Enable use of latest formula version in resource livecheck URLs

[nandahkrishna]

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See Homebrew/homebrew-core#112155 (comment) for a great explanation from @samford. In short, we require access to the formula's latest version as found by livecheck in URLs for resource livecheck blocks – this is required for multiple formulae including influxdb, flux, luvit, etc.

This PR:

• Adds a method to the livecheck DSL to return a placeholder string (<FORMULA_LATEST_VERSION>)
• Enables the replacement of this placeholder string with a formula's latest version in its resources' livecheck URLs

I briefly tested this with the pkg-config-wrapper resource in influxdb and it seems to work fine.

The livecheck block for the resource:

livecheck do
  url "https://raw.githubusercontent.com/influxdata/influxdb/v#{latest_version}/go.mod"
  regex(/(pkg-config\sv)+(\d+(?:\.\d+)+)/i)
  strategy :page_match do |page, regex|
    page.scan(regex).map { |match| match[1] }
  end
end

The result:

...

Resource:         pkg-config-wrapper
Livecheckable?:   Yes

URL:              https://raw.githubusercontent.com/influxdata/influxdb/v<FORMULA_LATEST_VERSION>/go.mod
URL (processed):  https://raw.githubusercontent.com/influxdata/influxdb/v2.6.0/go.mod
Strategies:       PageMatch
Strategy:         PageMatch
Regex:            /(pkg-config\sv)+(\d+(?:\.\d+)+)/i

Matched Versions:
0.2.11 => #<Version:0x00007fc40c9d4608 @version="0.2.11", @detected_from_url=false>

...

  pkg-config-wrapper: 0.2.11 ==> 0.2.11

[BrewTestBot]

Review period will end on 2022-12-20 at 05:21:52 UTC.

[samford]

Nice work. This is in line with what I was thinking from our past discussion and it works as expected when I manually tested it.

Going through the changes, there may be something to be said for extracting the <FORMULA_LATEST_VERSION> string into a constant that can be used in both the Livecheck class (i.e., the DSL methods) and Livecheck#resource_version. I tinkered with this and the approach I ended up with involves creating a separate Homebrew::Livecheck::Constants module that can be loaded in both places.

That setup got me thinking about whether we should simply use the constant in resource livecheck blocks instead of a latest_version method. All the method buys us at that point is a lowercase name. However, it's a bit tricky to make the constant available within a livecheck block.

Luckily, that's something I experimented with in the past when I was trying to figure out how we could provide regex constants (i.e., regex-escaped strings that can be readily interpolated as well as full regex literals). I dug up my work from last year and basically I was taking a similar approach, where the constants were in a module. The way I made them available to livecheck blocks was by including the module in the Formula#livecheck method (after the early return but before @livecheck.instance_eval(&block) is called).

I've pushed a branch with a follow-up commit that uses this approach in my brew fork, if you would like to take a look and let me know what you think. The only public-facing difference is that the URL in the example livecheck block would be "https://raw.githubusercontent.com/influxdata/influxdb/v#{LATEST_VERSION}/go.mod" instead.

My thinking is that the constant approach would also lay the groundwork for regex constants (if we want to go that route) and it would be as simple as adding more values to Homebrew::Livecheck::Constants (i.e., no need to add methods to the DSL). It would also end up being more consistent for maintainers/contributors, as these would all be constants (i.e., we wouldn't have to remember that latest_version is a method but the regexes are constants).

[nandahkrishna]

My thinking is that the constant approach would also lay the groundwork for regex constants (if we want to go that route) and it would be as simple as adding more values to Homebrew::Livecheck::Constants (i.e., no need to add methods to the DSL). It would also end up being more consistent for maintainers/contributors, as these would all be constants (i.e., we wouldn't have to remember that latest_version is a method but the regexes are constants).

I agree with this, we should make it as easy as possible for contributors to look at/modify/use these placeholders. Using constants (uppercase) instead of methods (lowercase) isn't less elegant at all, in my opinion.

I've pushed your commit to this PR branch as I'm in favour of the approach you suggested.

[samford]

Thanks, Nanda!

I believe we'll have to wait for this to be included in a new brew release before we can start using it in formulae but I'll try to take another pass through the related homebrew/core PRs sometime tomorrow (unless you beat me to it).

[BrewTestBot]

Review period ended.