This problem kinda sucks because the original YAML will have quotes that are not preserved between the conversion of kustomize template->JSON->YAML, and if we desire to keep them, there's no way to indicate to kustomize that they shouldn't be removed. Suppose we start with this YAML kustomize template:

apiVersion: v1
kind: ConfigMap
data:
  test1: "${TEMPLATE_VAR1}"
  test2: "{TEMPLATE_VAR2}"
  test3: "true"
  test4: "test4"
metadata:
  name: config

When executing kustomize build, it'll read the kustomization.yaml file and render the manifest. I've narrowed the path down to the JSONToYAML() function used by the build command here: https://github.com/kubernetes-sigs/kustomize/blob/33caee50cb25954e1889cea30e3a5b6283e0bfef/api/resource/resource.go#L371

This function is supposed to take a JSON byte array and turns it into valid YAML:

JSON:

{"apiVersion":"v1","data":{"test1":"${TEMPLATE_VAR1}","test2":"{TEMPLATE_VAR2}","test3":"true","test4":"test4"},"kind":"ConfigMap","metadata":{"name":"config"}}

FINAL RESULT

apiVersion: v1
data:
  test1: ${TEMPLATE_VAR1}
  test2: '{TEMPLATE_VAR2}'
  test3: "true"
  test4: test4
kind: ConfigMap
metadata:
  name: config

My assumption is that any string that isn’t already a valid JSON type (such as a sub structure with {} or a boolean true) gets its quotes stripped (You can test the code out in this playground link: https://go.dev/play/p/rFUUlFjPk5f):

Our investigation suggests that the YAML parser is actually doing the "right thing", but the fact that quotes are removed when we explicitly need them, and there appears to be no way to force them to be kept, is very problematic in our use case.

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I'm also seeing this but its causing "1234" to be turned into 1234 in resource annotations and labels, which is ironically making kubernetes' own YAML parser not compliant with its own resource manifest schemas.

/remove-lifecycle stale

This could be solved if the output of JSONToYAML() included explicit yaml type tags, although that is a pretty ugly solution eg.

apiVersion: v1
data:
  test1: !!str ${TEMPLATE_VAR1}
  test2: !!str '{TEMPLATE_VAR2}'
  test3: !!str "true"
  test4: !!str test4
kind: ConfigMap
metadata:
  name: config

Kustomize at least knows that the values must all be strings

---
apiVersion: v1
kind: ConfigMap
data:
  test1: !!str "${TEMPLATE_VAR1}"
  test2: "{TEMPLATE_VAR2}"
  test3: "true"
  test4: "test4"
  test5: !!str no
  test6: !!bool no
metadata:
  name: config

❯ kustomize build .
Error: map[string]interface {}(nil): yaml: cannot decode !!str `no` as a !!bool

Could you instead prepend !!str to the value of your ${TEMPLATE_VAR1} as a workaround like this?

❯ kustomize build . | TEMPLATE_VAR1='!!str 1234' envsubst
apiVersion: v1
data:
  test1: !!str 1234
  test2: '{TEMPLATE_VAR2}'
  test3: "true"
  test4: test4
  test5: "no"
kind: ConfigMap
metadata:
  name: config

So yaml.encode.stringv() has an internal variable canUsePlain, that's set for multi-line strings or for strings aliasing reserved words like "true". We'd like to pass in on override that always sets canUsePlain to false.

So we'd need to have a parameters object passed through yaml.JSONToYAML(), then yaml.Marshal(), then encoder.newEncoder(). And I guess we'd need a new top-level function name like yaml.JSONToYAMLWithOptions() to keep compatibility with callers, since Go doesn't have optional arguments?

My concern is that while we do use a fork of go-yaml, there's this notice:

In this fork, we plan to support only critical changes required for kubernetes, such as small bug fixes and regressions. Larger, general-purpose feature requests should be made in the upstream go-yaml library, and we will reject such changes in this fork unless we are pulling them from upstream.

I doubt the upstream maintainer would want to support a relative niche feature in their general-purpose library. But on the other hand, I don't see how kustomize can do the right thing without it, and the point of having this library is to support projects like kustomize. Could we make this change in the fork anyway?

Tagging @niemeyer as the upstream maintainer, about whether either of the changes mentioned in my last comment would be accepted upstream.

1. Pass a params object to yaml.Marshal() (e.g. yaml.MarshalWithOptions()) with a parameter to always enclose string values in quotes.
2. Improve behavior of encoder.stringv() for special cases like "- this is not a list, it's a string". [ignore, this actually works]