Move async execution from celluloid to concurrent-ruby

This removes 8 runtime gem dependencies from Rails:

```
Using hitimes 1.2.3
Using timers 4.1.1
Using celluloid-essentials 0.20.5
Using celluloid-extras 0.20.5
Using celluloid-fsm 0.20.5
Using celluloid-pool 0.20.5
Using celluloid-supervision 0.20.5
Using celluloid 0.17.2
```

8 files changed, 47 insertions, 29 deletions

diff --git a/actioncable/actioncable.gemspec b/actioncable/actioncable.gemspec
index 74c21bd24d..b6e529e879 100644
--- a/actioncable/actioncable.gemspec
+++ b/actioncable/actioncable.gemspec
@@ -23,7 +23,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'coffee-rails',     '~> 4.1.0'
   s.add_dependency 'faye-websocket',   '~> 0.10.0'
   s.add_dependency 'websocket-driver', '~> 0.6.1'
-  s.add_dependency 'celluloid',        '~> 0.17.2'
+  s.add_dependency 'concurrent-ruby',  '~> 1.0.0'
   s.add_dependency 'em-hiredis',       '~> 0.3.0'
   s.add_dependency 'redis',            '~> 3.0'
 
diff --git a/actioncable/lib/action_cable/channel/periodic_timers.rb b/actioncable/lib/action_cable/channel/periodic_timers.rb
index 25fe8e5e54..7f0fb37afc 100644
--- a/actioncable/lib/action_cable/channel/periodic_timers.rb
+++ b/actioncable/lib/action_cable/channel/periodic_timers.rb
@@ -28,7 +28,7 @@ module ActionCable
         def start_periodic_timers
           self.class.periodic_timers.each do |callback, options|
             active_periodic_timers << EventMachine::PeriodicTimer.new(options[:every]) do
-              connection.worker_pool.async.run_periodic_timer(self, callback)
+              connection.worker_pool.async_run_periodic_timer(self, callback)
             end
           end
         end
diff --git a/actioncable/lib/action_cable/connection/base.rb b/actioncable/lib/action_cable/connection/base.rb
index 977856d656..a8cfdf90f3 100644
--- a/actioncable/lib/action_cable/connection/base.rb
+++ b/actioncable/lib/action_cable/connection/base.rb
@@ -103,7 +103,7 @@ module ActionCable
 
       # Invoke a method on the connection asynchronously through the pool of thread workers.
       def send_async(method, *arguments)
-        worker_pool.async.invoke(self, method, *arguments)
+        worker_pool.async_invoke(self, method, *arguments)
       end
 
       # Return a basic hash of statistics for the connection keyed with `identifier`, `started_at`, and `subscriptions`.
diff --git a/actioncable/lib/action_cable/process/logging.rb b/actioncable/lib/action_cable/process/logging.rb
index 72b1a080d1..dce637b3ca 100644
--- a/actioncable/lib/action_cable/process/logging.rb
+++ b/actioncable/lib/action_cable/process/logging.rb
@@ -1,10 +1,7 @@
 require 'action_cable/server'
 require 'eventmachine'
-require 'celluloid'
 
 EM.error_handler do |e|
   puts "Error raised inside the event loop: #{e.message}"
   puts e.backtrace.join("\n")
 end
-
-Celluloid.logger = ActionCable.server.logger
diff --git a/actioncable/lib/action_cable/server/base.rb b/actioncable/lib/action_cable/server/base.rb
index 740e4b301e..cfd0a65f0f 100644
--- a/actioncable/lib/action_cable/server/base.rb
+++ b/actioncable/lib/action_cable/server/base.rb
@@ -1,6 +1,3 @@
-# FIXME: Cargo culted fix from https://github.com/celluloid/celluloid-pool/issues/10
-require 'celluloid/current'
-
 require 'em-hiredis'
 
 module ActionCable
diff --git a/actioncable/lib/action_cable/server/worker.rb b/actioncable/lib/action_cable/server/worker.rb
index e063b2a2e1..6cddb4e7a5 100644
--- a/actioncable/lib/action_cable/server/worker.rb
+++ b/actioncable/lib/action_cable/server/worker.rb
@@ -1,39 +1,70 @@
-require 'celluloid'
 require 'active_support/callbacks'
+require 'concurrent'
 
 module ActionCable
   module Server
     # Worker used by Server.send_async to do connection work in threads. Only for internal use.
     class Worker
       include ActiveSupport::Callbacks
-      include Celluloid
 
-      attr_reader :connection
       define_callbacks :work
       include ActiveRecordConnectionManagement
 
+      def initialize(max_size=5)
+        @pool = Concurrent::ThreadPoolExecutor.new(
+          min_threads: 1,
+          max_threads: max_size,
+          max_queue: 0,
+        )
+      end
+
+      def connection
+        Thread.current[:connection] || raise("No connection set")
+      end
+
+      def async_invoke(receiver, method, *args)
+        @pool.post do
+          invoke(receiver, method, *args)
+        end
+      end
+
       def invoke(receiver, method, *args)
-        @connection = receiver
+        begin
+          Thread.current[:connection] = receiver
+
+          run_callbacks :work do
+            receiver.send method, *args
+          end
+        rescue Exception => e
+          logger.error "There was an exception - #{e.class}(#{e.message})"
+          logger.error e.backtrace.join("\n")
 
-        run_callbacks :work do
-          receiver.send method, *args
+          receiver.handle_exception if receiver.respond_to?(:handle_exception)
+        ensure
+          Thread.current[:connection] = nil
         end
-      rescue Exception => e
-        logger.error "There was an exception - #{e.class}(#{e.message})"
-        logger.error e.backtrace.join("\n")
+      end
 
-        receiver.handle_exception if receiver.respond_to?(:handle_exception)
+      def async_run_periodic_timer(channel, callback)
+        @pool.post do
+          run_periodic_timer(channel, callback)
+        end
       end
 
       def run_periodic_timer(channel, callback)
-        @connection = channel.connection
+        begin
+          Thread.current[:connection] = channel.connection
 
-        run_callbacks :work do
-          callback.respond_to?(:call) ? channel.instance_exec(&callback) : channel.send(callback)
+          run_callbacks :work do
+            callback.respond_to?(:call) ? channel.instance_exec(&callback) : channel.send(callback)
+          end
+        ensure
+          Thread.current[:connection] = nil
         end
       end
 
       private
+
         def logger
           ActionCable.server.logger
         end
diff --git a/actioncable/test/test_helper.rb b/actioncable/test/test_helper.rb
index 12dcd98402..325305939f 100644
--- a/actioncable/test/test_helper.rb
+++ b/actioncable/test/test_helper.rb
@@ -14,11 +14,6 @@ require 'rack/mock'
 # Require all the stubs and models
 Dir[File.dirname(__FILE__) + '/stubs/*.rb'].each {|file| require file }
 
-$CELLULOID_DEBUG = false
-$CELLULOID_TEST  = false
-require 'celluloid'
-Celluloid.logger = Logger.new(StringIO.new)
-
 require 'faye/websocket'
 class << Faye::WebSocket
   remove_method :ensure_reactor_running
diff --git a/actioncable/test/worker_test.rb b/actioncable/test/worker_test.rb
index 69c4b6529d..9911a3b98b 100644
--- a/actioncable/test/worker_test.rb
+++ b/actioncable/test/worker_test.rb
@@ -17,8 +17,6 @@ class WorkerTest < ActiveSupport::TestCase
   end
 
   setup do
-    Celluloid.boot
-
     @worker = ActionCable::Server::Worker.new
     @receiver = Receiver.new
   end