Use schema.never() for dev/!dist config parameters

[afharo]

Related discussion: #107663 (comment)

Should we set schema.never() instead of schema.boolean({ validate: (rawValue) => if (rawValue === true) return 'validation error message'; }) for those config entries that are not supported in production?

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

Imho having an explicit error message is more important than an technical implementation detail, so id I had to choose, I'd say to keep the current code.

[afharo]

That brings me to the next question: since the only thing stopping us from using schema.never() is that the message is not clear enough, should we improve it to allow custom validation messages?

[pgayvallet]

is that the message is not clear enough, should we improve it to allow custom validation messages?

Not sure I really like allowing custom error messages for joi types, as it can lead to some overcomplicated things. OTOH, for never, there is only one rule (forbidden), so I guess we could add a customMessage option to the type. It would probably be fine

kibana/packages/kbn-config-schema/src/types/never_type.ts

Lines 13 to 22 in 4584a8b

	constructor() {
	super(internals.any().forbidden());
	}
	protected handleError(type: string) {
	switch (type) {
	case 'any.unknown':
	return "a value wasn't expected to be present";
	}
	}

[afharo]

Alternatively, we could improve the default error message to [key]: is not allowed. However, since we normally use it inside a conditional, we may need the conditional to report why ... when 'dist' is 'true'.

That sounds like a lot of work (and users probably will wonder how to set dist to false) 😅

I think the customMessage approach for this type would be good enough.

What do you think?

[pgayvallet]

we may need the conditional to report why ... when 'dist' is 'true'.

Realistically, it can't be automated, as the amount of work to parse the JOI schema tree would be enormous. So the only option for such specific message would be the customMessage option