1 files changed, 111 insertions, 0 deletions
diff --git a/lib/gitlab/utils/override.rb b/lib/gitlab/utils/override.rb
new file mode 100644
index 00000000000..8bf6bcb1fe2
--- /dev/null
+++ b/lib/gitlab/utils/override.rb
@@ -0,0 +1,111 @@
+module Gitlab
+  module Utils
+    module Override
+      class Extension
+        def self.verify_class!(klass, method_name)
+          instance_method_defined?(klass, method_name) ||
+            raise(
+              NotImplementedError.new(
+                "#{klass}\##{method_name} doesn't exist!"))
+        end
+
+        def self.instance_method_defined?(klass, name, include_super: true)
+          klass.instance_methods(include_super).include?(name) ||
+            klass.private_instance_methods(include_super).include?(name)
+        end
+
+        attr_reader :subject
+
+        def initialize(subject)
+          @subject = subject
+        end
+
+        def add_method_name(method_name)
+          method_names << method_name
+        end
+
+        def add_class(klass)
+          classes << klass
+        end
+
+        def verify!
+          classes.each do |klass|
+            index = klass.ancestors.index(subject)
+            parents = klass.ancestors.drop(index + 1)
+
+            method_names.each do |method_name|
+              parents.any? do |parent|
+                self.class.instance_method_defined?(
+                  parent, method_name, include_super: false)
+              end ||
+                raise(
+                  NotImplementedError.new(
+                    "#{klass}\##{method_name} doesn't exist!"))
+            end
+          end
+        end
+
+        private
+
+        def method_names
+          @method_names ||= []
+        end
+
+        def classes
+          @classes ||= []
+        end
+      end
+
+      # Instead of writing patterns like this:
+      #
+      #     def f
+      #       raise NotImplementedError unless defined?(super)
+      #
+      #       true
+      #     end
+      #
+      # We could write it like:
+      #
+      #     extend ::Gitlab::Utils::Override
+      #
+      #     override :f
+      #     def f
+      #       true
+      #     end
+      #
+      # This would make sure we're overriding something. See:
+      # https://gitlab.com/gitlab-org/gitlab-ee/issues/1819
+      def override(method_name)
+        return unless ENV['STATIC_VERIFICATION']
+
+        if is_a?(Class)
+          Extension.verify_class!(self, method_name)
+        else # We delay the check for modules
+          Override.extensions[self] ||= Extension.new(self)
+          Override.extensions[self].add_method_name(method_name)
+        end
+      end
+
+      def included(base = nil)
+        return super if base.nil? # Rails concern, ignoring it
+
+        super
+
+        if base.is_a?(Class) # We could check for Class in `override`
+          # This could be `nil` if `override` was never called
+          Override.extensions[self]&.add_class(base)
+        end
+      end
+
+      alias_method :prepended, :included
+
+      def self.extensions
+        @extensions ||= {}
+      end
+
+      def self.verify!
+        extensions.values.each(&:verify!)
+      end
+    end
+  end
+end