4b

	func toJSON(v interface{}) string {
	data, err := json.Marshal(v)
	if err != nil {
	// Swallow errors inside of a template.
	return ""
	}
	return string(data)
	}

[mcg1969]

The problem is that I don't know for sure if the culprit is sprig or Helm's use of it. The fact that the toString form is correct but toJson is not suggests it's the latter. Again, no worries, I have a workaround for this already, but I do think it's worth figuring out for future users.

I suppose I should have tested toYaml as well...

[github-actions]

This issue has been marked as stale because it has been open for 90 days with no activity. This thread will be automatically closed in 30 days if no further activity occurs.

[Tommyf]

I just ran into this exact issue, but with toYaml.

For now, I need to use a workaround if I am likely to be using concat on potentially empty lists.

Do we know if this is a problem with the concat function in Sprig, or the toYaml/toJson functions in Helm?

[joejulian]

Doesn't look to be a helm problem. See this go-template playground example.