ActiveSupport's class_attribute is not thread-safe

class_attribute.rb

# This is a script that reproduces https://github.com/rails/rails/issues/14021

require 'active_support/all'

ActiveSupport::VERSION::STRING
# => "4.1.1"
 
RUBY_ENGINE
# => "jruby"

class Foo
  class_attribute :bar
end

Thread.abort_on_exception = true

(1..100).map do |i| 
  Thread.new do
    Foo.bar = i
  end
end

# NameError: Undefined method bar for 'Foo'
# 	from org/jruby/RubyModule.java:2297:in `undef_method'
# 	from /Users/dwbutler/.rvm/gems/jruby-1.7.12/gems/activesupport-4.1.1/lib/active_support/core_ext/module/remove_method.rb:4:in `remove_possible_method'
# 	from /Users/dwbutler/.rvm/gems/jruby-1.7.12/gems/activesupport-4.1.1/lib/active_support/core_ext/class/attribute.rb:85:in `bar='
# 	from org/jruby/RubyModule.java:2304:in `module_eval'
# 	from /Users/dwbutler/.rvm/gems/jruby-1.7.12/gems/activesupport-4.1.1/lib/active_support/core_ext/class/attribute.rb:84:in `bar='
# 	from (irb):21:in `evaluate'