Emit a warning when env loading bug is being relied on

[KnVerey]

Problem

It recently came to our attention that the configMapGenerator has a bug where it will load values from the environment when a line in an env file has a key but no value. This bug dates back to the origins of Kustomize, when some of the original code was copied out of kubectl, and this undesirable behaviour (for kustomize, not kubectl) went unnoticed in the adaptation.

This behaviour is a clear violation of one of Kustomize's core principles, as documented in our Eschewed Features list:

Very unfortunately, someone not only discovered this bug, but made a PR to document it as a feature on kubernetes.io: kubernetes/website#30348. Unfortunately no Kustomize maintainers were tagged on the PR, and it has been live since January 2022. 😞

As stated in Kustomize documentation screenshot above, any need for environment reading should be handled as part of a pre-build workflow. We can consider adding a kustomize edit subcommand for this specifically if there is demand from those currently relying on the bug. For example, it could look like kustomize edit add configmap my-configmap --from-env-file=env/path.env --from-env=ONE,TWO.

Proposal

• Immediately revert the docs PR incorrectly describing this bug as a feature: Revert documentation of Kustomize bug as a feature kubernetes/website#35522
• Start emitting a warning to those relying on this bug (because it has existed for so long, and was in the docs for 7 months) -- That's this PR
• Patch the bug in the next major version of Kustomize, i.e. v5: ConfigMapGenerator should not load values from the build environment #4731

I don't think there's a nice way to unit test the printing to sdterr like this, barring making invasive changes to the kv loader method signatures (lmk if you have an idea for something nice). Here are some manual tests:

/cc @natasha41575 @annasong20

[k8s-ci-robot]

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In response to this:

Problem

It recently came to our attention that the configMapGenerator has a bug where it will load values from the environment when a line in an env file has a key but no value. This bug dates back to the origins of Kustomize, when some of the original code was copied out of kubectl, and this undesirable behaviour (for kustomize, not kubectl) went unnoticed in the adaptation.

This behaviour is a clear violation of one of Kustomize's core principles, as documented in our Eschewed Features list:

Very unfortunately, someone not only discovered this bug, but made a PR to document it as a feature on kubernetes.io: kubernetes/website#30348. Unfortunately no Kustomize maintainers were tagged on the PR, and it has been live since January 2022. 😞

As stated in Kustomize documentation screenshot above, any need for environment reading should be handled as part of a pre-build workflow. We can consider adding a kustomize edit subcommand for this specifically if there is demand from those currently relying on the bug. For example, it could look like kustomize edit add configmap my-configmap --from-env-file=env/path.env --from-env=ONE,TWO.

Proposal

• Immediately revert the docs PR incorrectly describing this bug as a feature
• Start emitting a warning to those relying on this bug (because it has existed for so long, and was in the docs for 7 months) -- That's this PR
• Patch the bug in the next major version of Kustomize, i.e. v5

I don't think there's a nice way to unit test the printing to sdterr like this, barring making invasive changes to the kv loader method signatures (lmk if you have an idea for something nice). Here are some manual tests:

/cc @natasha41575 @annasong20

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository.

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[natasha41575]

/lgtm

I can retag after you fix the lint error

[KnVerey]

I can retag after you fix the lint error

bah, line length! 🤦‍♀️ Fixed, and I re-ran my manual tests:

[natasha41575]

/lgtm

[sftim]

BTW, kubectl emits warnings in yellow; see https://kubernetes.io/blog/2020/09/03/warnings/

Consider copying that style.

[annasong20]

I don't know the difference between os.Stderr and log, but I recently learned that you can use log.SetOutput to specify the output location of log functions. This made it really easy for me to check logging in tests.

[KnVerey]

This PR does not change behaviour; it simply emits a warning. The provided samples build fine for me with the latest release of Kustomize, and do not hit the warning case in this PR. If you're able to reproduce, please open a new issue and include the Kustomize version and error message.