Add thread_m/cattr_accessor/reader/writer suite of methods for declaring class and module variables that live per-thread

5 files changed, 302 insertions, 0 deletions

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 07f7fae5d5..386ef821a7 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,52 @@
+*   Add thread_m/cattr_accessor/reader/writer suite of methods for declaring class and module variables that live per-thread.
+    This makes it easy to declare per-thread globals that are encapsulated. Note: This is a sharp edge. A wild proliferation
+    of globals is A Bad Thing. But like other sharp tools, when it's right, it's right.
+
+    Here's an example of a simple event tracking system where the object being tracked needs not pass a creator that it
+    doesn't need itself along:
+
+    module Current
+      thread_mattr_accessor :account
+      thread_mattr_accessor :user
+      
+      def self.reset() self.account = self.user = nil end
+    end
+
+    class ApplicationController < ActiveController::Base
+      before_action :set_current
+      after_action { Current.reset }
+    
+      private
+        def set_current
+          Current.account = Account.find(params[:account_id])
+          Current.person  = Current.account.people.find(params[:person_id])
+        end
+    end
+
+    class MessagesController < ApplicationController
+      def create
+        @message = Message.create!(message_params)
+      end
+    end
+
+    class Message < ApplicationRecord
+      has_many :events
+      after_create :track_created
+    
+      private
+        def track_created
+          events.create! origin: self, action: :create
+        end
+    end
+
+    class Event < ApplicationRecord
+      belongs_to :creator, class_name: 'Person'
+      before_validation { self.creator ||= Current.person }
+    end
+
+    *DHH*
+
+
 *   Deprecated `Module#qualified_const_` in favour of the builtin Module#const_
     methods.
 
diff --git a/activesupport/lib/active_support/core_ext/module.rb b/activesupport/lib/active_support/core_ext/module.rb
index b4efff8b24..ef038331c2 100644
--- a/activesupport/lib/active_support/core_ext/module.rb
+++ b/activesupport/lib/active_support/core_ext/module.rb
@@ -3,6 +3,7 @@ require 'active_support/core_ext/module/introspection'
 require 'active_support/core_ext/module/anonymous'
 require 'active_support/core_ext/module/reachable'
 require 'active_support/core_ext/module/attribute_accessors'
+require 'active_support/core_ext/module/attribute_accessors_per_thread'
 require 'active_support/core_ext/module/attr_internal'
 require 'active_support/core_ext/module/concerning'
 require 'active_support/core_ext/module/delegation'
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
new file mode 100644
index 0000000000..36fe52cea1
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
@@ -0,0 +1,140 @@
+require 'active_support/core_ext/array/extract_options'
+
+# Extends the module object with class/module and instance accessors for
+# class/module attributes, just like the native attr* accessors for instance
+# attributes, but does so on a per-thread basis.
+# 
+# So the values are scoped within the Thread.current space under the class name
+# of the module.
+class Module
+  # Defines a per-thread class attribute and creates class and instance reader methods.
+  # The underlying per-thread class variable is set to +nil+, if it is not previously defined.
+  #
+  #   module Current
+  #     thread_mattr_reader :user
+  #   end
+  #
+  #   Current.user # => nil
+  #   Thread.current[:attr_Current_user] = "DHH"
+  #   Current.user # => "DHH"
+  #
+  # The attribute name must be a valid method name in Ruby.
+  #
+  #   module Foo
+  #     thread_mattr_reader :"1_Badname"
+  #   end
+  #   # => NameError: invalid attribute name: 1_Badname
+  #
+  # If you want to opt out the creation on the instance reader method, pass
+  # <tt>instance_reader: false</tt> or <tt>instance_accessor: false</tt>.
+  #
+  #   class Current
+  #     thread_mattr_reader :user, instance_reader: false
+  #   end
+  #
+  #   Current.new.user # => NoMethodError
+  def thread_mattr_reader(*syms)
+    options = syms.extract_options!
+
+    syms.each do |sym|
+      raise NameError.new("invalid attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
+      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
+        def self.#{sym}
+          Thread.current[:"attr_#{name}_#{sym}"]
+        end
+      EOS
+
+      unless options[:instance_reader] == false || options[:instance_accessor] == false
+        class_eval(<<-EOS, __FILE__, __LINE__ + 1)
+          def #{sym}
+            Thread.current[:"attr_#{self.class.name}_#{sym}"]
+          end
+        EOS
+      end
+    end
+  end
+  alias :thread_cattr_reader :thread_mattr_reader
+
+  # Defines a per-thread class attribute and creates a class and instance writer methods to
+  # allow assignment to the attribute.
+  #
+  #   module Current
+  #     thread_mattr_writer :user
+  #   end
+  #
+  #   Current.user = "DHH"
+  #   Thread.current[:attr_Current_user] # => "DHH"
+  #
+  # If you want to opt out the instance writer method, pass
+  # <tt>instance_writer: false</tt> or <tt>instance_accessor: false</tt>.
+  #
+  #   class Current
+  #     thread_mattr_writer :user, instance_writer: false
+  #   end
+  #
+  #   Current.new.user = "DHH" # => NoMethodError
+  def thread_mattr_writer(*syms)
+    options = syms.extract_options!
+    syms.each do |sym|
+      raise NameError.new("invalid attribute name: #{sym}") unless sym =~ /^[_A-Za-z]\w*$/
+      class_eval(<<-EOS, __FILE__, __LINE__ + 1)
+        def self.#{sym}=(obj)
+          Thread.current[:"attr_#{name}_#{sym}"] = obj
+        end
+      EOS
+
+      unless options[:instance_writer] == false || options[:instance_accessor] == false
+        class_eval(<<-EOS, __FILE__, __LINE__ + 1)
+          def #{sym}=(obj)
+            Thread.current[:"attr_#{self.class.name}_#{sym}"] = obj
+          end
+        EOS
+      end
+    end
+  end
+  alias :thread_cattr_writer :thread_mattr_writer
+
+  # Defines both class and instance accessors for class attributes.
+  #
+  #   class Current
+  #     thread_mattr_accessor :user
+  #   end
+  #
+  #   Current.user = "DHH"
+  #   Current.user     # => "DHH"
+  #   Current.new.user # => "DHH"
+  #
+  # If a subclass changes the value then that will not change the value for
+  # parent class. Similarly if parent class changes the value then that will not
+  # change the value of subclasses either.
+  #
+  #   class Customer < Account
+  #   end
+  #
+  #   Customer.user = "CHH"
+  #   Account.user # => "DHH"
+  #
+  # To opt out of the instance writer method, pass <tt>instance_writer: false</tt>.
+  # To opt out of the instance reader method, pass <tt>instance_reader: false</tt>.
+  #
+  #   class Current
+  #     thread_mattr_accessor :user, instance_writer: false, instance_reader: false
+  #   end
+  #
+  #   Current.new.user = "DHH"  # => NoMethodError
+  #   Current.new.user          # => NoMethodError
+  #
+  # Or pass <tt>instance_accessor: false</tt>, to opt out both instance methods.
+  #
+  #   class Current
+  #     mattr_accessor :user, instance_accessor: false
+  #   end
+  #
+  #   Current.new.user = "DHH"  # => NoMethodError
+  #   Current.new.user          # => NoMethodError
+  def thread_mattr_accessor(*syms, &blk)
+    thread_mattr_reader(*syms, &blk)
+    thread_mattr_writer(*syms, &blk)
+  end
+  alias :thread_cattr_accessor :thread_mattr_accessor
+end
diff --git a/activesupport/lib/active_support/per_thread_registry.rb b/activesupport/lib/active_support/per_thread_registry.rb
index a909a65bb6..88e2b12cc7 100644
--- a/activesupport/lib/active_support/per_thread_registry.rb
+++ b/activesupport/lib/active_support/per_thread_registry.rb
@@ -1,6 +1,9 @@
 require 'active_support/core_ext/module/delegation'
 
 module ActiveSupport
+  # NOTE: This approach has been deprecated for end-user code in favor of thread_mattr_accessor and friends.
+  # Please use that approach instead.
+  #
   # This module is used to encapsulate access to thread local variables.
   #
   # Instead of polluting the thread locals namespace:
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
new file mode 100644
index 0000000000..b8d6c00ff7
--- /dev/null
+++ b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
@@ -0,0 +1,109 @@
+require 'abstract_unit'
+require 'active_support/core_ext/module/attribute_accessors_per_thread'
+
+class ModuleAttributeAccessorTest < ActiveSupport::TestCase
+  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
+    end
+
+    @object = @class.new
+  end
+
+  def test_should_use_mattr_default
+    Thread.new do
+      assert_nil @class.foo
+      assert_nil @object.foo
+    end.join
+  end
+
+  def test_should_set_mattr_value
+    Thread.new do
+      @class.foo = :test
+      assert_equal :test, @class.foo
+
+      @class.foo = :test2
+      assert_equal :test2, @class.foo
+    end.join
+  end
+
+  def test_should_not_create_instance_writer
+    Thread.new do
+      assert_respond_to @class, :foo
+      assert_respond_to @class, :foo=
+      assert_respond_to @object, :bar
+      assert !@object.respond_to?(:bar=)
+    end.join
+  end
+
+  def test_should_not_create_instance_reader
+    Thread.new do
+      assert_respond_to @class, :shaq
+      assert !@object.respond_to?(:shaq)
+    end.join
+  end
+
+  def test_should_not_create_instance_accessors
+    Thread.new do
+      assert_respond_to @class, :camp
+      assert !@object.respond_to?(:camp)
+      assert !@object.respond_to?(:camp=)
+    end.join
+  end
+
+  def test_values_should_not_bleed_between_threads
+    threads = []
+    threads << Thread.new do
+      @class.foo = 'things'
+      sleep 1
+      assert_equal 'things', @class.foo
+    end
+
+    threads << Thread.new do
+      @class.foo = 'other things'
+      sleep 1
+      assert_equal 'other things', @class.foo      
+    end
+    
+    threads << Thread.new do
+      @class.foo = 'really other things'
+      sleep 1
+      assert_equal 'really other things', @class.foo      
+    end
+    
+    threads.each { |t| t.join }
+  end
+
+  def test_should_raise_name_error_if_attribute_name_is_invalid
+    exception = assert_raises NameError do
+      Class.new do
+        thread_cattr_reader "1nvalid"
+      end
+    end
+    assert_equal "invalid attribute name: 1nvalid", exception.message
+
+    exception = assert_raises NameError do
+      Class.new do
+        thread_cattr_writer "1nvalid"
+      end
+    end
+    assert_equal "invalid attribute name: 1nvalid", exception.message
+
+    exception = assert_raises NameError do
+      Class.new do
+        thread_mattr_reader "1valid_part"
+      end
+    end
+    assert_equal "invalid attribute name: 1valid_part", exception.message
+
+    exception = assert_raises NameError do
+      Class.new do
+        thread_mattr_writer "2valid_part"
+      end
+    end
+    assert_equal "invalid attribute name: 2valid_part", exception.message
+  end
+end