Add URL parameter to force enable experiments

lib/gitlab/experimentation.rb

@@ -43,7 +43,7 @@ module Gitlab
       end
 
       def experiment_enabled?(experiment_key)
-        Experimentation.enabled_for_user?(experiment_key, experimentation_subject_index)
+        Experimentation.enabled_for_user?(experiment_key, experimentation_subject_index) || forced_enabled?(experiment_key)
       end
 
       def track_experiment_event(experiment_key, action)
@@ -94,6 +94,10 @@ module Gitlab
 
         experiment_enabled?(experiment_key) ? 'experimental_group' : 'control_group'
       end
+
+      def forced_enabled?(experiment_key)
+        params.has_key?(:force_experiment) && params[:force_experiment] == experiment_key.to_s
+      end
     end
 
     class << self

spec/lib/gitlab/experimentation_spec.rb

@@ -71,6 +71,24 @@ describe Gitlab::Experimentation do
           controller.experiment_enabled?(:test_experiment)
         end
       end
+
+      describe 'URL parameter to force enable experiment' do
+        context 'is not present' do
+          it 'returns false' do
+            get :index, params: { force_experiment: :test_experiment2 }
+
+            expect(controller.experiment_enabled?(:test_experiment)).to be_falsey
+          end
+        end
+
+        context 'is present' do
+          it 'returns true' do
+            get :index, params: { force_experiment: :test_experiment }
+
+            expect(controller.experiment_enabled?(:test_experiment)).to be_truthy
+          end
+        end
+      end
     end
 
     describe '#track_experiment_event' do