Feature flag for variables with potentially long values

app/models/ci/build.rb

@@ -568,7 +568,7 @@ module Ci
 
     def environment_changed_page_variables
       Gitlab::Ci::Variables::Collection.new.tap do |variables|
-        break variables unless environment_status
+        break variables unless environment_status && Feature.enabled?(:modifed_path_ci_variables, project)
 
         variables.append(key: 'CI_MERGE_REQUEST_CHANGED_PAGE_PATHS', value: environment_status.changed_paths.join(','))
         variables.append(key: 'CI_MERGE_REQUEST_CHANGED_PAGE_URLS', value: environment_status.changed_urls.join(','))

spec/models/ci/build_spec.rb

@@ -2495,6 +2495,17 @@ describe Ci::Build do
           end
         end
       end
+
+      context 'with the :modified_path_ci_variables feature flag disabled' do
+        before do
+          stub_feature_flags(modified_path_ci_variables: false)
+        end
+
+        it 'does not set CI_MERGE_REQUEST_CHANGED_PAGES_* variables' do
+          expect(subject.find { |var| var[:key] == 'CI_MERGE_REQUEST_CHANGED_PAGE_PATHS' }).to be_nil
+          expect(subject.find { |var| var[:key] == 'CI_MERGE_REQUEST_CHANGED_PAGE_URLS' }).to be_nil
+        end
+      end
     end
 
     context 'when build has user' do