Separate `dup` from `deep_dup` in the attributes hash

I'm looking to move towards a tree-like structure for dirty checking
that involves an attribute holding onto the attribute that it was
created from. This means that `changed?` can be fully encapsulated on
that object. Since the objects are immutable, in `changes_applied`, we
can simply perform a shallow dup, instead of a deep one.

I'm not sure if that will actually end up in a performance boost, but
I'd like to semantically separate these concepts regardless

activerecord/lib/active_record/attribute_methods/dirty.rb

@@ -47,7 +47,7 @@ module ActiveRecord
       def initialize_dup(other) # :nodoc:
         super
         @mutation_tracker = AttributeMutationTracker.new(@attributes,
-                                                         self.class._default_attributes.dup)
+                                                         self.class._default_attributes.deep_dup)
       end
 
       def changes_applied

activerecord/lib/active_record/attribute_set.rb

@@ -60,8 +60,14 @@ module ActiveRecord
       super
     end
 
+    def deep_dup
+      dup.tap do |copy|
+        copy.instance_variable_set(:@attributes, attributes.deep_dup)
+      end
+    end
+
     def initialize_dup(_)
-      @attributes = attributes.deep_dup
+      @attributes = attributes.dup
       super
     end
 

activerecord/lib/active_record/attribute_set/builder.rb

@@ -47,8 +47,14 @@ module ActiveRecord
       delegate_hash[key] = value
     end
 
+    def deep_dup
+      dup.tap do |copy|
+        copy.instance_variable_set(:@delegate_hash, delegate_hash.transform_values(&:dup))
+      end
+    end
+
     def initialize_dup(_)
-      @delegate_hash = delegate_hash.transform_values(&:dup)
+      @delegate_hash = Hash[delegate_hash]
       super
     end
 

activerecord/lib/active_record/core.rb

@@ -296,7 +296,7 @@ module ActiveRecord
     #   # Instantiates a single new object
     #   User.new(first_name: 'Jamie')
     def initialize(attributes = nil)
-      @attributes = self.class._default_attributes.dup
+      @attributes = self.class._default_attributes.deep_dup
       self.class.define_attribute_methods
 
       init_internals
@@ -366,7 +366,7 @@ module ActiveRecord
 
     ##
     def initialize_dup(other) # :nodoc:
-      @attributes = @attributes.dup
+      @attributes = @attributes.deep_dup
       @attributes.reset(self.class.primary_key)
 
       _run_initialize_callbacks

activerecord/test/cases/attribute_set_test.rb

@@ -29,7 +29,7 @@ module ActiveRecord
       assert_equal :bar, attributes[:bar].name
     end
 
-    test "duping creates a new hash and dups each attribute" do
+    test "duping creates a new hash, but does not dup the attributes" do
       builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::String.new)
       attributes = builder.build_from_database(foo: 1, bar: 'foo')
 
@@ -41,6 +41,24 @@ module ActiveRecord
       duped.write_from_database(:foo, 2)
       duped[:bar].value << 'bar'
 
+      assert_equal 1, attributes[:foo].value
+      assert_equal 2, duped[:foo].value
+      assert_equal 'foobar', attributes[:bar].value
+      assert_equal 'foobar', duped[:bar].value
+    end
+
+    test "deep_duping creates a new hash and dups each attribute" do
+      builder = AttributeSet::Builder.new(foo: Type::Integer.new, bar: Type::String.new)
+      attributes = builder.build_from_database(foo: 1, bar: 'foo')
+
+      # Ensure the type cast value is cached
+      attributes[:foo].value
+      attributes[:bar].value
+
+      duped = attributes.deep_dup
+      duped.write_from_database(:foo, 2)
+      duped[:bar].value << 'bar'
+
       assert_equal 1, attributes[:foo].value
       assert_equal 2, duped[:foo].value
       assert_equal 'foo', attributes[:bar].value