3 files changed, 65 insertions, 0 deletions
diff --git a/activesupport/lib/active_support.rb b/activesupport/lib/active_support.rb
index 7ebb3c48e0..62d538e2d5 100644
--- a/activesupport/lib/active_support.rb
+++ b/activesupport/lib/active_support.rb
@@ -32,6 +32,7 @@ module ActiveSupport
   autoload :BufferedLogger, 'active_support/buffered_logger'
   autoload :Cache, 'active_support/cache'
   autoload :Callbacks, 'active_support/callbacks'
+  autoload :ConcurrentHash, 'active_support/concurrent_hash'
   autoload :Deprecation, 'active_support/deprecation'
   autoload :Duration, 'active_support/duration'
   autoload :Gzip, 'active_support/gzip'
diff --git a/activesupport/lib/active_support/concurrent_hash.rb b/activesupport/lib/active_support/concurrent_hash.rb
new file mode 100644
index 0000000000..c9f9b16da3
--- /dev/null
+++ b/activesupport/lib/active_support/concurrent_hash.rb
@@ -0,0 +1,26 @@
+module ActiveSupport
+  class ConcurrentHash
+    def initialize(hash = {})
+      @backup_cache = hash.dup
+      @frozen_cache = hash.dup.freeze
+      @mutex = Mutex.new
+    end
+
+    def []=(k,v)
+      @mutex.synchronize { @backup_cache[k] = v }
+      @frozen_cache = @backup_cache.dup.freeze
+    end
+
+    def [](k)
+      if @frozen_cache.key?(k)
+        @frozen_cache[k]
+      else
+        @mutex.synchronize { @backup_cache[k] }
+      end
+    end
+
+    def empty?
+      @backup_cache.empty?
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/memoizable.rb b/activesupport/lib/active_support/memoizable.rb
index 4ffb5d9520..8e9abeaf91 100644
--- a/activesupport/lib/active_support/memoizable.rb
+++ b/activesupport/lib/active_support/memoizable.rb
@@ -1,4 +1,42 @@
 module ActiveSupport
+  class ConcurrentHash
+    def initialize(hash = {})
+      @backup_cache = hash.dup
+      @frozen_cache = hash.dup.freeze
+      @mutex = Mutex.new
+    end
+
+    def []=(k,v)
+      @mutex.synchronize { @backup_cache[k] = v }
+      @frozen_cache = @backup_cache.dup.freeze
+    end
+
+    def [](k)
+      if @frozen_cache.key?(k)
+        @frozen_cache[k]
+      else
+        @mutex.synchronize { @backup_cache[k] }
+      end
+    end
+    
+    def empty?
+      @backup_cache.empty?
+    end
+  end
+  
+  module SafelyMemoizable
+    def safely_memoize(*symbols)
+      symbols.each do |symbol|
+        class_eval <<-RUBY, __FILE__, __LINE__ + 1
+          def #{symbol}(*args)
+            memoized = @_memoized_#{symbol} || ::ActiveSupport::ConcurrentHash.new
+            memoized[args] ||= memoized_#{symbol}(*args)
+          end
+        RUBY
+      end
+    end
+  end
+  
   module Memoizable
     def self.memoized_ivar_for(symbol)
       "@_memoized_#{symbol.to_s.sub(/\?\Z/, '_query').sub(/!\Z/, '_bang')}".to_sym