1 files changed, 114 insertions, 0 deletions
diff --git a/rubocop/cop/experiments_test_coverage.rb b/rubocop/cop/experiments_test_coverage.rb
new file mode 100644
index 00000000000..4bb2832030c
--- /dev/null
+++ b/rubocop/cop/experiments_test_coverage.rb
@@ -0,0 +1,114 @@
+# frozen_string_literal: true
+
+module RuboCop
+  module Cop
+    # Check for test coverage for GitLab experiments.
+    class ExperimentsTestCoverage < RuboCop::Cop::Base
+      CLASS_OFFENSE = 'Make sure experiment class has test coverage for all the variants.'
+      BLOCK_OFFENSE = 'Make sure experiment block has test coverage for all the variants.'
+
+      # Validates classes inherited from ApplicationExperiment
+      # These classes are located under app/experiments or ee/app/experiments
+      def on_class(node)
+        return if node.parent_class&.const_name != 'ApplicationExperiment'
+        return if covered_with_tests?(node)
+
+        add_offense(node, message: CLASS_OFFENSE)
+      end
+
+      # Validates experiments block in *.rb and *.haml files:
+      # experiment(:experiment_name) do |e|
+      #   e.candidate { 'candidate' }
+      #   e.run
+      # end
+      def on_block(node)
+        return if node.method_name != :experiment
+        return if covered_with_tests?(node)
+
+        add_offense(node, message: BLOCK_OFFENSE)
+      end
+
+      private
+
+      def covered_with_tests?(node)
+        tests_code = test_files_code(node)
+
+        return false if tests_code.blank?
+        return false unless tests_code.match?(stub_experiments_matcher)
+        return false unless tests_code.include?(experiment_name(node))
+
+        experiment_variants(node).map { |variant| tests_code.include?(variant) }.all?(&:present?)
+      end
+
+      def test_files_code(node)
+        # haml-lint add .rb extension to *.haml files
+        # https://gitlab.com/gitlab-org/gitlab/-/issues/415330#caveats
+        test_file_path = filepath(node).gsub('app/', 'spec/').gsub('.rb', '_spec.rb')
+        "#{read_file(test_file_path)}\n#{additional_tests_code(test_file_path)}"
+      end
+
+      def additional_tests_code(test_file_path)
+        # rubocop:disable Gitlab/NoCodeCoverageComment
+        # :nocov: File paths stubed in tests
+        if test_file_path.include?('/controllers/')
+          read_file(test_file_path.gsub('/controllers/', '/requests/'))
+        elsif test_file_path.include?('/lib/api/')
+          read_file(test_file_path.gsub('/lib/', '/spec/requests/'))
+        end
+        # :nocov:
+        # rubocop:enable Gitlab/NoCodeCoverageComment
+      end
+
+      def read_file(file_path)
+        File.exist?(file_path) ? File.new(file_path).read : ''
+      end
+
+      def experiment_name(node)
+        if node.is_a?(RuboCop::AST::ClassNode)
+          File.basename(filepath(node), '_experiment.rb')
+        else
+          block_node_value(node)
+        end
+      end
+
+      def experiment_variants(node)
+        node.body.children.filter_map do |child|
+          next unless child.is_a?(RuboCop::AST::SendNode) || child.is_a?(RuboCop::AST::BlockNode)
+
+          extract_variant(child)
+        end
+      end
+
+      def extract_variant(node)
+        # control enabled by default for tests
+        case node.method_name
+        when :candidate then 'candidate'
+        when :variant then variant_name(node)
+        end
+      end
+
+      def variant_name(node)
+        return send_node_value(node) if node.is_a?(RuboCop::AST::SendNode)
+
+        block_node_value(node)
+      end
+
+      def block_node_value(node)
+        send_node_value(node.children[0])
+      end
+
+      def send_node_value(node)
+        node.children[2].value.to_s
+      end
+
+      def filepath(node)
+        node.location.expression.source_buffer.name
+      end
+
+      def stub_experiments_matcher
+        # validates test files contains uncommented stub_experiments(...
+        /^([^#]|\s*|\w*)stub_experiments\(/
+      end
+    end
+  end
+end