Able to initalize default value for thread_mattr_*

Added the ability to initialize `thread_mattr_*` methods with default
values like so:

``` ruby
  class MyClass
    thread_attr_reader :foo, default: :foo
    thread_attr_writer :bar, default: :bar
    thread_attr_accessor: baz do
      "baz"
    end
  end
```

This is consistent with the api exposed by `mattr_accessor`.

3 files changed, 46 insertions, 16 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index f20c7c92e6..dad7005eb1 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,11 @@
+*   Allow initializing `thread_mattr_*` attributes via `:default` option
+
+        class Scraper
+          thread_mattr_reader :client, default: Api::Client.new
+        end
+
+    *Guilherme Mansur*
+
 *   Add `compact_blank` for those times when you want to remove #blank? values from
     an Enumerable (also `compact_blank!` on Hash, Array, ActionController::Parameters)
 
diff --git a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
index a6e87aeb68..452a456a52 100644
--- a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
+++ b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
@@ -33,7 +33,7 @@ class Module
   #   end
   #
   #   Current.new.user # => NoMethodError
-  def thread_mattr_reader(*syms, instance_reader: true, instance_accessor: true) # :nodoc:
+  def thread_mattr_reader(*syms, instance_reader: true, instance_accessor: true, default: nil) # :nodoc:
     syms.each do |sym|
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
 
@@ -52,6 +52,9 @@ class Module
           end
         EOS
       end
+
+      default_val = (block_given? && default.nil?) ? yield : default
+      Thread.current["attr_" + name + "_#{sym}"] = default_val unless default_val.nil?
     end
   end
   alias :thread_cattr_reader :thread_mattr_reader
@@ -74,7 +77,7 @@ class Module
   #   end
   #
   #   Current.new.user = "DHH" # => NoMethodError
-  def thread_mattr_writer(*syms, instance_writer: true, instance_accessor: true) # :nodoc:
+  def thread_mattr_writer(*syms, instance_writer: true, instance_accessor: true, default: nil) # :nodoc:
     syms.each do |sym|
       raise NameError.new("invalid attribute name: #{sym}") unless /^[_A-Za-z]\w*$/.match?(sym)
 
@@ -93,6 +96,9 @@ class Module
           end
         EOS
       end
+
+      default_val = (block_given? && default.nil?) ? yield : default
+      Thread.current["attr_" + name + "_#{sym}"] = default_val unless default_val.nil?
     end
   end
   alias :thread_cattr_writer :thread_mattr_writer
@@ -136,9 +142,9 @@ class Module
   #
   #   Current.new.user = "DHH"  # => NoMethodError
   #   Current.new.user          # => NoMethodError
-  def thread_mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true)
-    thread_mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor)
-    thread_mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor)
+  def thread_mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)
+    thread_mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor, default: default, &blk)
+    thread_mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor, default: default)
   end
   alias :thread_cattr_accessor :thread_mattr_accessor
 end
diff --git a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
index e0e331fc91..c73ca0be0c 100644
--- a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
+++ b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
@@ -4,21 +4,37 @@ require "abstract_unit"
 require "active_support/core_ext/module/attribute_accessors_per_thread"
 
 class ModuleAttributeAccessorPerThreadTest < ActiveSupport::TestCase
+  class MyClass
+    thread_mattr_accessor :foo
+    thread_mattr_accessor :bar,  instance_writer: false
+    thread_mattr_reader   :shaq, instance_reader: false
+    thread_mattr_accessor :camp, instance_accessor: false
+  end
+
+  class SubMyClass < MyClass
+  end
+
   def setup
-    @class = Class.new do
-      thread_mattr_accessor :foo
-      thread_mattr_accessor :bar,  instance_writer: false
-      thread_mattr_reader   :shaq, instance_reader: false
-      thread_mattr_accessor :camp, instance_accessor: false
+    @class = MyClass
+    @subclass = SubMyClass
+    @object = @class.new
+  end
 
-      def self.name; "MyClass" end
-    end
+  def test_can_initialize_with_default_value
+    Thread.new do
+      @class.thread_mattr_accessor :baz, default: "default_value"
+      assert_equal "default_value", @class.baz
+    end.join
+  end
 
-    @subclass = Class.new(@class) do
-      def self.name; "SubMyClass" end
-    end
+  def test_can_initialize_with_a_block_as_default_value
+    Thread.new do
+      @class.thread_mattr_accessor :baz do
+        "default_value"
+      end
 
-    @object = @class.new
+      assert_equal "default_value", @class.baz
+    end.join
   end
 
   def test_should_use_mattr_default