Add Govuk-Use-Recommended-Links Vary response header and value header

This commit adds the `Govuk-Use-Recommended-Links` Vary response header and value header to responses generated by the `ContentItemsController`. This is to enable cache invalidation of previous responses if the incoming `Govuk-Use-Recommended-Links` request header changes state.

app/controllers/content_items_controller.rb

@@ -14,6 +14,7 @@ def show
     load_content_item
 
     set_expiry
+    set_use_recommended_related_links_header
     set_access_control_allow_origin_header if request.format.atom?
     set_guide_draft_access_token if @content_item.is_a?(GuidePresenter)
     render_template
@@ -112,6 +113,11 @@ def set_access_control_allow_origin_header
     response.headers["Access-Control-Allow-Origin"] = "*"
   end
 
+  def set_use_recommended_related_links_header
+    response.headers['Vary'] = [response.headers['Vary'], FeatureFlagNames.recommended_related_links].compact.join(', ')
+    response.headers[FeatureFlagNames.recommended_related_links] = Services.feature_toggler.feature_flags.get_feature_flag(FeatureFlagNames.recommended_related_links)
+  end
+
   def set_expiry
     expires_in(@content_item.cache_control_max_age(request.format),
                public: @content_item.cache_control_public?)

app/models/feature_toggler.rb

@@ -1,4 +1,6 @@
 class FeatureToggler
+  attr_reader :feature_flags
+
   def initialize(feature_flags)
     @feature_flags = feature_flags
   end

app/models/http_feature_flags.rb

@@ -7,6 +7,10 @@ def add_http_feature_flag(header_name, val)
     @feature_flags[header_name] = val
   end
 
+  def get_feature_flag(header_name)
+    @feature_flags[header_name]
+  end
+
   def feature_enabled?(header_name, request_headers)
     @feature_flags.has_key?(header_name) && @feature_flags[header_name] == request_headers[header_name]
   end

test/controllers/content_items_controller_test.rb

@@ -131,6 +131,7 @@ class ContentItemsControllerTest < ActionController::TestCase
   end
 
   test "gets item from content store and keeps existing ordered_related_items when feature flag header not specified" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'true')
     content_item = content_store_has_schema_example('case_study', 'case_study')
 
     get :show, params: { path: path_for(content_item) }
@@ -139,6 +140,7 @@ class ContentItemsControllerTest < ActionController::TestCase
   end
 
   test "gets item from content store and keep existing ordered_related_items when feature flag header is specified but links already exist" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'true')
     request.headers[FeatureFlagNames.recommended_related_links] = 'true'
 
     content_item = content_store_has_schema_example('guide', 'guide')
@@ -150,7 +152,21 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal content_item['links']['ordered_related_items'], assigns[:content_item].content_item['links']['ordered_related_items']
   end
 
+  test "gets item from content store and keeps ordered_related_items when feature flag header is specified but recommended links turned off" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'false')
+    request.headers[FeatureFlagNames.recommended_related_links] = 'true'
+
+    content_item = content_store_has_schema_example('case_study', 'case_study')
+
+    get :show, params: { path: path_for(content_item) }
+    assert_response :success
+    assert_empty content_item['links']['ordered_related_items'], 'Content item should have existing related links'
+    refute_empty content_item['links']['suggested_ordered_related_items'], 'Content item should have existing suggested related links'
+    assert_equal [], assigns[:content_item].content_item['links']['ordered_related_items']
+  end
+
   test "gets item from content store and replaces ordered_related_items when feature flag header is specified and there are no existing links" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'true')
     request.headers[FeatureFlagNames.recommended_related_links] = 'true'
 
     content_item = content_store_has_schema_example('case_study', 'case_study')
@@ -162,6 +178,26 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal assigns[:content_item].content_item['links']['ordered_related_items'], content_item['links']['suggested_ordered_related_items']
   end
 
+  test "sets the Govuk-Use-Recommended-Links header to true when using recommended links" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'true')
+    content_item = content_store_has_schema_example('case_study', 'case_study')
+
+    get :show, params: { path: path_for(content_item) }
+
+    assert_includes response.headers['Vary'], FeatureFlagNames.recommended_related_links
+    assert_equal 'true', response.headers[FeatureFlagNames.recommended_related_links]
+  end
+
+  test "sets the Govuk-Use-Recommended-Links header to false when not using recommended links" do
+    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, 'false')
+    content_item = content_store_has_schema_example('case_study', 'case_study')
+
+    get :show, params: { path: path_for(content_item) }
+
+    assert_includes response.headers['Vary'], FeatureFlagNames.recommended_related_links
+    assert_equal 'false', response.headers[FeatureFlagNames.recommended_related_links]
+  end
+
   test "sets the expiry as sent by content-store" do
     content_item = content_store_has_schema_example('coming_soon', 'coming_soon')
     content_store_has_item(content_item['base_path'], content_item, max_age: 20)

test/models/feature_toggler_test.rb

@@ -46,6 +46,13 @@ class FeatureTogglerTest < ActiveSupport::TestCase
     assert_equal(true, use_recommended_links)
   end
 
+  test 'feature_flags attr_reader delegates to instance of feature_flags' do
+    feature_flags = HttpFeatureFlags.new
+    feature_toggler = FeatureToggler.new(feature_flags)
+
+    assert_equal feature_flags, feature_toggler.feature_flags
+  end
+
   def setup
     @request_headers = { 'HTTP_GOVUK_USE_RECOMMENDED_RELATED_LINKS': 'true' }
   end

test/models/http_feature_flags_test.rb

@@ -52,4 +52,21 @@ class HttpFeatureFlagsTest < ActiveSupport::TestCase
     feature_enabled = instance.feature_enabled?('USE_MAGIC', 'USE_MAGIC' => 'only_at_weekends')
     assert_equal(true, feature_enabled)
   end
+
+  test 'get_feature_flag returns nil when feature flag does not exist' do
+    instance = HttpFeatureFlags.new
+
+    feature_flag_value = instance.get_feature_flag('USE_MAGIC')
+
+    assert_equal nil, feature_flag_value
+  end
+
+  test 'get_feature_flag returns feature flag value when feature flag exists' do
+    instance = HttpFeatureFlags.new
+
+    instance.add_http_feature_flag('USE_MAGIC', 'only_at_weekends')
+    feature_flag_value = instance.get_feature_flag('USE_MAGIC')
+
+    assert_equal 'only_at_weekends', feature_flag_value
+  end
 end