Eliminate instance level writers for class accessors

Instance level writers can have an impact on how the Active Model /
Record objects are saved.  Specifically, they can be used to bypass
validations.  This is a problem if mass assignment protection is
disabled and specific attributes are passed to the constructor.

CVE-2016-0753

activemodel/lib/active_model/serializers/json.rb

@@ -10,7 +10,7 @@ module ActiveModel
       included do
         extend ActiveModel::Naming
 
-        class_attribute :include_root_in_json
+        class_attribute :include_root_in_json, instance_writer: false
         self.include_root_in_json = false
       end
 

activemodel/lib/active_model/validations.rb

@@ -47,9 +47,10 @@ module ActiveModel
       include HelperMethods
 
       attr_accessor :validation_context
+      private :validation_context=
       define_callbacks :validate, scope: :name
 
-      class_attribute :_validators
+      class_attribute :_validators, instance_writer: false
       self._validators = Hash.new { |h,k| h[k] = [] }
     end
 

activerecord/lib/active_record/enum.rb

@@ -95,7 +95,7 @@ module ActiveRecord
 
   module Enum
     def self.extended(base) # :nodoc:
-      base.class_attribute(:defined_enums)
+      base.class_attribute(:defined_enums, instance_writer: false)
       base.defined_enums = {}
     end
 

activerecord/lib/active_record/reflection.rb

@@ -7,8 +7,8 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     included do
-      class_attribute :_reflections
-      class_attribute :aggregate_reflections
+      class_attribute :_reflections, instance_writer: false
+      class_attribute :aggregate_reflections, instance_writer: false
       self._reflections = {}
       self.aggregate_reflections = {}
     end

activerecord/lib/active_record/scoping/default.rb

@@ -6,7 +6,7 @@ module ActiveRecord
       included do
         # Stores the default scope for the class.
         class_attribute :default_scopes, instance_writer: false, instance_predicate: false
-        class_attribute :default_scope_override, instance_predicate: false
+        class_attribute :default_scope_override, instance_writer: false, instance_predicate: false
 
         self.default_scopes = []
         self.default_scope_override = nil

activesupport/lib/active_support/callbacks.rb

@@ -71,7 +71,7 @@ module ActiveSupport
     # halt the entire callback chain and display a deprecation message.
     # If false, callback chains will only be halted by calling +throw :abort+.
     # Defaults to +true+.
-    mattr_accessor(:halt_and_display_warning_on_return_false) { true }
+    mattr_accessor(:halt_and_display_warning_on_return_false, instance_writer: false) { true }
 
     # Runs the callbacks for the given event.
     #
@@ -742,7 +742,7 @@ module ActiveSupport
         options = names.extract_options!
 
         names.each do |name|
-          class_attribute "_#{name}_callbacks"
+          class_attribute "_#{name}_callbacks", instance_writer: false
           set_callbacks name, CallbackChain.new(name, options)
 
           module_eval <<-RUBY, __FILE__, __LINE__ + 1