livecheck: error on invalid url symbol

[samford]

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Up to now, we haven't been accounting for #url symbol arguments in livecheck blocks that don't reference a checkable URL. This can either be an invalid symbol (e.g., using the :stable formula symbol in a cask) or a valid symbol where the referenced URL doesn't exist (e.g., using :head when there's no head URL). [Almost all of the valid symbols are required URLs but head is optional.]

Over the years, we've had a handful of slips where we've used :url in formulae (when it's only valid in casks) and :stable in casks (when it's only valid in formulae). In this scenario, livecheck_url_string is nil, so livecheck falls back to #checkable_urls. stable and url are the first checkable URLs for formulae and casks (respectively), so the checks ended up working as expected merely by chance. This isn't obvious in the output and even the debug output looks normal. It only becomes apparent that livecheck isn't working as expected if it iterates through more than one checkable URL before reaching one that works (not the case in those instances).

With that in mind, this adds an error when a #url symbol is used but it doesn't correspond to a checkable URL. This will account for both of the mentioned scenarios (invalid symbols and valid ones referencing a non-existent URL) and prevent livecheck from quietly proceeding in an unexpected manner.

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It's worth mentioning that I will also be creating a RuboCop for this in a later PR, so we can catch this with brew style/brew audit (not just at runtime).

I'm currently reworking the livecheck RuboCops to share cops between formulae, resources, and casks. We originally implemented them as FormulaCops before livecheck blocks were available in casks (and later resource blocks) but we didn't have a way of sharing cops at the time. We now have some shared cops, so it's just a matter of migrating the cops to that pattern. I have most of that done (it works for formulae/resources and most casks) but I'm working on writing tests and handling less common cask setups. Once that's done, I'll create a PR for the refactor and added RuboCops.

[samford]

This latest push includes the following changes:

• Added the error to #resource_version, to align it with #latest_version
• Expanded tests for #livecheck_url_to_string to reach 100% coverage
• Expanded tests for #checkable_urls to cover everything except branches that shouldn't ever be reached

[MikeMcQuaid]

Thanks @samford, a few thoughts.

[MikeMcQuaid]

Looks good thanks @samford! One suggested refactoring.