Try not to hold env and release the controller

after the request. This way, we could release the
project referred from the controller, which potentially
referred a repository which potentially allocated a lot of
memories.

Before this change, we could hold the last request data
and cannot release the memory. After this change, the
largest request data should be able to be collected from GC.

This might not impact the instances having heavy load,
as the last request should be changing all the time,
and GC won't kick in for each request anyway.

However it could still potentially allow us to free more
memories for each GC runs, because now we could free one
more request anyway.

config.ru

@@ -23,5 +23,6 @@ warmup do |app|
 end
 
 map ENV['RAILS_RELATIVE_URL_ROOT'] || "/" do
+  use Gitlab::ReleaseController
   run Gitlab::Application
 end

lib/gitlab/middleware/read_only.rb

@@ -28,7 +28,7 @@ module Gitlab
           end
         end
 
-        @app.call(env)
+        @app.call(env).tap { @env = nil }
       end
 
       private

lib/gitlab/middleware/release_controller.rb

@@ -0,0 +1,9 @@
+module Gitlab
+  module Middleware
+    ReleaseController = Struct.new(:app) do
+      def call(env)
+        app.call(env).tap { env.delete('action_controller.instance') }
+      end
+    end
+  end
+end

spec/lib/gitlab/middleware/release_controller_spec.rb

@@ -0,0 +1,20 @@
+require 'spec_helper'
+
+describe Gitlab::Middleware::ReleaseController do
+  let(:inner_app) { double(:app) }
+  let(:app) { described_class.new(inner_app) }
+  let(:env) { { 'action_controller.instance' => 'something' } }
+
+  before do
+    expect(inner_app).to receive(:call).with(env).and_return('yay')
+  end
+
+  describe '#call' do
+    it 'calls the app and delete the controller' do
+      result = app.call(env)
+
+      expect(result).to eq('yay')
+      expect(env).to be_empty
+    end
+  end
+end