Support Action Cable on GCP Memorystore

This patches Action Cable so that an id that is explicitly set to nil
in cable.yml is not overwritten.

A nil id is needed so that the Redis client does not call CLIENT SETNAME
which is not supported in GCP Memorystore

Changelog: fixed

config/initializers/action_cable.rb

@@ -9,6 +9,8 @@ Rails.application.configure do
   config.action_cable.worker_pool_size = Gitlab::ActionCable::Config.worker_pool_size
 end
 
+ActionCable::SubscriptionAdapter::Base.prepend(Gitlab::Patch::ActionCableSubscriptionAdapterIdentifier)
+
 # https://github.com/rails/rails/blob/bb5ac1623e8de08c1b7b62b1368758f0d3bb6379/actioncable/lib/action_cable/subscription_adapter/redis.rb#L18
 ActionCable::SubscriptionAdapter::Redis.redis_connector = lambda do |config|
   args = config.except(:adapter, :channel_prefix)

lib/gitlab/patch/action_cable_subscription_adapter_identifier.rb

+# frozen_string_literal: true
+
+# Modifies https://github.com/rails/rails/blob/v6.1.4.1/actioncable/lib/action_cable/subscription_adapter/base.rb so
+# that we do not overwrite an id that was explicitly set to `nil` in cable.yml.
+# This is needed to support GCP Memorystore. See https://github.com/rails/rails/issues/38244.
+
+module Gitlab
+  module Patch
+    module ActionCableSubscriptionAdapterIdentifier
+      def identifier
+        @server.config.cable.has_key?(:id) ? @server.config.cable[:id] : "ActionCable-PID-#{$$}" # rubocop:disable Gitlab/ModuleWithInstanceVariables
+      end
+    end
+  end
+end