Helm: a YAML merge key (<<) in Chart.yaml gives Helm another name and version than Forge reads (follow-up to #232, #422) #428

What

Forge reads Chart.yaml with yaml-rust2 (parse_chart_yaml in plugins/helm/src/chart.rs). yaml-rust2 does not implement YAML merge keys: a top-level << is an ordinary key, and Forge ignores it.

Helm parses Chart.yaml with sigs.k8s.io/yaml, which is based on a fork of go-yaml v2. go-yaml v2 does merge: the mapping (or each mapping of a sequence) under << is decoded into the surrounding mapping where the << stands. Its keys overwrite the keys written before it.

Found while fixing #422 (#427). Checked with a throwaway Go harness that runs Helm 3.17.0's loader.LoadFiles on a Chart.yaml:

Unlike #422, this can change the chart's name, which is the class of #232: a publisher whose package selector allows only good publishes a chart that helm install loads as evil. A merged mapping can also bring in a key that #427's folding check never sees, such as <<: {verſion: 6.6.6}.

Failure scenario

• Push: a publisher allowed only good pushes name: good / version: 1.0.0 / <<: [{name: evil}]. Forge stores and lists good 1.0.0. Helm installs evil 1.0.0.
• Proxy: an upstream chart built this way is cached under the identity Forge reads. The identity check against foreign charts (Helm push and proxy read the first Chart.yaml of an archive, while Helm loads the last one (two top-level directories or duplicate entries), the Helm counterpart of #34 #232, crates/core/src/repo/proxy.rs) compares Forge's reading, so it does not see the mismatch.

Already safe:

• requirements.yaml accepts only the exact key dependencies, so a << there is refused.
• An index entry carries only the exact METADATA_KEYS.

Suggested fix

In parse_chart_yaml, refuse a Chart.yaml whose top-level mapping has a << key (400 on a push, a rejection on a proxy download). A chart has no use for one.

• After loading, yaml-rust2 cannot tell a plain << from a quoted or !!str-tagged one. Refusing every top-level << is the simple, safe rule; Helm ignores the quoted forms anyway.
• Add tests for each row of the table, including an alias (base: &b {name: evil} … <<: *b).

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