Add feature flag to toggle autocomplete

This adds a feature flag to disable autocomplete in case of unforeseen
problems by returning an empty set of completion results in the
response.

This is controlled by the env var `ENABLE_SEARCH_AUTOCOMPLETE`, and
will allow us to consolidate the logic behind disabling autocomplete in
a single place instead of having it scattered across several apps (as
the component will not render completions if no results are returned).

app/controllers/autocompletes_controller.rb

@@ -1,10 +1,16 @@
 class AutocompletesController < ApplicationController
   def show
-    render json: DiscoveryEngine::Autocomplete::Complete.new(query).completion_result
+    render json: completion_result
   end
 
 private
 
+  def completion_result
+    return CompletionResult.new(suggestions: []) unless Rails.configuration.enable_autocomplete
+
+    DiscoveryEngine::Autocomplete::Complete.new(query).completion_result
+  end
+
   def query
     params[:q]
   end

config/application.rb

@@ -40,5 +40,11 @@ class Application < Rails::Application
     ## currently only using a single instance (the Publishing "shared" Redis). If we ever need to
     ## use multiple Redis instances, this is the only place that needs updating.
     config.redlock_redis_instances = [config.redis_url]
+
+    # Feature flags
+    def self.feature_flag(name, default: false)
+      ActiveModel::Type::Boolean.new.cast(ENV.fetch(name, default))
+    end
+    config.enable_autocomplete = feature_flag("GOVUK_SEARCH_ENABLE_AUTOCOMPLETE", default: true)
   end
 end

spec/requests/autocomplete_request_spec.rb

@@ -17,4 +17,19 @@
       })
     end
   end
+
+  context "when autocomplete is disabled through the feature flag" do
+    before do
+      allow(Rails.configuration).to receive(:enable_autocomplete).and_return(false)
+    end
+
+    it "returns empty suggestions" do
+      get "/autocomplete.json?q=foo"
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)).to eq({
+        "suggestions" => [],
+      })
+    end
+  end
 end