Add specs for override

lib/gitlab/patch/action_cable_subscription_adapter_identifier.rb

@@ -8,7 +8,7 @@ module Gitlab
   module Patch
     module ActionCableSubscriptionAdapterIdentifier
       def identifier
-        @server.config.cable.has_key?(:id) ? @server.config.cable[:id] : "ActionCable-PID-#{$$}" # rubocop:disable Gitlab/ModuleWithInstanceVariables
+        @server.config.cable.has_key?(:id) ? @server.config.cable[:id] : super # rubocop:disable Gitlab/ModuleWithInstanceVariables
       end
     end
   end

spec/initializers/action_cable_subscription_adapter_identifier_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe 'ActionCableSubscriptionAdapterIdentifier override' do
+  describe '#identifier' do
+    context 'when id key is nil on cable.yml' do
+      it 'does not override server config id with action cable pid' do
+        config = {
+          adapter: 'redis',
+          url: 'unix:/home/localuser/redis/redis.socket',
+          channel_prefix: 'test_',
+          id: nil
+        }
+        ::ActionCable::Server::Base.config.cable = config
+
+        sub = ActionCable.server.pubsub.send(:redis_connection)
+
+        expect(sub.connection[:id]).to eq('redis:///home/localuser/redis/redis.socket/0')
+        expect(ActionCable.server.config.cable[:id]).to be_nil
+      end
+    end
+  end
+end