Audit validation tags for configuration properties

[bluekeyes]

Is your feature request related to a problem? Please describe.

For some properties (I hit it on DownloadMode, but may apply to others), the zero value is invalid and the property is effectively required, but is not marked as required for validation.

Describe the solution you'd like

Audit the configuration type definitions to make sure the validation tags are correct for properties. I think the required tag is the most relevant, since a missing required property gives a clear error at startup, while the zero value for the type may give a confusing error later on.

Describe alternatives you've considered

Introduce another method to validate configuration. This seems non-ideal, since validation tags are already used for some properties.

Additional context

This issue is split out of #1336, which has some additional information.

[arschles]

@bluekeyes We should certainly get better at validating configurations. Since there are a lot of options, it's easy to forget something and make a mistake, so I think we should look holistically at the configuration. For example, if DownloadMode is set to "redirect", then DownloadURL needs to be set. We should check that and report it and disallow Athens from starting.

What do you think?

[bluekeyes]

Yeah, it makes sense to me that the application would need a way to validate relationships between properties and that that would provide a better user experience than just saying if an option is required or not.

From a pure code perspective, I personally find it a bit confusing when there are multiple validation methods at play in a project, so long term, I think it could be valuable to decide on when to use the validator.v9 library (if at all) vs. when to use any new method introduced here. But I'm also not very familiar with the Athens code base or development style.

[arschles]

@bluekeyes I don't have any experience with validator.v9, but it looks nice. The idea I have in mind is to centralize all validation of configuration in one place in the code, regardless of where the configuration values came from - file, environment variables, or anywhere else we define later on.

Ideally, the validations would come with good error messages too, when Athens is misconfigured. I'll check out validator.v9 to see if I can get it working in #1492 too - thanks!