Add documentation-related sources to helm_chart

[alonsodomin]

Closes #15622

[Eric-Arellano]

So unfortunately it's an antipattern and not likely to work to have multiple source fields on a target :( See https://pantsbuild.slack.com/archives/C01CQHVDMMW/p1653441966623019

Can these all be in the same sources field?

Good catch by Eric, I overlooked that all fields were based on source field.

[alonsodomin]

I see what you mean, I guess I could include several of those in the sources field... however I was considering this a stepping stone into being able to generate the README.md by adding support for a tool like helm-docs.

So I was creating the readme field because I will need a specific field to be used in the output field for GenerateSourcesRequest and then I presume I got carried away and included a field for each of the other ones.

So, considering the possibility of adding support to generate the README.md file, what could be a better approach?

[kaos]

I'm not sure if this is best-practice, but it works for the Docker backend.

pants/src/python/pants/backend/docker/util_rules/dockerfile.py

Lines 15 to 19 in 03d746a

	class GenerateDockerfileRequest(GenerateSourcesRequest):
	# This will always run codegen when hydrating `docker_image`s, performing source validations but
	# does not generate anything if there are no `instructions` defined on the target.
	input = DockerImageSourceField
	output = DockerImageSourceField

In this case, using the above, you could have a readme StringField, and when set, run helm-docs to create the file (or just a bool field if the output name should be fixed).

helm_chart(
  name="my-chart",
  generate_docs=True,
  ...
)

[alonsodomin]

Actually the field name needs to be fixed, and similar thing happens with the other fields...

Another thing I'm considering is if it's worth adding a helm_docs target because the helm-docs tool supports an optional source file to be used as the template.

Maybe in that case I can plug the GenerateSourcesRequest output field to a FileSourceField, which should already be supported by current helm backend.

Review was addressed: This now only adds default sources, not additional source fields.

[Eric-Arellano]

This now only adds default sources, not additional source fields.

Looks good! If you haven't yet, double check the PR title still is appropriate. Seems fine to me, but not sure if you'd reword it since the approach changed.