Wrap config's data hash in mutex

lib/active_model/serializer/config.rb

@@ -1,27 +1,39 @@
+require 'thread'
+
 module ActiveModel
   class Serializer
     class Config
       def initialize
         @data = {}
+        @mutex = Mutex.new
       end
 
       def [](key)
+        @mutex.synchronize do
           @data[key.to_s]
         end
+      end
 
       def []=(key, value)
+        @mutex.synchronize do
           @data[key.to_s] = value
         end
+      end
 
       def each(&block)
+        @mutex.synchronize do
           @data.each(&block)
         end
+      end
 
       def clear
+        @mutex.synchronize do
           @data.clear
         end
+      end
 
       def method_missing(name, *args)
+        @mutex.synchronize do
           name = name.to_s
           return @data[name] if @data.include?(name)
           match = name.match(/\A(.*?)([?=]?)\Z/)
@@ -33,6 +45,7 @@ module ActiveModel
           end
         end
       end
+    end
 
     CONFIG = Config.new
   end