record: Move validation by type out of CustomizeDiff.

[andrewsomething]

Fixes: #670

We're doing validation inside of a CustomizeDiff function. The interpolated values are not available there on plan. This prevents user from doing things like referencing a database cluster's port in a SRV record. This is similar to hashicorp/terraform-provider-consul#260 This PR moves that validation to later in the process.

This has one side effect. The reason for using a CustomizeDiff for something like this is that it allows you to error out early before any of the plan runs. Now the error will not happen until the record is being created. I don't love that, but not sure I see a better way to do it.

[bentranter]

LGTM!

[andrewsomething]

@andrewsomething does this happen when running a plan for the first time since the interpolated values won't be known until after the apply is run? Or does it happen on every plan?
Is there a case where the referenced value would be known when running plan?

@scotchneat It only happens if the resource being referenced is being created at the same time. For example, if you already have the database, you can add a domain record pointing to its port.

Thinking about this one some more... CustomizeDiff functions are not really meant for this kind of validation, but the fact that they run so early makes them super useful for that. It's much nicer to get the validation error in the plan and before other resources get created. Doing the validation in the create function means you can error out mid-apply.

Maybe just moving the validation for port make sense? It seems to be the one that is most likely to come from interpolation (and the only one we've had a bug reported for). This would allow us to retain the current behavior for other attributes and still unblock the use case in #670.

[andrewsomething]

@bentranter, @scotchneat - I decided to take the more limited approach describe in my last comment. Need a new review due to the changes.

[scotchneat]

I haven't tested this manually, but the tests you included cover it well. LGTM