Replaced `ActiveSupport::Concurrency::Latch` with concurrent-ruby.

The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.

11 files changed, 70 insertions, 58 deletions

diff --git a/Gemfile.lock b/Gemfile.lock
index 960a77af92..e44a2fc2ed 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -64,6 +64,7 @@ PATH
     actionpack (5.0.0.alpha)
       actionview (= 5.0.0.alpha)
       activesupport (= 5.0.0.alpha)
+      concurrent-ruby (~> 0.9.0)
       rack (~> 1.6)
       rack-test (~> 0.6.3)
       rails-dom-testing (~> 1.0, >= 1.0.5)
@@ -85,6 +86,7 @@ PATH
       activesupport (= 5.0.0.alpha)
       arel (= 7.0.0.alpha)
     activesupport (5.0.0.alpha)
+      concurrent-ruby (~> 0.9.0)
       i18n (~> 0.7)
       json (~> 1.7, >= 1.7.7)
       minitest (~> 5.1)
@@ -135,6 +137,7 @@ GEM
       execjs
     coffee-script-source (1.9.0)
     columnize (0.9.0)
+    concurrent-ruby (0.9.0)
     connection_pool (2.1.1)
     dalli (2.7.2)
     dante (0.1.5)
diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index 46856ffc5d..6c6f27ce90 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Replaced `ActiveSupport::Concurrency::Latch` with `Concurrent::CountDownLatch`
+    from the concurrent-ruby gem.
+
+    *Jerry D'Antonio*
+
 *   Add ability to filter parameters based on parent keys.
 
         # matches {credit_card: {code: "xxxx"}}
diff --git a/actionpack/actionpack.gemspec b/actionpack/actionpack.gemspec
index 1bba9df969..7830242af5 100644
--- a/actionpack/actionpack.gemspec
+++ b/actionpack/actionpack.gemspec
@@ -26,6 +26,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'rails-html-sanitizer', '~> 1.0', '>= 1.0.2'
   s.add_dependency 'rails-dom-testing', '~> 1.0', '>= 1.0.5'
   s.add_dependency 'actionview', version
+  s.add_dependency 'concurrent-ruby', '~> 0.9.0'
 
   s.add_development_dependency 'activemodel', version
 end
diff --git a/actionpack/test/controller/live_stream_test.rb b/actionpack/test/controller/live_stream_test.rb
index 0c65270ec1..6161e3f47a 100644
--- a/actionpack/test/controller/live_stream_test.rb
+++ b/actionpack/test/controller/live_stream_test.rb
@@ -1,5 +1,5 @@
 require 'abstract_unit'
-require 'active_support/concurrency/latch'
+require 'concurrent/atomics'
 Thread.abort_on_exception = true
 
 module ActionController
@@ -145,7 +145,7 @@ module ActionController
         response.headers['Content-Type'] = 'text/event-stream'
         %w{ hello world }.each do |word|
           response.stream.write word
-          latch.await
+          latch.wait
         end
         response.stream.close
       end
@@ -212,7 +212,7 @@ module ActionController
         # .. plus one more, because the #each frees up a slot:
         response.stream.write '.'
 
-        latch.release
+        latch.count_down
 
         # This write will block, and eventually raise
         response.stream.write 'x'
@@ -233,7 +233,7 @@ module ActionController
         end
 
         logger.info 'Work complete'
-        latch.release
+        latch.count_down
       end
     end
 
@@ -278,7 +278,7 @@ module ActionController
     def test_async_stream
       rubinius_skip "https://github.com/rubinius/rubinius/issues/2934"
 
-      @controller.latch = ActiveSupport::Concurrency::Latch.new
+      @controller.latch = Concurrent::CountDownLatch.new
       parts             = ['hello', 'world']
 
       @controller.request  = @request
@@ -289,8 +289,8 @@ module ActionController
         resp.stream.each do |part|
           assert_equal parts.shift, part
           ol = @controller.latch
-          @controller.latch = ActiveSupport::Concurrency::Latch.new
-          ol.release
+          @controller.latch = Concurrent::CountDownLatch.new
+          ol.count_down
         end
       }
 
@@ -300,23 +300,23 @@ module ActionController
     end
 
     def test_abort_with_full_buffer
-      @controller.latch = ActiveSupport::Concurrency::Latch.new
+      @controller.latch = Concurrent::CountDownLatch.new
 
       @request.parameters[:format] = 'plain'
       @controller.request  = @request
       @controller.response = @response
 
-      got_error = ActiveSupport::Concurrency::Latch.new
+      got_error = Concurrent::CountDownLatch.new
       @response.stream.on_error do
         ActionController::Base.logger.warn 'Error while streaming'
-        got_error.release
+        got_error.count_down
       end
 
       t = Thread.new(@response) { |resp|
         resp.await_commit
         _, _, body = resp.to_a
         body.each do |part|
-          @controller.latch.await
+          @controller.latch.wait
           body.close
           break
         end
@@ -325,13 +325,13 @@ module ActionController
       capture_log_output do |output|
         @controller.process :overfill_buffer_and_die
         t.join
-        got_error.await
+        got_error.wait
         assert_match 'Error while streaming', output.rewind && output.read
       end
     end
 
     def test_ignore_client_disconnect
-      @controller.latch = ActiveSupport::Concurrency::Latch.new
+      @controller.latch = Concurrent::CountDownLatch.new
 
       @controller.request  = @request
       @controller.response = @response
@@ -349,7 +349,7 @@ module ActionController
         @controller.process :ignore_client_disconnect
         t.join
         Timeout.timeout(3) do
-          @controller.latch.await
+          @controller.latch.wait
         end
         assert_match 'Work complete', output.rewind && output.read
       end
diff --git a/actionpack/test/dispatch/live_response_test.rb b/actionpack/test/dispatch/live_response_test.rb
index 512f3a8a7a..5cfa5f7b3b 100644
--- a/actionpack/test/dispatch/live_response_test.rb
+++ b/actionpack/test/dispatch/live_response_test.rb
@@ -1,5 +1,5 @@
 require 'abstract_unit'
-require 'active_support/concurrency/latch'
+require 'concurrent/atomics'
 
 module ActionController
   module Live
@@ -27,18 +27,18 @@ module ActionController
       end
 
       def test_parallel
-        latch = ActiveSupport::Concurrency::Latch.new
+        latch = Concurrent::CountDownLatch.new
 
         t = Thread.new {
           @response.stream.write 'foo'
-          latch.await
+          latch.wait
           @response.stream.close
         }
 
         @response.await_commit
         @response.each do |part|
           assert_equal 'foo', part
-          latch.release
+          latch.count_down
         end
         assert t.join
       end
@@ -62,15 +62,15 @@ module ActionController
 
       def test_headers_cannot_be_written_after_webserver_reads
         @response.stream.write 'omg'
-        latch = ActiveSupport::Concurrency::Latch.new
+        latch = Concurrent::CountDownLatch.new
 
         t = Thread.new {
           @response.stream.each do |chunk|
-            latch.release
+            latch.count_down
           end
         }
 
-        latch.await
+        latch.wait
         assert @response.headers.frozen?
         e = assert_raises(ActionDispatch::IllegalStateError) do
           @response.headers['Content-Length'] = "zomg"
diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index e4a6734009..e3ad9ff3b1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Replaced `ActiveSupport::Concurrency::Latch` with `Concurrent::CountDownLatch`
+    from the concurrent-ruby gem.
+
+    *Jerry D'Antonio*
+
 *   Fix through associations using scopes having the scope merged multiple
     times.
 
diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index 31c31e4329..382adbbdc7 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1,7 +1,6 @@
 # -*- coding: utf-8 -*-
 
 require "cases/helper"
-require 'active_support/concurrency/latch'
 require 'models/post'
 require 'models/author'
 require 'models/topic'
@@ -29,6 +28,7 @@ require 'models/bird'
 require 'models/car'
 require 'models/bulb'
 require 'rexml/document'
+require 'concurrent/atomics'
 
 class FirstAbstractClass < ActiveRecord::Base
   self.abstract_class = true
@@ -1506,20 +1506,20 @@ class BasicsTest < ActiveRecord::TestCase
     orig_handler = klass.connection_handler
     new_handler = ActiveRecord::ConnectionAdapters::ConnectionHandler.new
     after_handler = nil
-    latch1 = ActiveSupport::Concurrency::Latch.new
-    latch2 = ActiveSupport::Concurrency::Latch.new
+    latch1 = Concurrent::CountDownLatch.new
+    latch2 = Concurrent::CountDownLatch.new
 
     t = Thread.new do
       klass.connection_handler = new_handler
-      latch1.release
-      latch2.await
+      latch1.count_down
+      latch2.wait
       after_handler = klass.connection_handler
     end
 
-    latch1.await
+    latch1.wait
 
     klass.connection_handler = orig_handler
-    latch2.release
+    latch2.count_down
     t.join
 
     assert_equal after_handler, new_handler
diff --git a/activerecord/test/cases/connection_pool_test.rb b/activerecord/test/cases/connection_pool_test.rb
index c905772193..7ef5c93a48 100644
--- a/activerecord/test/cases/connection_pool_test.rb
+++ b/activerecord/test/cases/connection_pool_test.rb
@@ -1,5 +1,5 @@
 require "cases/helper"
-require 'active_support/concurrency/latch'
+require 'concurrent/atomics'
 
 module ActiveRecord
   module ConnectionAdapters
@@ -133,15 +133,15 @@ module ActiveRecord
       end
 
       def test_reap_inactive
-        ready = ActiveSupport::Concurrency::Latch.new
+        ready = Concurrent::CountDownLatch.new
         @pool.checkout
         child = Thread.new do
           @pool.checkout
           @pool.checkout
-          ready.release
+          ready.count_down
           Thread.stop
         end
-        ready.await
+        ready.wait
 
         assert_equal 3, active_connections(@pool).size
 
@@ -360,13 +360,13 @@ module ActiveRecord
       def test_concurrent_connection_establishment
         assert_operator @pool.connections.size, :<=, 1
 
-        all_threads_in_new_connection = ActiveSupport::Concurrency::Latch.new(@pool.size - @pool.connections.size)
-        all_go                        = ActiveSupport::Concurrency::Latch.new
+        all_threads_in_new_connection = Concurrent::CountDownLatch.new(@pool.size - @pool.connections.size)
+        all_go                        = Concurrent::CountDownLatch.new
 
         @pool.singleton_class.class_eval do
           define_method(:new_connection) do
-            all_threads_in_new_connection.release
-            all_go.await
+            all_threads_in_new_connection.count_down
+            all_go.wait
             super()
           end
         end
@@ -381,14 +381,14 @@ module ActiveRecord
             # the kernel of the whole test is here, everything else is just scaffolding,
             # this latch will not be released unless conn. pool allows for concurrent
             # connection creation
-            all_threads_in_new_connection.await
+            all_threads_in_new_connection.wait
           end
         rescue Timeout::Error
           flunk 'pool unable to establish connections concurrently or implementation has ' <<
                 'changed, this test then needs to patch a different :new_connection method'
         ensure
           # clean up the threads
-          all_go.release
+          all_go.count_down
           connecting_threads.map(&:join)
         end
       end
@@ -441,11 +441,11 @@ module ActiveRecord
         with_single_connection_pool do |pool|
           [:disconnect, :disconnect!, :clear_reloadable_connections, :clear_reloadable_connections!].each do |group_action_method|
             conn               = pool.connection # drain the only available connection
-            second_thread_done = ActiveSupport::Concurrency::Latch.new
+            second_thread_done = Concurrent::CountDownLatch.new
 
             # create a first_thread and let it get into the FIFO queue first
             first_thread = Thread.new do
-              pool.with_connection { second_thread_done.await }
+              pool.with_connection { second_thread_done.wait }
             end
 
             # wait for first_thread to get in queue
@@ -456,7 +456,7 @@ module ActiveRecord
             # first_thread when a connection is made available
             second_thread = Thread.new do
               pool.send(group_action_method)
-              second_thread_done.release
+              second_thread_done.count_down
             end
 
             # wait for second_thread to get in queue
@@ -471,7 +471,7 @@ module ActiveRecord
             failed = true unless second_thread.join(2)
 
             #--- post test clean up start
-            second_thread_done.release if failed
+            second_thread_done.count_down if failed
 
             # after `pool.disconnect()` the first thread will be left stuck in queue, no need to wait for
             # it to timeout with ConnectionTimeoutError
diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 334eebc268..17c2f0eb27 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,3 +1,8 @@
+*   Removed `ActiveSupport::Concurrency::Latch`, superseded by `Concurrent::CountDownLatch`
+    from the concurrent-ruby gem.
+
+    *Jerry D'Antonio*
+
 *   Fix not calling `#default` on `HashWithIndifferentAccess#to_hash` when only
     `default_proc` is set, which could raise.
 
diff --git a/activesupport/activesupport.gemspec b/activesupport/activesupport.gemspec
index 12c6a4d449..c18c1c87c9 100644
--- a/activesupport/activesupport.gemspec
+++ b/activesupport/activesupport.gemspec
@@ -25,4 +25,5 @@ Gem::Specification.new do |s|
   s.add_dependency 'tzinfo',     '~> 1.1'
   s.add_dependency 'minitest',   '~> 5.1'
   s.add_dependency 'thread_safe','~> 0.3', '>= 0.3.4'
+  s.add_dependency 'concurrent-ruby', '~> 0.9.0'
 end
diff --git a/activesupport/lib/active_support/concurrency/latch.rb b/activesupport/lib/active_support/concurrency/latch.rb
index 1507de433e..7b8df0df04 100644
--- a/activesupport/lib/active_support/concurrency/latch.rb
+++ b/activesupport/lib/active_support/concurrency/latch.rb
@@ -1,26 +1,18 @@
-require 'thread'
-require 'monitor'
+require 'concurrent/atomics'
 
 module ActiveSupport
   module Concurrency
-    class Latch
-      def initialize(count = 1)
-        @count = count
-        @lock = Monitor.new
-        @cv = @lock.new_cond
-      end
+    class Latch < Concurrent::CountDownLatch
 
-      def release
-        @lock.synchronize do
-          @count -= 1 if @count > 0
-          @cv.broadcast if @count.zero?
-        end
+      def initialize(count = 1)
+        ActiveSupport::Deprecation.warn("ActiveSupport::Concurrency::Latch is deprecated. Please use Concurrent::CountDownLatch instead.")
+        super(count)
       end
+      
+      alias_method :release, :count_down
 
       def await
-        @lock.synchronize do
-          @cv.wait_while { @count > 0 }
-        end
+        wait(nil)
       end
     end
   end