HIP: Default to strict Chart.yaml loading

Signed-off-by: George Jenkins <[email protected]>

hips/hip-9999.md

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+---
+hip: 9999
+title: "Default to strict `Chart.yaml` loading"
+authors: [ "George Jenkins <[email protected]>" ]
+created: "2024-11-18"
+type: "feature"
+status: "draft"
+---
+
+## Abstract
+
+Helm should default to using strict-mode yaml loading for loading charts ie. `yaml.UnmarshalStrict(..)` rather than `yaml.Unmarshal(..)`.
+
+
+## Motivation
+
+Currently Helm loads chart's Chart.yaml in "non-strict" yaml loading mode.
+The main consequence of this behavior is that Helm ignores unknown and duplicate fields in `Chart.yaml`.
+Either unknown to the present version of Helm, or simply typos.
+In that the behavior the user expects from Helm as described by unknown fields is effectively ignored.
+Presenting a confusing experience to the user.
+
+
+## Rationale
+
+Also see: [HIP-9999][hip-9999] ("Forward compatibility: `Chart.yaml` `minimumHelmVersion`"), for a complementary HIP that allows further tightening of Helm's forward compatibility behavior.
+
+
+## Specification
+
+Helm will default to using strict-mode yaml loading for loading charts ie. ie. `yaml.UnmarshalStrict(..)` rather than `yaml.Unmarshal(..)`.
+
+The CLI/SDK will contain an option to allow non-strict loading.
+This is to allow users to continue to utilize charts that are malformed.
+
+
+## Backwards compatibility
+
+Helm 4 allows breaking behavioral changes.
+However it is a core premise that Helm 4 will remain compatibility with existing chart.
+Moving to strict mode for loading yaml will prevent charts from being loaded that contain unknown/invalid fields.
+Requiring a command line flag / SDK option allows fallback compatibility if required.
+
+## Security implications
+
+N/A
+
+
+## How to teach this
+
+Update `Chart.yaml` docs
+
+
+## Reference implementation
+
+See PR links in [References]
+
+
+## References
+
+- \[chart/loader\] use strict yaml unmarshaling for chart files <https://github.com/helm/helm/pull/11818>
+- fix(pkg/lint): unmarshals Chart.yaml strictly <https://github.com/helm/helm/pull/12382>
+
+[hip-9999]: ./hip-9999.md