Introduce block syntax for url stanza

[claui]

Changes to the core

• Followed hacking.md.

---

Issue

Some providers use disposable URLs, which need to be constructed from some sort of landing site (recent example). Doing such expensive things in the middle of a Cask definition has proven difficult in the past. Loading a Cask would become unacceptably slow if HTTP/S requests were done in Cask definitions.

Solution

Albeit rarely, such cases do pop up every now and then; so I decided to write this PR, which introduces an optional block syntax for the url stanza:

url do
  # No known stable URL; fetching disposable URL from landing site
  open('https://example.com/app/landing') do |landing_page|
    content = landing_page.read
    parse(content) # => https://example.com/download?23309800482283
  end
end

The block is only evaluated when needed, for example on download time or when auditing a Cask. It is never evaluated when loading a Cask.

The return value of the block is either a String, or a String, Hash tuple; the latter case is for additional options such as referrer.

Implementation note

For stanza_proxy.rb, I could have used SimpleDelegator but it turned out to be much faster to roll my own logic.

The proxy is designed to be easily reused for other stanzas if needed.

Credits

Thanks @tobadia for contributing the regex/open-uri snippet in audacity.rb, which also triggered the idea for this PR.

Feedback

Pinging @caskroom/maintainers for opinions (and for suggestions to improve my convoluted English in the docs). Thanks!

[vitorgalvao]

Won’t this happen on every cask load? Since this is exclusively needed for url, why not have the require inside the url block? Any reason why that wouldn’t work?

[claui]

It does work, and makes for a good improvement.
Changing this, thanks!

[claui]

@vitorgalvao Changed as per your suggestions, thanks.
I also cut short redundant comments and added the required line:

# fosshub.com/Audacity.html was verified as official when first introduced to the cask

[vitorgalvao]

Why remove the FossHub example?

[claui]

The example has not been removed. I moved it to the next line:

Some providers do not actively block command-line HTTP clients but use URLs that change periodically, or even on each visit (example: FossHub).

I did this for two reasons:

1. My new text wants to establish a clear difference between hosting providers that actively block CLI clients (e. g. using captchas) vs. those that just use disposable URLs for whatever reason. There are certainly many possible reasons for a provider to go for disposable URLs, even if that choice means breaking CLIs as a side effect. I feel it wouldn’t be unreasonable for us to politely err on the side of the accused, rather than implying that FossHub made their choices for a certain reason without giving actual evidence.
2. To me, the current wording would imply that both Audacity update url #21952 and Introduce block syntax for url stanza #22014 are acts of circumventing a technological barrier in a way FossHub does not wish to allow. While I am willing to take responsibility for my actions, I certainly don’t feel good about making assumptions which in the worst case might someday be used against the project and its participants. On the other hand, IANAL; my comment is just an account of how I currently feel about the whole thing.

That all said, I dislike how FossHub’s choices makes our lives harder. I’d be glad if someone would inform them about the issue and make them aware of the hoops through which their technical decision makes us jump.

Regarding the docs, I am completely open for feedback/suggestions how to reword this.

[vitorgalvao]

Sounds good. Great detail, as always.

[claui]

My verbosity can be both a blessing and a curse. 😄

[vitorgalvao]

Everything looks great.

This could provide immense value in the caskroom/versions repo. We could switch all nightlies to use :latest and :no_check and make url always grab the most up-to-date version. Most nightlies users should prefer that to permanently outdated casks.

[claui]

Committed fixes to url.md regarding typography, spelling, and the link anchor.

[claui]

Rebased.

Leaving the PR open for another few hours before I proceed to merge.

[vitorgalvao]

I wonder in which fun way cask-repair will blow up with these casks. I’ll likely just make it abort if it finds a cask with a block url.