livecheck: Add Json strategy

This adds a generic `Json` strategy to livecheck that requires a
`strategy` block to operate. This is primarily intended as a
replacement for existing `strategy` blocks in formulae/casks that
use `JSON#parse`, as it allows us to internalize/standardize that
boilerplate while improving error-handling.

Additionally, future strategies that parse JSON data can use the
`Json#find_versions` method instead of having to reinvent the wheel
(similar to how we currently have a number of strategies that
leverage `PageMatch#find_versions`).

Library/Homebrew/livecheck/strategy.rb

@@ -153,9 +153,13 @@ module Homebrew
       def from_url(url, livecheck_strategy: nil, url_provided: false, regex_provided: false, block_provided: false)
         usable_strategies = strategies.values.select do |strategy|
           if strategy == PageMatch
-            # Only treat the `PageMatch` strategy as usable if a regex is
+            # Only treat the strategy as usable if the `livecheck` block
-            # present in the `livecheck` block
+            # contains a regex and/or `strategy` block
             next if !regex_provided && !block_provided
+          elsif strategy == Json
+            # Only treat the strategy as usable if the `livecheck` block
+            # contains a `strategy` block
+            next unless block_provided
           elsif strategy.const_defined?(:PRIORITY) &&
                 !strategy::PRIORITY.positive? &&
                 from_symbol(livecheck_strategy) != strategy
@@ -273,6 +277,7 @@ require_relative "strategy/gnome"
 require_relative "strategy/gnu"
 require_relative "strategy/hackage"
 require_relative "strategy/header_match"
+require_relative "strategy/json"
 require_relative "strategy/launchpad"
 require_relative "strategy/npm"
 require_relative "strategy/page_match"

Library/Homebrew/livecheck/strategy/json.rb

@@ -0,0 +1,126 @@
+# typed: true
+# frozen_string_literal: true
+
+module Homebrew
+  module Livecheck
+    module Strategy
+      # The {Json} strategy fetches content at a URL, parses it as JSON, and
+      # provides the parsed data to a `strategy` block. If a regex is present
+      # in the `livecheck` block, it should be passed as the second argument to
+      # the  `strategy` block.
+      #
+      # This is a generic strategy that doesn't contain any logic for finding
+      # versions, as the structure of JSON data varies. Instead, a `strategy`
+      # block must be used to extract version information from the JSON data.
+      #
+      # This strategy is not applied automatically and it is necessary to use
+      # `strategy :json` in a `livecheck` block (in conjunction with a
+      # `strategy` block) to use it.
+      #
+      # This strategy's {find_versions} method can be used in other strategies
+      # that work with JSON content, so it should only be necessary to write
+      # the version-finding logic that works with the parsed JSON data.
+      #
+      # @api public
+      class Json
+        extend T::Sig
+
+        NICE_NAME = "JSON"
+
+        # A priority of zero causes livecheck to skip the strategy. We do this
+        # for {Json} so we can selectively apply it only when a strategy block
+        # is provided in a `livecheck` block.
+        PRIORITY = 0
+
+        # The `Regexp` used to determine if the strategy applies to the URL.
+        URL_MATCH_REGEX = %r{^https?://}i.freeze
+
+        # Whether the strategy can be applied to the provided URL.
+        # {Json} will technically match any HTTP URL but is only usable with
+        # a `livecheck` block containing a `strategy` block.
+        #
+        # @param url [String] the URL to match against
+        # @return [Boolean]
+        sig { params(url: String).returns(T::Boolean) }
+        def self.match?(url)
+          URL_MATCH_REGEX.match?(url)
+        end
+
+        # Parses JSON text and identifies versions using a `strategy` block.
+        # If a regex is provided, it will be passed as the second argument to
+        # the  `strategy` block (after the parsed JSON data).
+        # @param content [String] the JSON text to parse and check
+        # @param regex [Regexp, nil] a regex used for matching versions in the
+        #   content
+        # @return [Array]
+        sig {
+          params(
+            content: String,
+            regex:   T.nilable(Regexp),
+            block:   T.untyped,
+          ).returns(T::Array[String])
+        }
+        def self.versions_from_content(content, regex = nil, &block)
+          return [] if content.blank? || block.blank?
+
+          require "json"
+          json = begin
+            JSON.parse(content)
+          rescue JSON::ParserError
+            raise "Content could not be parsed as JSON."
+          end
+
+          block_return_value = if regex.present?
+            yield(json, regex)
+          elsif block.arity == 2
+            raise "Two arguments found in `strategy` block but no regex provided."
+          else
+            yield(json)
+          end
+          Strategy.handle_block_return(block_return_value)
+        end
+
+        # Checks the JSON content at the URL for versions, using the provided
+        # `strategy` block to extract version information.
+        #
+        # @param url [String] the URL of the content to check
+        # @param regex [Regexp, nil] a regex used for matching versions
+        # @param provided_content [String, nil] page content to use in place of
+        #   fetching via `Strategy#page_content`
+        # @param homebrew_curl [Boolean] whether to use brewed curl with the URL
+        # @return [Hash]
+        sig {
+          params(
+            url:              String,
+            regex:            T.nilable(Regexp),
+            provided_content: T.nilable(String),
+            homebrew_curl:    T::Boolean,
+            _unused:          T.nilable(T::Hash[Symbol, T.untyped]),
+            block:            T.untyped,
+          ).returns(T::Hash[Symbol, T.untyped])
+        }
+        def self.find_versions(url:, regex: nil, provided_content: nil, homebrew_curl: false, **_unused, &block)
+          raise ArgumentError, "#{T.must(name).demodulize} requires a `strategy` block" if block.blank?
+
+          match_data = { matches: {}, regex: regex, url: url }
+          return match_data if url.blank? || block.blank?
+
+          content = if provided_content.is_a?(String)
+            match_data[:cached] = true
+            provided_content
+          else
+            match_data.merge!(Strategy.page_content(url, homebrew_curl: homebrew_curl))
+            match_data[:content]
+          end
+          return match_data if content.blank?
+
+          versions_from_content(content, regex, &block).each do |match_text|
+            match_data[:matches][match_text] = Version.new(match_text)
+          end
+
+          match_data
+        end
+      end
+    end
+  end
+end

Library/Homebrew/test/livecheck/strategy/json_spec.rb

@@ -0,0 +1,149 @@
+# typed: false
+# frozen_string_literal: true
+
+require "livecheck/strategy"
+
+describe Homebrew::Livecheck::Strategy::Json do
+  subject(:json) { described_class }
+
+  let(:http_url) { "https://brew.sh/blog/" }
+  let(:non_http_url) { "ftp://brew.sh/" }
+
+  let(:regex) { /^v?(\d+(?:\.\d+)+)$/i }
+
+  let(:simple_content) { '{"version":"1.2.3"}' }
+  let(:content) {
+    <<~EOS
+      {
+        "versions": [
+          { "version": "1.1.2" },
+          { "version": "1.1.2b" },
+          { "version": "1.1.2a" },
+          { "version": "1.1.1" },
+          { "version": "1.1.0" },
+          { "version": "1.1.0-rc3" },
+          { "version": "1.1.0-rc2" },
+          { "version": "1.1.0-rc1" },
+          { "version": "1.0.x-last" },
+          { "version": "1.0.3" },
+          { "version": "1.0.3-rc3" },
+          { "version": "1.0.3-rc2" },
+          { "version": "1.0.3-rc1" },
+          { "version": "1.0.2" },
+          { "version": "1.0.2-rc1" },
+          { "version": "1.0.1" },
+          { "version": "1.0.1-rc1" },
+          { "version": "1.0.0" },
+          { "version": "1.0.0-rc1" },
+          { "other": "version is omitted from this object for testing" }
+        ]
+      }
+    EOS
+  }
+
+  let(:simple_content_matches) { ["1.2.3"] }
+  let(:content_matches) { ["1.1.2", "1.1.1", "1.1.0", "1.0.3", "1.0.2", "1.0.1", "1.0.0"] }
+
+  let(:find_versions_return_hash) {
+    {
+      matches: {
+        "1.1.2" => Version.new("1.1.2"),
+        "1.1.1" => Version.new("1.1.1"),
+        "1.1.0" => Version.new("1.1.0"),
+        "1.0.3" => Version.new("1.0.3"),
+        "1.0.2" => Version.new("1.0.2"),
+        "1.0.1" => Version.new("1.0.1"),
+        "1.0.0" => Version.new("1.0.0"),
+      },
+      regex:   regex,
+      url:     http_url,
+    }
+  }
+
+  let(:find_versions_cached_return_hash) {
+    find_versions_return_hash.merge({ cached: true })
+  }
+
+  describe "::match?" do
+    it "returns true for an HTTP URL" do
+      expect(json.match?(http_url)).to be true
+    end
+
+    it "returns false for a non-HTTP URL" do
+      expect(json.match?(non_http_url)).to be false
+    end
+  end
+
+  describe "::versions_from_content" do
+    it "returns an empty array when given a block but content is blank" do
+      expect(json.versions_from_content("", regex) { "1.2.3" }).to eq([])
+    end
+
+    it "errors if provided content is not valid JSON" do
+      expect { json.versions_from_content("not valid JSON") { [] } }
+        .to raise_error(RuntimeError, "Content could not be parsed as JSON.")
+    end
+
+    it "returns an array of version strings when given content and a block" do
+      # Returning a string from block
+      expect(json.versions_from_content(simple_content) { |json| json["version"] }).to eq(simple_content_matches)
+      expect(json.versions_from_content(simple_content, regex) do |json|
+        json["version"][regex, 1]
+      end).to eq(simple_content_matches)
+
+      # Returning an array of strings from block
+      expect(json.versions_from_content(content, regex) do |json, regex|
+        json["versions"].select { |item| item["version"]&.match?(regex) }
+                        .map { |item| item["version"][regex, 1] }
+      end).to eq(content_matches)
+    end
+
+    it "allows a nil return from a block" do
+      expect(json.versions_from_content(content, regex) { next }).to eq([])
+    end
+
+    it "errors if a block uses two arguments but a regex is not given" do
+      expect { json.versions_from_content(simple_content) { |json, regex| json["version"][regex, 1] } }
+        .to raise_error("Two arguments found in `strategy` block but no regex provided.")
+    end
+
+    it "errors on an invalid return type from a block" do
+      expect { json.versions_from_content(content, regex) { 123 } }
+        .to raise_error(TypeError, Homebrew::Livecheck::Strategy::INVALID_BLOCK_RETURN_VALUE_MSG)
+    end
+  end
+
+  describe "::find_versions?" do
+    it "finds versions in provided_content using a block" do
+      expect(json.find_versions(url: http_url, regex: regex, provided_content: content) do |json, regex|
+        json["versions"].select { |item| item["version"]&.match?(regex) }
+                        .map { |item| item["version"][regex, 1] }
+      end).to eq(find_versions_cached_return_hash)
+
+      # NOTE: A regex should be provided using the `#regex` method in a
+      # `livecheck` block but we're using a regex literal in the `strategy`
+      # block here simply to ensure this method works as expected when a
+      # regex isn't provided.
+      expect(json.find_versions(url: http_url, provided_content: content) do |json|
+        regex = /^v?(\d+(?:\.\d+)+)$/i.freeze
+        json["versions"].select { |item| item["version"]&.match?(regex) }
+                        .map { |item| item["version"][regex, 1] }
+      end).to eq(find_versions_cached_return_hash.merge({ regex: nil }))
+    end
+
+    it "errors if a block is not provided" do
+      expect { json.find_versions(url: http_url, provided_content: content) }
+        .to raise_error(ArgumentError, "Json requires a `strategy` block")
+    end
+
+    it "returns default match_data when url is blank" do
+      expect(json.find_versions(url: "") { "1.2.3" })
+        .to eq({ matches: {}, regex: nil, url: "" })
+    end
+
+    it "returns default match_data when content is blank" do
+      expect(json.find_versions(url: http_url, provided_content: "") { "1.2.3" })
+        .to eq({ matches: {}, regex: nil, url: http_url, cached: true })
+    end
+  end
+end

docs/Brew-Livecheck.md

@@ -141,6 +141,21 @@ livecheck do
 end
 ```
 
+#### `Json` `strategy` block
+
+A `strategy` block for `Json` receives parsed JSON data and, if provided, a regex. For example, if we have an object containing an array of objects with a `version` string, we can select only the members that match the regex and isolate the relevant version text as follows:
+
+```ruby
+livecheck do
+  url "https://www.example.com/example.json"
+  regex(/^v?(\d+(?:\.\d+)+)$/i)
+  strategy :json do |json, regex|
+    json["versions"].select { |item| item["version"]&.match?(regex) }
+                    .map { |item| item["version"][regex, 1] }
+  end
+end
+```
+
 #### `Sparkle` `strategy` block
 
 A `strategy` block for `Sparkle` receives an `item` which has methods for the `short_version`, `version`, `url` and `title`.