Allow passing custom config to ActionCable::Server::Base

That allows us to create a separate, isolated Action Cable server
instance within the same app.

actioncable/CHANGELOG.md

@@ -1,3 +1,26 @@
+*   Allow passing custom configuration to `ActionCable::Server::Base`.
+
+    You can now create a standalone Action Cable server with a custom configuration
+    (e.g. to run it in isolation from the default one):
+
+    ```ruby
+    config = ActionCable::Server::Configuration.new
+    config.cable = { adapter: "redis", channel_prefix: "custom_" }
+
+    CUSTOM_CABLE = ActionCable::Server::Base.new(config: config)
+    ```
+
+    Then you can mount it in the `routes.rb` file:
+
+    ```ruby
+    Rails.application.routes.draw do
+      mount CUSTOM_CABLE => "/custom_cable"
+      # ...
+    end
+    ```
+
+    *Vladimir Dementyev*
+
 *   Add `:action_cable_connection` and `:action_cable_channel` load hooks.
 
     You can use them to extend `ActionCable::Connection::Base` and `ActionCable::Channel::Base`

actioncable/lib/action_cable/server/base.rb

@@ -12,14 +12,17 @@ class Base
       include ActionCable::Server::Broadcasting
       include ActionCable::Server::Connections
 
-      cattr_accessor :config, instance_accessor: true, default: ActionCable::Server::Configuration.new
+      cattr_accessor :config, instance_accessor: false, default: ActionCable::Server::Configuration.new
+
+      attr_reader :config
 
       def self.logger; config.logger; end
       delegate :logger, to: :config
 
       attr_reader :mutex
 
-      def initialize
+      def initialize(config: self.class.config)
+        @config = config
         @mutex = Monitor.new
         @remote_connections = @event_loop = @worker_pool = @pubsub = nil
       end

actioncable/test/subscription_adapter/channel_prefix.rb

@@ -2,17 +2,9 @@
 
 require "test_helper"
 
-class ActionCable::Server::WithIndependentConfig < ActionCable::Server::Base
-  # ActionCable::Server::Base defines config as a class variable.
-  # Need config to be an instance variable here as we're testing 2 separate configs
-  def config
-    @config ||= ActionCable::Server::Configuration.new
-  end
-end
-
 module ChannelPrefixTest
   def test_channel_prefix
-    server2 = ActionCable::Server::WithIndependentConfig.new
+    server2 = ActionCable::Server::Base.new(config: ActionCable::Server::Configuration.new)
     server2.config.cable = alt_cable_config
     server2.config.logger = Logger.new(StringIO.new).tap { |l| l.level = Logger::UNKNOWN }