Switch the default redis adapter to a single-stream model

This new adapter does get a little more intimate with the redis-rb gem's
implementation than I would like, but it's the least bad of the
approaches I've come up with.

Gemfile

@@ -65,6 +65,7 @@ group :cable do
   gem 'puma', require: false
 
   gem 'em-hiredis', require: false
+  gem 'hiredis', require: false
   gem 'redis', require: false
 
   gem 'faye-websocket', require: false

Gemfile.lock

@@ -286,6 +286,7 @@ DEPENDENCIES
   delayed_job_active_record
   em-hiredis
   faye-websocket
+  hiredis
   jquery-rails
   json
   kindlerb (= 0.1.1)

actioncable/lib/action_cable/subscription_adapter/evented_redis.rb

@@ -0,0 +1,67 @@
+require 'thread'
+
+gem 'em-hiredis', '~> 0.3.0'
+gem 'redis', '~> 3.0'
+require 'em-hiredis'
+require 'redis'
+
+EventMachine.epoll  if EventMachine.epoll?
+EventMachine.kqueue if EventMachine.kqueue?
+
+module ActionCable
+  module SubscriptionAdapter
+    class EventedRedis < Base # :nodoc:
+      @@mutex = Mutex.new
+
+      def initialize(*)
+        super
+        @redis_connection_for_broadcasts = @redis_connection_for_subscriptions = nil
+      end
+
+      def broadcast(channel, payload)
+        redis_connection_for_broadcasts.publish(channel, payload)
+      end
+
+      def subscribe(channel, message_callback, success_callback = nil)
+        redis_connection_for_subscriptions.pubsub.subscribe(channel, &message_callback).tap do |result|
+          result.callback { |reply| success_callback.call } if success_callback
+        end
+      end
+
+      def unsubscribe(channel, message_callback)
+        redis_connection_for_subscriptions.pubsub.unsubscribe_proc(channel, message_callback)
+      end
+
+      def shutdown
+        redis_connection_for_subscriptions.pubsub.close_connection
+        @redis_connection_for_subscriptions = nil
+      end
+
+      private
+        def redis_connection_for_subscriptions
+          ensure_reactor_running
+          @redis_connection_for_subscriptions || @server.mutex.synchronize do
+            @redis_connection_for_subscriptions ||= EM::Hiredis.connect(@server.config.cable[:url]).tap do |redis|
+              redis.on(:reconnect_failed) do
+                @logger.info "[ActionCable] Redis reconnect failed."
+              end
+            end
+          end
+        end
+
+        def redis_connection_for_broadcasts
+          @redis_connection_for_broadcasts || @server.mutex.synchronize do
+            @redis_connection_for_broadcasts ||= ::Redis.new(@server.config.cable)
+          end
+        end
+
+        def ensure_reactor_running
+          return if EventMachine.reactor_running?
+          @@mutex.synchronize do
+            Thread.new { EventMachine.run } unless EventMachine.reactor_running?
+            Thread.pass until EventMachine.reactor_running?
+          end
+        end
+    end
+  end
+end

actioncable/lib/action_cable/subscription_adapter/redis.rb

@@ -1,52 +1,40 @@
 require 'thread'
 
-gem 'em-hiredis', '~> 0.3.0'
 gem 'redis', '~> 3.0'
-require 'em-hiredis'
 require 'redis'
 
-EventMachine.epoll  if EventMachine.epoll?
-EventMachine.kqueue if EventMachine.kqueue?
-
 module ActionCable
   module SubscriptionAdapter
     class Redis < Base # :nodoc:
-      @@mutex = Mutex.new
-
       def initialize(*)
         super
-        @redis_connection_for_broadcasts = @redis_connection_for_subscriptions = nil
+        @listener = nil
+        @redis_connection_for_broadcasts = nil
       end
 
       def broadcast(channel, payload)
         redis_connection_for_broadcasts.publish(channel, payload)
       end
 
-      def subscribe(channel, message_callback, success_callback = nil)
-        redis_connection_for_subscriptions.pubsub.subscribe(channel, &message_callback).tap do |result|
-          result.callback { |reply| success_callback.call } if success_callback
-        end
+      def subscribe(channel, callback, success_callback = nil)
+        listener.add_subscriber(channel, callback, success_callback)
       end
 
-      def unsubscribe(channel, message_callback)
-        redis_connection_for_subscriptions.pubsub.unsubscribe_proc(channel, message_callback)
+      def unsubscribe(channel, callback)
+        listener.remove_subscriber(channel, callback)
       end
 
       def shutdown
-        redis_connection_for_subscriptions.pubsub.close_connection
-        @redis_connection_for_subscriptions = nil
+        @listener.shutdown if @listener
+      end
+
+      def redis_connection_for_subscriptions
+        ::Redis.new(@server.config.cable)
       end
 
       private
-        def redis_connection_for_subscriptions
-          ensure_reactor_running
-          @redis_connection_for_subscriptions || @server.mutex.synchronize do
-            @redis_connection_for_subscriptions ||= EM::Hiredis.connect(@server.config.cable[:url]).tap do |redis|
-              redis.on(:reconnect_failed) do
-                @logger.info "[ActionCable] Redis reconnect failed."
-              end
-            end
-          end
+        def listener
+          @listener || @server.mutex.synchronize { @listener ||= Listener.new(self) }
         end
 
         def redis_connection_for_broadcasts
@@ -55,12 +43,120 @@ module ActionCable
           end
         end
 
-        def ensure_reactor_running
-          return if EventMachine.reactor_running?
-          @@mutex.synchronize do
-            Thread.new { EventMachine.run } unless EventMachine.reactor_running?
-            Thread.pass until EventMachine.reactor_running?
+        class Listener < SubscriberMap
+          def initialize(adapter)
+            super()
+
+            @adapter = adapter
+
+            @subscribe_callbacks = Hash.new { |h, k| h[k] = [] }
+            @subscription_lock = Mutex.new
+
+            @raw_client = nil
+
+            @when_connected = []
+
+            @thread = nil
           end
+
+          def listen(conn)
+            conn.without_reconnect do
+              original_client = conn.client
+
+              conn.subscribe('_action_cable_internal') do |on|
+                on.subscribe do |chan, count|
+                  @subscription_lock.synchronize do
+                    if count == 1
+                      @raw_client = original_client
+
+                      until @when_connected.empty?
+                        @when_connected.shift.call
+                      end
+                    end
+
+                    if callbacks = @subscribe_callbacks[chan]
+                      next_callback = callbacks.shift
+                      Concurrent.global_io_executor << next_callback if next_callback
+                      @subscribe_callbacks.delete(chan) if callbacks.empty?
+                    end
+                  end
+                end
+
+                on.message do |chan, message|
+                  broadcast(chan, message)
+                end
+
+                on.unsubscribe do |chan, count|
+                  if count == 0
+                    @subscription_lock.synchronize do
+                      @raw_client = nil
+                    end
+                  end
+                end
+              end
+            end
+          end
+
+          def shutdown
+            @subscription_lock.synchronize do
+              return if @thread.nil?
+
+              when_connected do
+                send_command('unsubscribe')
+                @raw_client = nil
+              end
+            end
+
+            Thread.pass while @thread.alive?
+          end
+
+          def add_channel(channel, on_success)
+            @subscription_lock.synchronize do
+              ensure_listener_running
+              @subscribe_callbacks[channel] << on_success
+              when_connected { send_command('subscribe', channel) }
+            end
+          end
+
+          def remove_channel(channel)
+            @subscription_lock.synchronize do
+              when_connected { send_command('unsubscribe', channel) }
+            end
+          end
+
+          def invoke_callback(*)
+            Concurrent.global_io_executor.post { super }
+          end
+
+          private
+            def ensure_listener_running
+              @thread ||= Thread.new do
+                Thread.current.abort_on_exception = true
+
+                conn = @adapter.redis_connection_for_subscriptions
+                listen conn
+              end
+            end
+
+            def when_connected(&block)
+              if @raw_client
+                block.call
+              else
+                @when_connected << block
+              end
+            end
+
+            def send_command(*command)
+              @raw_client.write(command)
+
+              very_raw_connection =
+                @raw_client.connection.instance_variable_defined?(:@connection) &&
+                @raw_client.connection.instance_variable_get(:@connection)
+
+              if very_raw_connection && very_raw_connection.respond_to?(:flush)
+                very_raw_connection.flush
+              end
+            end
         end
     end
   end

actioncable/test/subscription_adapter/evented_redis_test.rb

@@ -0,0 +1,10 @@
+require 'test_helper'
+require_relative './common'
+
+class EventedRedisAdapterTest < ActionCable::TestCase
+  include CommonSubscriptionAdapterTest
+
+  def cable_config
+    { adapter: 'evented_redis', url: 'redis://127.0.0.1:6379/12' }
+  end
+end

actioncable/test/subscription_adapter/redis_test.rb

@@ -5,6 +5,12 @@ class RedisAdapterTest < ActionCable::TestCase
   include CommonSubscriptionAdapterTest
 
   def cable_config
-    { adapter: 'redis', url: 'redis://127.0.0.1:6379/12' }
+    { adapter: 'redis', driver: 'ruby', url: 'redis://127.0.0.1:6379/12' }
+  end
+end
+
+class RedisAdapterTest::Hiredis < RedisAdapterTest
+  def cable_config
+    super.merge(driver: 'hiredis')
   end
 end

railties/lib/rails/generators/app_base.rb

@@ -344,7 +344,6 @@ module Rails
         return [] if options[:skip_action_cable]
         comment = 'Action Cable dependencies for the Redis adapter'
         gems = []
-        gems << GemfileEntry.new("em-hiredis", '~> 0.3.0', comment)
         gems << GemfileEntry.new("redis", '~> 3.0', comment)
         gems
       end

railties/test/generators/app_generator_test.rb

@@ -400,7 +400,6 @@ class AppGeneratorTest < Rails::Generators::TestCase
       assert_no_match(/action_cable_meta_tag/, content)
     end
     assert_file "Gemfile" do |content|
-      assert_no_match(/em-hiredis/, content)
       assert_no_match(/redis/, content)
     end
   end
@@ -412,14 +411,12 @@ class AppGeneratorTest < Rails::Generators::TestCase
     assert_no_file "app/assets/javascripts/cable.coffee"
     assert_no_file "app/channels"
     assert_file "Gemfile" do |content|
-      assert_no_match(/em-hiredis/, content)
       assert_no_match(/redis/, content)
     end
   end
 
   def test_action_cable_redis_gems
     run_generator
-    assert_gem 'em-hiredis'
     assert_gem 'redis'
   end