Remove HTTP feature flags

The rendering of recommended related links was conditional on a feature flag,
which is set via a header within our Fastly VCL.

As an outcome of RFC-163, we are trying to simplify our CDN configuration by
removing functionality that’s no longer used, or by moving logic to other parts
of the stack.

This change was suggested in a comment on RFC-163:

  "I think the flag for Whitehall recommended links should be removed. It was put
  in as a safety mechanism for the introduction of the links, in case something
  really inappropriate was found. There are ways to manually override the links
  now, and we've never used the feature flag."

Related content is component is defined in govuk_publishing_components gem and
this is where the overriding is happening (see:
https://github.com/alphagov/govuk_publishing_components/blob/5d713ff637de14bec6abc784c35ee63a0cbc2c75/lib/govuk_publishing_components/presenters/content_item.rb#L70-L74)

It's now recommended to create feature flags as environment variables in helm
charts. See the docs:
https://github.com/alphagov/govuk-developer-docs/blob/33f1f0434aa988419df0687a829367e3f769816f/source/kubernetes/manage-app/set-env-var/index.html.md?plain=1#L7-L21

The value can then be read in Rails config:

```
config.unreleased_features = ENV["UNRELEASED_FEATURES"].present?
```

app/controllers/content_items_controller.rb

@@ -31,7 +31,6 @@ def show
     elsif is_history_page?
       show_history_page
     else
-      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
@@ -139,9 +138,8 @@ def set_guide_draft_access_token
 
   def load_content_item
     content_item = Services.content_store.content_item(content_item_path)
-    if Services.feature_toggler.use_recommended_related_links?(content_item["links"], request.headers)
-      content_item["links"]["ordered_related_items"] = content_item["links"].fetch("suggested_ordered_related_items", [])
-    end
+
+    content_item["links"]["ordered_related_items"] = ordered_related_items(content_item["links"]) if content_item["links"]
 
     @content_item = PresenterBuilder.new(
       content_item,
@@ -150,6 +148,14 @@ def load_content_item
     ).presenter
   end
 
+  def ordered_related_items(links)
+    return [] if links["ordered_related_items_overrides"].present?
+
+    links["ordered_related_items"].presence || links.fetch(
+      "suggested_ordered_related_items", []
+    )
+  end
+
   def format_banner_links(links, type)
     links.each.with_index(1).map do |(title, base_path), index|
       view_context.link_to(
@@ -205,14 +211,6 @@ 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(", ")
-
-    related_links_request_header = RequestHelper.get_header(FeatureFlagNames.recommended_related_links, request.headers)
-    required_header_value = Services.feature_toggler.feature_flags.get_feature_flag(FeatureFlagNames.recommended_related_links)
-    response.headers[FeatureFlagNames.recommended_related_links] = (related_links_request_header == required_header_value).to_s
-  end
-
   def set_expiry
     expires_in(
       @content_item.cache_control_max_age(request.format),

app/lib/services.rb

@@ -12,12 +12,6 @@ def self.content_store
     )
   end
 
-  def self.feature_toggler
-    @feature_toggler ||= FeatureToggler.new(
-      HttpFeatureFlags.instance,
-    )
-  end
-
   def self.publishing_api
     @publishing_api ||= GdsApi::PublishingApi.new(
       Plek.find("publishing-api"),

test/controllers/content_items_controller_test.rb

@@ -132,19 +132,7 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_equal content_item["title"], assigns[:content_item].title
   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) }
-    assert_response :success
-    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 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["HTTP_GOVUK_USE_RECOMMENDED_RELATED_LINKS"] = "true"
-
+  test "gets item from content store and keeps existing ordered_related_items when links already exist" do
     content_item = content_store_has_schema_example("guide", "guide")
 
     get :show, params: { path: path_for(content_item) }
@@ -154,10 +142,7 @@ 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 does not change ordered_related_items when feature flag header is specified but link overrides exist" do
-    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, "true")
-    request.headers["HTTP_GOVUK_USE_RECOMMENDED_RELATED_LINKS"] = "true"
-
+  test "gets item from content store and does not change ordered_related_items when link overrides exist" do
     content_item = content_store_has_schema_example("guide", "guide-with-related-link-overrides")
 
     get :show, params: { path: path_for(content_item) }
@@ -168,23 +153,7 @@ class ContentItemsControllerTest < ActionController::TestCase
     assert_nil 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["HTTP_GOVUK_USE_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"
-    assert_not_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 or overrides" do
-    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, "true")
-    request.headers["HTTP_GOVUK_USE_RECOMMENDED_RELATED_LINKS"] = "true"
-
+  test "gets item from content store and replaces ordered_related_items there are no existing links or overrides" do
     content_item = content_store_has_schema_example("case_study", "case_study")
 
     get :show, params: { path: path_for(content_item) }
@@ -194,38 +163,6 @@ 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 response header to false when request header is not set" 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 "false", response.headers[FeatureFlagNames.recommended_related_links]
-  end
-
-  test "sets the Govuk-Use-Recommended-Links response header to false when request header is set to false" do
-    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, "true")
-    request.headers["HTTP_GOVUK_USE_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 Govuk-Use-Recommended-Links response header to true when request header is set to true" do
-    HttpFeatureFlags.instance.add_http_feature_flag(FeatureFlagNames.recommended_related_links, "true")
-    request.headers["HTTP_GOVUK_USE_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 expiry as sent by content-store" do
     content_item = content_store_has_schema_example("case_study", "case_study")
     stub_content_store_has_item(content_item["base_path"], content_item, max_age: 20)