Freeze formula definition once first instance is created

Library/Homebrew/build_environment.rb

@@ -33,9 +33,17 @@ class BuildEnvironment
   module DSL
     extend T::Sig
 
+    # Initialise @env for each class which may use this DSL (e.g. each formula subclass).
+    # `env` may never be called, and it needs to be initialised before the class is frozen.
+    def inherited(child)
+      super
+      child.instance_eval do
+        @env = BuildEnvironment.new
+      end
+    end
+
     sig { params(settings: Symbol).returns(BuildEnvironment) }
     def env(*settings)
-      @env ||= BuildEnvironment.new
       @env.merge(settings)
     end
   end

Library/Homebrew/dependency_collector.rb

@@ -26,18 +26,25 @@ class DependencyCollector
 
   sig { void }
   def initialize
+    # Ensure this is synced with `initialize_dup` and `freeze` (excluding simple objects like integers and booleans)
     @deps = Dependencies.new
     @requirements = Requirements.new
 
     init_global_dep_tree_if_needed!
   end
 
-  def initialize_copy(other)
+  def initialize_dup(other)
     super
     @deps = @deps.dup
     @requirements = @requirements.dup
   end
 
+  def freeze
+    @deps.freeze
+    @requirements.freeze
+    super
+  end
+
   def add(spec)
     case dep = fetch(spec)
     when Dependency

Library/Homebrew/extend/module.rb

@@ -5,11 +5,17 @@ class Module
   def attr_rw(*attrs)
     attrs.each do |attr|
       module_eval <<-EOS, __FILE__, __LINE__+1
-        def #{attr}(val=nil)           # def prefix(val=nil)
+        def #{attr}(val=nil)                         # def prefix(val=nil)
-          @#{attr} ||= nil             #   @prefix ||= nil
+          if val.nil?                                #   if val.nil?
-          return @#{attr} if val.nil?  #   return @prefix if val.nil?
+            if instance_variable_defined?(:@#{attr}) #      if instance_variable_defined?(:@prefix)
-          @#{attr} = val               #   @prefix = val
+              return @#{attr}                        #        return @prefix
-        end                            # end
+            else                                     #      else
+              return nil                             #        return nil
+            end                                      #      end
+          end                                        #    end
+                                                     #
+          @#{attr} = val                             #   @prefix = val
+        end                                          # end
       EOS
     end
   end

Library/Homebrew/formula.rb

@@ -184,6 +184,11 @@ class Formula
     # Only allow instances of subclasses. The base class does not hold any spec information (URLs etc).
     raise "Do not call `Formula.new' directly without a subclass." unless self.class < Formula
 
+    # Stop any subsequent modification of a formula's definition.
+    # Changes do not propagate to existing instances of formulae.
+    # Now that we have an instance, it's too late to make any changes to the class-level definition.
+    self.class.freeze
+
     @name = name
     @path = path
     @alias_path = alias_path
@@ -2699,6 +2704,22 @@ class Formula
     include BuildEnvironment::DSL
     include OnSystem::MacOSAndLinux
 
+    # Initialise instance variables for each subclass. These need to be initialised before the class is frozen,
+    # and some DSL may never be called so it can't be done lazily.
+    def inherited(child)
+      super
+      child.instance_eval do
+        # Ensure this is synced with `freeze`
+        @stable = SoftwareSpec.new(flags: build_flags)
+        @head = HeadSoftwareSpec.new(flags: build_flags)
+        @livecheck = Livecheck.new(self)
+        @conflicts = []
+        @skip_clean_paths = Set.new
+        @link_overwrite_paths = Set.new
+        @allowed_missing_libraries = Set.new
+      end
+    end
+
     def method_added(method)
       super
 
@@ -2710,6 +2731,16 @@ class Formula
       end
     end
 
+    def freeze
+      specs.each(&:freeze)
+      @livecheck.freeze
+      @conflicts.freeze
+      @skip_clean_paths.freeze
+      @link_overwrite_paths.freeze
+      @allowed_missing_libraries.freeze
+      super
+    end
+
     # Whether this formula contains OS/arch-specific blocks
     # (e.g. `on_macos`, `on_arm`, `on_monterey :or_older`, `on_system :linux, macos: :big_sur_or_newer`).
     # @private
@@ -2790,6 +2821,18 @@ class Formula
     # @private
     attr_reader :plist_manual
 
+    # @private
+    attr_reader :conflicts
+
+    # @private
+    attr_reader :skip_clean_paths
+
+    # @private
+    attr_reader :link_overwrite_paths
+
+    # @private
+    attr_reader :allowed_missing_libraries
+
     # If `pour_bottle?` returns `false` the user-visible reason to display for
     # why they cannot use the bottle.
     # @private
@@ -2820,7 +2863,7 @@ class Formula
     # A list of the {.stable} and {.head} {SoftwareSpec}s.
     # @private
     def specs
-      @specs ||= [stable, head].freeze
+      [stable, head].freeze
     end
 
     # @!attribute [w] url
@@ -2937,7 +2980,6 @@ class Formula
     #   depends_on "libffi"
     # end</pre>
     def stable(&block)
-      @stable ||= SoftwareSpec.new(flags: build_flags)
       return @stable unless block
 
       @stable.instance_eval(&block)
@@ -2956,7 +2998,6 @@ class Formula
     # or (if autodetect fails):
     # <pre>head "https://hg.is.awesome.but.git.has.won.example.com/", using: :hg</pre>
     def head(val = nil, specs = {}, &block)
-      @head ||= HeadSoftwareSpec.new(flags: build_flags)
       if block
         @head.instance_eval(&block)
       elsif val
@@ -3107,11 +3148,6 @@ class Formula
       @plist_manual = options[:manual]
     end
 
-    # @private
-    def conflicts
-      @conflicts ||= []
-    end
-
     # One or more formulae that conflict with this one and why.
     # <pre>conflicts_with "imagemagick", because: "both install `convert` binaries"</pre>
     def conflicts_with(*names)
@@ -3132,11 +3168,6 @@ class Formula
       skip_clean_paths.merge(paths)
     end
 
-    # @private
-    def skip_clean_paths
-      @skip_clean_paths ||= Set.new
-    end
-
     # Software that will not be symlinked into the `brew --prefix` and will
     # only live in its Cellar. Other formulae can depend on it and Homebrew
     # will add the necessary includes, libraries, and other paths while
@@ -3240,7 +3271,6 @@ class Formula
     #   regex /foo-(\d+(?:\.\d+)+)\.tar/
     # end</pre>
     def livecheck(&block)
-      @livecheck ||= Livecheck.new(self)
       return @livecheck unless block
 
       @livecheckable = true
@@ -3396,11 +3426,6 @@ class Formula
       link_overwrite_paths.merge(paths)
     end
 
-    # @private
-    def link_overwrite_paths
-      @link_overwrite_paths ||= Set.new
-    end
-
     # Permit links to certain libraries that don't exist. Available on Linux only.
     def ignore_missing_libraries(*libs)
       unless Homebrew::SimulateSystem.simulating_or_running_on_linux?
@@ -3414,11 +3439,6 @@ class Formula
 
       allowed_missing_libraries.merge(libraries)
     end
-
-    # @private
-    def allowed_missing_libraries
-      @allowed_missing_libraries ||= Set.new
-    end
   end
 end
 

Library/Homebrew/options.rb

@@ -82,9 +82,20 @@ class Options
   end
 
   def initialize(*args)
+    # Ensure this is synced with `initialize_dup` and `freeze` (excluding simple objects like integers and booleans)
     @options = Set.new(*args)
   end
 
+  def initialize_dup(other)
+    super
+    @options = @options.dup
+  end
+
+  def freeze
+    @options.dup
+    super
+  end
+
   def each(*args, &block)
     @options.each(*args, &block)
   end

Library/Homebrew/resource.rb

@@ -29,6 +29,7 @@ class Resource
   attr_accessor :name
 
   def initialize(name = nil, &block)
+    # Ensure this is synced with `initialize_dup` and `freeze` (excluding simple objects like integers and booleans)
     @name = name
     @url = nil
     @version = nil
@@ -37,11 +38,35 @@ class Resource
     @checksum = nil
     @using = nil
     @patches = []
-    @livecheck = nil
+    @livecheck = Livecheck.new(self)
     @livecheckable = false
     instance_eval(&block) if block
   end
 
+  def initialize_dup(other)
+    super
+    @name = @name.dup
+    @version = @version.dup
+    @mirrors = @mirrors.dup
+    @specs = @specs.dup
+    @checksum = @checksum.dup
+    @using = @using.dup
+    @patches = @patches.dup
+    @livecheck = @livecheck.dup
+  end
+
+  def freeze
+    @name.freeze
+    @version.freeze
+    @mirrors.freeze
+    @specs.freeze
+    @checksum.freeze
+    @using.freeze
+    @patches.freeze
+    @livecheck.freeze
+    super
+  end
+
   def owner=(owner)
     @owner = owner
     patches.each { |p| p.owner = owner }
@@ -185,7 +210,6 @@ class Resource
   #   regex /foo-(\d+(?:\.\d+)+)\.tar/
   # end</pre>
   def livecheck(&block)
-    @livecheck ||= Livecheck.new(self) if block
     return @livecheck unless block
 
     @livecheckable = true

Library/Homebrew/software_spec.rb

@@ -36,6 +36,7 @@ class SoftwareSpec
   def_delegators :@resource, :sha256
 
   def initialize(flags: [])
+    # Ensure this is synced with `initialize_dup` and `freeze` (excluding simple objects like integers and booleans)
     @resource = Resource.new
     @resources = {}
     @dependency_collector = DependencyCollector.new
@@ -50,9 +51,36 @@ class SoftwareSpec
     @uses_from_macos_elements = []
   end
 
-  def initialize_copy(other)
+  def initialize_dup(other)
     super
+    @resource = @resource.dup
+    @resources = @resources.dup
     @dependency_collector = @dependency_collector.dup
+    @bottle_specification = @bottle_specification.dup
+    @patches = @patches.dup
+    @options = @options.dup
+    @flags = @flags.dup
+    @deprecated_flags = @deprecated_flags.dup
+    @deprecated_options = @deprecated_options.dup
+    @build = @build.dup
+    @compiler_failures = @compiler_failures.dup
+    @uses_from_macos_elements = @uses_from_macos_elements.dup
+  end
+
+  def freeze
+    @resource.freeze
+    @resources.freeze
+    @dependency_collector.freeze
+    @bottle_specification.freeze
+    @patches.freeze
+    @options.freeze
+    @flags.freeze
+    @deprecated_flags.freeze
+    @deprecated_options.freeze
+    @build.freeze
+    @compiler_failures.freeze
+    @uses_from_macos_elements.freeze
+    super
   end
 
   def owner=(owner)

Library/Homebrew/test/resource_spec.rb

@@ -66,10 +66,6 @@ describe Resource do
   end
 
   describe "#livecheck" do
-    it "returns nil if livecheck block is not set in resource" do
-      expect(resource.livecheck).to be_nil
-    end
-
     specify "when livecheck block is set" do
       expect(livecheck_resource.livecheck.url).to eq("https://brew.sh/test/releases")
       expect(livecheck_resource.livecheck.regex).to eq(/foo[._-]v?(\d+(?:\.\d+)+)\.t/i)