Implemented module-level configuration.

* Implemented a module-level Configuration object
* Removed the $GLOBAL_THREAD_POOL global variable
* Added a global_thread_pool accessor to the module config
* Renamed NullThreadPool to PerThreadExecutor
* Moved PerThreadExecutor into its own file
* Updated UsesGlobalThreadPool to use new module config
* Updated all tests to use new module config
* Updated all tests with new name of PerThreadExecutor

Author: jdantonio

lib/concurrent.rb

@@ -1,4 +1,5 @@
 require 'concurrent/version'
+require 'concurrent/configuration'
 
 require 'concurrent/atomic'
 require 'concurrent/count_down_latch'
@@ -30,10 +31,11 @@
 require 'concurrent/timer_task'
 require 'concurrent/utilities'
 
-require 'concurrent/global_thread_pool'
 require 'concurrent/cached_thread_pool'
 require 'concurrent/fixed_thread_pool'
 require 'concurrent/immediate_executor'
+require 'concurrent/per_thread_executor'
+require 'concurrent/uses_global_thread_pool'
 
 # Modern concurrency tools for Ruby. Inspired by Erlang, Clojure, Scala, Haskell,
 # F#, C#, Java, and classic concurrency patterns.

lib/concurrent/agent.rb

@@ -2,7 +2,7 @@
 require 'observer'
 
 require 'concurrent/dereferenceable'
-require 'concurrent/global_thread_pool'
+require 'concurrent/uses_global_thread_pool'
 require 'concurrent/utilities'
 
 module Concurrent

lib/concurrent/configuration.rb

@@ -0,0 +1,22 @@
+require 'concurrent/per_thread_executor'
+
+module Concurrent
+  class << self
+    attr_accessor :configuration
+  end
+
+  def self.configure
+    yield(configuration)
+  end
+
+  class Configuration
+    attr_accessor :global_thread_pool
+
+    def initialize
+      @global_thread_pool = Concurrent::PerThreadExecutor.new
+    end
+  end
+
+  # create the default configuration on load
+  self.configuration = Configuration.new
+end

lib/concurrent/future.rb

@@ -1,7 +1,7 @@
 require 'thread'
 
 require 'concurrent/obligation'
-require 'concurrent/global_thread_pool'
+require 'concurrent/uses_global_thread_pool'
 require 'concurrent/safe_task_executor'
 
 module Concurrent

lib/concurrent/global_thread_pool.rb

@@ -0,0 +1,22 @@
+module Concurrent
+
+  class PerThreadExecutor
+
+    def self.post(*args)
+      Thread.new(*args) do
+        Thread.current.abort_on_exception = false
+        yield(*args)
+      end
+      return true
+    end
+
+    def post(*args, &block)
+      return PerThreadExecutor.post(*args, &block)
+    end
+
+    def <<(block)
+      PerThreadExecutor.post(&block)
+      return self
+    end
+  end
+end

lib/concurrent/per_thread_executor.rb

@@ -1,6 +1,6 @@
 require 'thread'
 
-require 'concurrent/global_thread_pool'
+require 'concurrent/uses_global_thread_pool'
 require 'concurrent/obligation'
 
 module Concurrent

lib/concurrent/promise.rb

@@ -0,0 +1,22 @@
+require 'concurrent/configuration'
+
+module Concurrent
+
+  module UsesGlobalThreadPool
+
+    def self.included(base)
+      class << base
+        def thread_pool
+          @thread_pool || Concurrent.configuration.global_thread_pool
+        end
+        def thread_pool=(pool)
+          if pool == Concurrent.configuration.global_thread_pool
+            @thread_pool = nil
+          else
+            @thread_pool = pool
+          end
+        end
+      end
+    end
+  end
+end

lib/concurrent/uses_global_thread_pool.rb

@@ -18,7 +18,7 @@ module Concurrent
     end
 
     before(:each) do
-      Agent.thread_pool = NullThreadPool.new
+      Agent.thread_pool = PerThreadExecutor.new
     end
 
     context 'behavior' do

spec/concurrent/agent_spec.rb

@@ -5,7 +5,7 @@ module Concurrent
   describe Async do
 
     before(:each) do
-      Concurrent::Future.thread_pool = Concurrent::NullThreadPool.new
+      Concurrent::Future.thread_pool = Concurrent::PerThreadExecutor.new
     end
 
     let(:async_class) do