Add GovukProxy::StaticProxy for development

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 4.9.0
+
+- Add GovukProxy::StaticProxy to forward Static asset requests by setting `GOVUK_PROXY_STATIC_ENABLED=true`.([#261](https://github.com/alphagov/govuk_app_config/pull/261))
+
 # 4.8.0
 
 - Enables Sentry environment names for EKS versions of integration, staging and production.([#260](https://github.com/alphagov/govuk_app_config/pull/260))

govuk_app_config.gemspec

@@ -21,8 +21,10 @@ Gem::Specification.new do |spec|
   spec.require_paths = %w[lib]
 
   spec.add_dependency "logstasher", "~> 2.1"
+  spec.add_dependency "plek", "~> 4"
   spec.add_dependency "prometheus_exporter", "~> 2.0"
   spec.add_dependency "puma", "~> 5.6"
+  spec.add_dependency "rack-proxy", "~> 0.7"
   spec.add_dependency "sentry-rails", "~> 5.3"
   spec.add_dependency "sentry-ruby", "~> 5.3"
   spec.add_dependency "statsd-ruby", "~> 1.5"

lib/govuk_app_config.rb

@@ -1,6 +1,7 @@
 require "govuk_app_config/version"
 require "govuk_app_config/govuk_statsd"
 require "govuk_app_config/govuk_error"
+require "govuk_app_config/govuk_proxy/static_proxy"
 require "govuk_app_config/govuk_healthcheck"
 require "govuk_app_config/govuk_i18n"
 # This require is deprecated and should be removed on next major version bump

lib/govuk_app_config/govuk_proxy/static_proxy.rb

@@ -0,0 +1,19 @@
+require "rack-proxy"
+
+module GovukProxy
+  class StaticProxy < Rack::Proxy
+    def perform_request(env)
+      request = Rack::Request.new(env)
+
+      # use rack proxy to forward any requests for /assets/static/*
+      # this regex needs to match the path set for `Rails.application.config.assets.prefix` in Static
+      # https://github.com/alphagov/static/blob/main/config/initializers/assets.rb
+      if request.path =~ %r{^/assets/static/}
+        env["HTTP_HOST"] = @backend.host
+        super(env)
+      else
+        @app.call(env)
+      end
+    end
+  end
+end

lib/govuk_app_config/railtie.rb

@@ -1,5 +1,14 @@
+require "plek"
+
 module GovukAppConfig
   class Railtie < Rails::Railtie
+    initializer "govuk_app_config.configure_govuk_proxy" do |app|
+      if ENV["GOVUK_PROXY_STATIC_ENABLED"] == "true"
+        static_url = Plek.new.find("static")
+        app.middleware.use GovukProxy::StaticProxy, backend: static_url
+      end
+    end
+
     config.before_initialize do
       GovukLogging.configure if Rails.env.production?
     end

lib/govuk_app_config/version.rb

@@ -1,3 +1,3 @@
 module GovukAppConfig
-  VERSION = "4.8.0".freeze
+  VERSION = "4.9.0".freeze
 end

spec/lib/govuk_proxy/static_proxy_spec.rb

@@ -0,0 +1,40 @@
+require "spec_helper"
+require "govuk_app_config/govuk_proxy/static_proxy"
+require "rack/mock"
+
+RSpec.describe GovukProxy::StaticProxy do
+  def test_request(path, proxied)
+    dest_host = (proxied ? static_domain : app_domain)
+
+    stub_request(:get, "https://#{dest_host}#{path}")
+        .with(headers: { "Host" => dest_host })
+        .to_return(status: 200, body: "", headers: {})
+
+    env = Rack::MockRequest.env_for("http://#{app_domain}#{path}")
+    status, _headers, _response = proxy.call(env)
+    expect(status.to_i).to eq(200)
+  end
+
+  # dummy app to validate success
+  let(:app) { ->(_env) { [200, {}, "success"] } }
+  let(:app_domain) { "app.domain" }
+  let(:static_domain) { "static.domain" }
+
+  let(:proxy) { GovukProxy::StaticProxy.new(app, backend: "https://static.domain", streaming: false) }
+
+  it "redirects the request if path begins with /asset/static" do
+    test_request("/assets/static/a.css", true)
+  end
+
+  it "ignores requests not with path prefix /asset/static" do
+    test_request("/assets/app/a.css", false)
+  end
+
+  it "ignores requests where /asset/static isn't a prefix" do
+    test_request("/another/prefix/assets/static/a.css", false)
+  end
+
+  it "ignores requests with no path" do
+    test_request("/", false)
+  end
+end