Further encapsulate dirty checking on `Attribute`

We can skip the allocation of a full `AttributeSet` by changing the
semantics of how we structure things. Instead of comparing two separate
`AttributeSet` objects, and `Attribute` is now a singly linked list of
every change that has happened to it. Since the attribute objects are
immutable, to apply the changes we simply need to copy the head of the
list.

It's worth noting that this causes one subtle change in the behavior of
AR. When a record is saved successfully, the `before_type_cast` version
of everything will be what was sent to the database. I honestly think
these semantics make more sense, as we could have just as easily had the
DB do `RETURNING *` and updated the record with those if we had things
like timestamps implemented at the DB layer.

This brings our performance closer to 4.2, but we're still not quite
there.

4 files changed, 61 insertions, 47 deletions

diff --git a/activerecord/lib/active_record/attribute.rb b/activerecord/lib/active_record/attribute.rb
index 74b44810d4..3c4c8f10ec 100644
--- a/activerecord/lib/active_record/attribute.rb
+++ b/activerecord/lib/active_record/attribute.rb
@@ -5,8 +5,8 @@ module ActiveRecord
         FromDatabase.new(name, value, type)
       end
 
-      def from_user(name, value, type)
-        FromUser.new(name, value, type)
+      def from_user(name, value, type, original_attribute = nil)
+        FromUser.new(name, value, type, original_attribute)
       end
 
       def with_cast_value(name, value, type)
@@ -26,10 +26,11 @@ module ActiveRecord
 
     # This method should not be called directly.
     # Use #from_database or #from_user
-    def initialize(name, value_before_type_cast, type)
+    def initialize(name, value_before_type_cast, type, original_attribute = nil)
       @name = name
       @value_before_type_cast = value_before_type_cast
       @type = type
+      @original_attribute = original_attribute
     end
 
     def value
@@ -38,21 +39,33 @@ module ActiveRecord
       @value
     end
 
+    def original_value
+      if assigned?
+        original_attribute.original_value
+      else
+        type_cast(value_before_type_cast)
+      end
+    end
+
     def value_for_database
       type.serialize(value)
     end
 
-    def changed_from?(old_value)
-      type.changed?(old_value, value, value_before_type_cast)
+    def changed?
+      changed_from_assignment? || changed_in_place?
+    end
+
+    def changed_in_place?
+      has_been_read? && type.changed_in_place?(original_value_for_database, value)
     end
 
-    def changed_in_place_from?(old_value)
-      has_been_read? && type.changed_in_place?(old_value, value)
+    def forgetting_assignment
+      with_value_from_database(value_for_database)
     end
 
     def with_value_from_user(value)
       type.assert_valid_value(value)
-      self.class.from_user(name, value, type)
+      self.class.from_user(name, value, type, self)
     end
 
     def with_value_from_database(value)
@@ -64,7 +77,7 @@ module ActiveRecord
     end
 
     def with_type(type)
-      self.class.new(name, value_before_type_cast, type)
+      self.class.new(name, value_before_type_cast, type, original_attribute)
     end
 
     def type_cast(*)
@@ -97,16 +110,39 @@ module ActiveRecord
 
     protected
 
+    attr_reader :original_attribute
+    alias_method :assigned?, :original_attribute
+
     def initialize_dup(other)
       if defined?(@value) && @value.duplicable?
         @value = @value.dup
       end
     end
 
+    def changed_from_assignment?
+      assigned? && type.changed?(original_value, value, value_before_type_cast)
+    end
+
+    def original_value_for_database
+      if assigned?
+        original_attribute.original_value_for_database
+      else
+        _original_value_for_database
+      end
+    end
+
+    def _original_value_for_database
+      value_for_database
+    end
+
     class FromDatabase < Attribute # :nodoc:
       def type_cast(value)
         type.deserialize(value)
       end
+
+      def _original_value_for_database
+        value_before_type_cast
+      end
     end
 
     class FromUser < Attribute # :nodoc:
diff --git a/activerecord/lib/active_record/attribute/user_provided_default.rb b/activerecord/lib/active_record/attribute/user_provided_default.rb
index 501590cf0e..fb8ad9163e 100644
--- a/activerecord/lib/active_record/attribute/user_provided_default.rb
+++ b/activerecord/lib/active_record/attribute/user_provided_default.rb
@@ -4,8 +4,7 @@ module ActiveRecord
   class Attribute # :nodoc:
     class UserProvidedDefault < FromUser
       def initialize(name, value, type, database_default)
-        super(name, value, type)
-        @database_default = database_default
+        super(name, value, type, database_default)
       end
 
       def type_cast(value)
@@ -16,17 +15,9 @@ module ActiveRecord
         end
       end
 
-      def changed_from?(old_value)
-        super || changed_from?(database_default.value)
-      end
-
       def with_type(type)
-        self.class.new(name, value_before_type_cast, type, database_default)
+        self.class.new(name, value_before_type_cast, type, original_attribute)
       end
-
-      protected
-
-      attr_reader :database_default
     end
   end
 end
diff --git a/activerecord/lib/active_record/attribute_methods/dirty.rb b/activerecord/lib/active_record/attribute_methods/dirty.rb
index 43c15841c2..a2e4e0ac31 100644
--- a/activerecord/lib/active_record/attribute_methods/dirty.rb
+++ b/activerecord/lib/active_record/attribute_methods/dirty.rb
@@ -41,13 +41,15 @@ module ActiveRecord
 
       def init_internals
         super
-        @mutation_tracker = AttributeMutationTracker.new(@attributes, @attributes.dup)
+        @mutation_tracker = AttributeMutationTracker.new(@attributes)
       end
 
       def initialize_dup(other) # :nodoc:
         super
-        @mutation_tracker = AttributeMutationTracker.new(@attributes,
-                                                         self.class._default_attributes.deep_dup)
+        @attributes = self.class._default_attributes.map do |attr|
+          attr.with_value_from_user(@attributes.fetch_value(attr.name))
+        end
+        @mutation_tracker = AttributeMutationTracker.new(@attributes)
       end
 
       def changes_applied
@@ -122,7 +124,8 @@ module ActiveRecord
       end
 
       def store_original_attributes
-        @mutation_tracker = @mutation_tracker.now_tracking(@attributes)
+        @attributes = @attributes.map(&:forgetting_assignment)
+        @mutation_tracker = AttributeMutationTracker.new(@attributes)
       end
 
       def previous_mutation_tracker
diff --git a/activerecord/lib/active_record/attribute_mutation_tracker.rb b/activerecord/lib/active_record/attribute_mutation_tracker.rb
index 08eb8ba7ac..ba7348918b 100644
--- a/activerecord/lib/active_record/attribute_mutation_tracker.rb
+++ b/activerecord/lib/active_record/attribute_mutation_tracker.rb
@@ -1,14 +1,13 @@
 module ActiveRecord
   class AttributeMutationTracker # :nodoc:
-    def initialize(attributes, original_attributes)
+    def initialize(attributes)
       @attributes = attributes
-      @original_attributes = original_attributes
     end
 
     def changed_values
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         if changed?(attr_name)
-          result[attr_name] = original_attributes.fetch_value(attr_name)
+          result[attr_name] = attributes[attr_name].original_value
         end
       end
     end
@@ -16,49 +15,34 @@ module ActiveRecord
     def changes
       attr_names.each_with_object({}.with_indifferent_access) do |attr_name, result|
         if changed?(attr_name)
-          result[attr_name] = [original_attributes.fetch_value(attr_name), attributes.fetch_value(attr_name)]
+          result[attr_name] = [attributes[attr_name].original_value, attributes.fetch_value(attr_name)]
         end
       end
     end
 
     def changed?(attr_name)
       attr_name = attr_name.to_s
-      modified?(attr_name) || changed_in_place?(attr_name)
+      attributes[attr_name].changed?
     end
 
     def changed_in_place?(attr_name)
-      original_database_value = original_attributes[attr_name].value_before_type_cast
-      attributes[attr_name].changed_in_place_from?(original_database_value)
+      attributes[attr_name].changed_in_place?
     end
 
     def forget_change(attr_name)
       attr_name = attr_name.to_s
-      original_attributes[attr_name] = attributes[attr_name].dup
-    end
-
-    def now_tracking(new_attributes)
-      AttributeMutationTracker.new(new_attributes, clean_copy_of(new_attributes))
+      attributes[attr_name] = attributes[attr_name].forgetting_assignment
     end
 
     protected
 
-    attr_reader :attributes, :original_attributes
+    attr_reader :attributes
 
     private
 
     def attr_names
       attributes.keys
     end
-
-    def modified?(attr_name)
-      attributes[attr_name].changed_from?(original_attributes.fetch_value(attr_name))
-    end
-
-    def clean_copy_of(attributes)
-      attributes.map do |attr|
-        attr.with_value_from_database(attr.value_for_database)
-      end
-    end
   end
 
   class NullMutationTracker # :nodoc: