tap: avoid class vars

Avoiding them also allows us to write proper tests.

Library/Homebrew/tap.rb

@@ -132,11 +132,10 @@ class Tap
     false
   end
 
-  # rubocop:disable Style/ClassVars
-  # We want the the class variables below to be global to all `Tap` classes and subclasses.
   sig { returns(T::Set[Tap]) }
   def self.allowed_taps
-    @@allowed_taps ||= begin
+    cache_key = "allowed_taps_#{Homebrew::EnvConfig.allowed_taps.to_s.gsub(" ", "_")}".to_sym
+    cache[cache_key] ||= begin
       allowed_tap_list = Homebrew::EnvConfig.allowed_taps.to_s.split
 
       Set.new(allowed_tap_list.filter_map do |tap|
@@ -144,15 +143,14 @@ class Tap
       rescue Tap::InvalidNameError
         opoo "Invalid tap name in `HOMEBREW_ALLOWED_TAPS`: #{tap}"
         nil
-      end)
+      end).freeze
     end
-
-    @@allowed_taps.freeze
   end
 
   sig { returns(T::Set[Tap]) }
   def self.forbidden_taps
-    @@forbidden_taps ||= begin
+    cache_key = "forbidden_taps_#{Homebrew::EnvConfig.forbidden_taps.to_s.gsub(" ", "_")}".to_sym
+    cache[cache_key] ||= begin
       forbidden_tap_list = Homebrew::EnvConfig.forbidden_taps.to_s.split
 
       Set.new(forbidden_tap_list.filter_map do |tap|
@@ -160,12 +158,9 @@ class Tap
       rescue Tap::InvalidNameError
         opoo "Invalid tap name in `HOMEBREW_FORBIDDEN_TAPS`: #{tap}"
         nil
-      end)
+      end).freeze
     end
-
-    @@forbidden_taps.freeze
   end
-  # rubocop:enable Style/ClassVars
 
   # @api public
   extend Enumerable

Library/Homebrew/test/tap_spec.rb

@@ -142,6 +142,24 @@ RSpec.describe Tap do
     end
   end
 
+  describe "::allowed_taps" do
+    before { allow(Homebrew::EnvConfig).to receive(:allowed_taps).and_return("homebrew/allowed") }
+
+    it "returns a set of allowed taps according to the environment" do
+      expect(described_class.allowed_taps)
+        .to contain_exactly(described_class.fetch("homebrew/allowed"))
+    end
+  end
+
+  describe "::forbidden_taps" do
+    before { allow(Homebrew::EnvConfig).to receive(:forbidden_taps).and_return("homebrew/forbidden") }
+
+    it "returns a set of forbidden taps according to the environment" do
+      expect(described_class.forbidden_taps)
+        .to contain_exactly(described_class.fetch("homebrew/forbidden"))
+    end
+  end
+
   specify "::names" do
     expect(described_class.names.sort).to eq(["homebrew/core", "homebrew/foo"])
   end