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superseded by similar tools in the more specialized gem.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.

3 files changed, 14 insertions, 16 deletions

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+*   Removed `ActiveSupport::Concurrency::Latch`, superseded by `Concurrent::CountDownLatch`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