Update backward compatibility promise in CONTRIBUTING.md to reflect HIP 0004

[banjoh]

In the Semantic Versioning section of CONTRIBUTING.md document, the backward compatibility promise does not align to HIP-0004

CONTRIBUTING.md

We also try very hard to not change publicly accessible Go library definitions inside of the pkg/ directory of our source code.

• This ought to say public functions MUST not be changed in the pkg/ directory

Go libraries inside of pkg/ SHOULD remain backward compatible, though code inside of cmd/ and internal/ may be changed from release to release without notice.

• SHOULD ought to be changed to MUST
• Question: Since cmd/ is meant to be internal code, should have all code that isn't meant to be a subcommand live in cmd/helm/internal, to allow ease of sharing code within the cmd/helm package, but ensure no one can import that code?

The section should point to HIP-0004

HIP-0004

programs using the Helm Go API will continue to compile

• Should this also include changing functionality? If a public function/method should probably not change overall internal functionality e.g if a change removes previous functionality and makes the function a no-op, perhaps this should not be allowed.

[scottrigby]

@banjoh thanks for making this follow-up issue! Adding my replies from slack here for better transparency.

Should this also include changing functionality?

HIP-0004 covers that point, indirectly, by linking to another article that covers it:

exported Go APIs will remain compatible as per this article

that article, under “Add configuration methods”, says:

So far we’ve talked about overt breaking changes, where changing a type or a function would cause users’ code to stop compiling. However, behavior changes can also break users, even if user code continues to compile…

[scottrigby]

how about changing this:

We also try very hard to not change publicly accessible Go library definitions inside of the pkg/ directory of our source code.
to something like:

We also remain committed to not change publicly accessible Go library definitions inside of the pkg/ directory of our source code in a non-backwards-compatible way.

and link to the HIP like you suggested, for more details on Helm’s minor and patch release backwards-compatibility rules before the summary bulletin list?

[banjoh]

I'll incorporate your suggestions in a PR.

Leaving the description as is, else comments won't make sense afterwards :)