Add Flipflop gem for feature-toggling

Add the Flipflop gem to enable feature-toggling within the publisher app.

The dashboard (available at `/flipflop`) provided is not secured in any
 way (but since we will be using the "Cookie" strategy, users will only
 be able to toggle features on/off for themselves). This could be added
 later, if desired.

Also provides a `FeatureConstraint` class to enable dynamic routing to
 be set up based on the value of the feature toggles.

Gemfile

@@ -7,6 +7,7 @@ gem "bootsnap", require: false
 gem "bootstrap-kaminari-views"
 gem "diffy"
 gem "erubis"
+gem "flipflop"
 gem "gds-api-adapters"
 gem "gds-sso"
 gem "govspeak"

Gemfile.lock

@@ -153,6 +153,9 @@ GEM
       ruby2_keywords (>= 0.0.4)
     faraday-net_http (3.0.2)
     ffi (1.15.5)
+    flipflop (2.7.1)
+      activesupport (>= 4.0)
+      terminal-table (>= 1.8)
     gds-api-adapters (91.1.0)
       addressable
       link_header
@@ -740,6 +743,8 @@ GEM
     statsd-ruby (1.5.0)
     strip_attributes (1.13.0)
       activemodel (>= 3.0, < 8.0)
+    terminal-table (3.0.2)
+      unicode-display_width (>= 1.1.1, < 3)
     terser (1.1.20)
       execjs (>= 0.3.0, < 3)
     thor (1.3.0)
@@ -789,6 +794,7 @@ DEPENDENCIES
   diffy
   erubis
   factory_bot_rails
+  flipflop
   gds-api-adapters
   gds-sso
   govspeak

app/constraints/feature_constraint.rb

@@ -0,0 +1,13 @@
+class FeatureConstraint
+  def initialize(feature_name)
+    @feature_name = feature_name
+  end
+
+  def matches?(request)
+    if request.cookies.key?(@feature_name)
+      request.cookies[@feature_name] == "1"
+    else
+      Flipflop.enabled?(@feature_name.to_sym)
+    end
+  end
+end

config/application.rb

@@ -18,6 +18,14 @@
 
 module Publisher
   class Application < Rails::Application
+    # Before filter for Flipflop dashboard. Replace with a lambda or method name
+    # defined in ApplicationController to implement access control.
+    config.flipflop.dashboard_access_filter = nil
+
+    # By default, when set to `nil`, strategy loading errors are suppressed in test
+    # mode. Set to `true` to always raise errors, or `false` to always warn.
+    config.flipflop.raise_strategy_errors = nil
+
     # Initialize configuration defaults for originally generated Rails version.
     config.load_defaults 7.0
 

config/environments/development.rb

@@ -1,6 +1,14 @@
 require "active_support/core_ext/integer/time"
 
 Rails.application.configure do
+  # Before filter for Flipflop dashboard. Replace with a lambda or method name
+  # defined in ApplicationController to implement access control.
+  config.flipflop.dashboard_access_filter = nil
+
+  # By default, when set to `nil`, strategy loading errors are suppressed in test
+  # mode. Set to `true` to always raise errors, or `false` to always warn.
+  config.flipflop.raise_strategy_errors = nil
+
   # Settings specified here will take precedence over those in config/application.rb.
 
   # In the development environment your application's code is reloaded any time

config/environments/test.rb

@@ -6,6 +6,14 @@
 # and recreated between test runs. Don't rely on the data there!
 
 Rails.application.configure do
+  # Before filter for Flipflop dashboard. Replace with a lambda or method name
+  # defined in ApplicationController to implement access control.
+  config.flipflop.dashboard_access_filter = nil
+
+  # By default, when set to `nil`, strategy loading errors are suppressed in test
+  # mode. Set to `true` to always raise errors, or `false` to always warn.
+  config.flipflop.raise_strategy_errors = nil
+
   # Settings specified here will take precedence over those in config/application.rb.
 
   # Turn false under Spring and add config.action_view.cache_template_loading = true.

config/features.rb

@@ -0,0 +1,11 @@
+Flipflop.configure do
+  # Strategies will be used in the order listed here.
+  strategy :cookie
+  strategy :default
+
+  if Rails.env.test?
+    feature :feature_for_tests,
+            default: true,
+            description: "A feature only used by tests; not to be used for any actual features."
+  end
+end

config/routes.rb

@@ -72,4 +72,5 @@
   get "/govuk-sitemap.xml" => "sitemap#index"
 
   mount GovukAdminTemplate::Engine, at: "/style-guide"
+  mount Flipflop::Engine => "/flipflop"
 end

test/unit/constraints/feature_constraint_test.rb

@@ -0,0 +1,65 @@
+require "test_helper"
+
+class FeatureConstraintTest < ActiveSupport::TestCase
+  context "Feature 'feature_for_tests' is enabled by default" do
+    setup do
+      test_strategy = Flipflop::FeatureSet.current.test!
+      test_strategy.switch!(:feature_for_tests, true)
+    end
+
+    should "match when a request cookie explicitly enables feature" do
+      request = stub(cookies: { "feature_for_tests" => "1" })
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal true, feature_constraint.matches?(request)
+    end
+
+    should "not match when a request cookie explicitly disables feature" do
+      request = stub(cookies: { "feature_for_tests" => "0" })
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal false, feature_constraint.matches?(request)
+    end
+
+    should "match when a request cookie does not override default feature status" do
+      request = stub(cookies: {})
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal true, feature_constraint.matches?(request)
+    end
+  end
+
+  context "Feature 'feature_for_tests' is disabled by default" do
+    setup do
+      test_strategy = Flipflop::FeatureSet.current.test!
+      test_strategy.switch!(:feature_for_tests, false)
+    end
+
+    should "match when a request cookie explicitly enables feature" do
+      request = stub(cookies: { "feature_for_tests" => "1" })
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal true, feature_constraint.matches?(request)
+    end
+
+    should "not match when a request cookie explicitly disables feature" do
+      request = stub(cookies: { "feature_for_tests" => "0" })
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal false, feature_constraint.matches?(request)
+    end
+
+    should "not match when a request cookie does not override default feature status" do
+      request = stub(cookies: {})
+
+      feature_constraint = FeatureConstraint.new("feature_for_tests")
+
+      assert_equal false, feature_constraint.matches?(request)
+    end
+  end
+end