Merge branch 'no-ivar-in-modules' into tmp

* no-ivar-in-modules:
  Cache allowed_ids
  Fix a few layout error
  Make it clear that this is an acceptable use
  Reword Instance variables in views
  Move ModuleWithInstanceVariables to Gitlab namespace
  Explain how to disable it in the doc
  Just define allowed_ids and override it with empty value
  Updates based on feedback
  Remove codes from bad merge
  Allow initialize method and single ivar
  Use StrongMemoize and enable/disable cops properly
  WIP
  Fix grammar: judge -> judgement
  Allow simple ivar ||= form. Update accordingly
  Add cop to make sure we don't use ivar in a module

.rubocop.yml

@@ -1190,7 +1190,20 @@ RSpec/SubjectStub:
 RSpec/VerifiedDoubles:
   Enabled: false
 
-# GitlabSecurity ##############################################################
+# Gitlab ###################################################################
+
+Gitlab/ModuleWithInstanceVariables:
+  Enable: true
+  Exclude:
+    # We ignore Rails helpers right now because it's hard to workaround it
+    - app/helpers/*_helper.rb
+    # We ignore Rails mailers right now because it's hard to workaround it
+    - app/mailers/emails/*.rb
+    # We ignore spec helpers because it usually doesn't matter
+    - spec/support/**/*.rb
+    - features/steps/**/*.rb
+
+# GitlabSecurity ###########################################################
 
 GitlabSecurity/DeepMunge:
   Enabled: true

app/controllers/concerns/boards_responses.rb

@@ -24,11 +24,11 @@ module BoardsResponses
   end
 
   def respond_with_boards
-    respond_with(@boards)
+    respond_with(@boards) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def respond_with_board
-    respond_with(@board)
+    respond_with(@board) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def respond_with(resource)

app/controllers/concerns/creates_commit.rb

 module CreatesCommit
   extend ActiveSupport::Concern
+  include Gitlab::Utils::StrongMemoize
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def create_commit(service, success_path:, failure_path:, failure_view: nil, success_notice: nil)
     if can?(current_user, :push_code, @project)
       @project_to_commit_into = @project
@@ -45,6 +47,7 @@ module CreatesCommit
       end
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def authorize_edit_tree!
     return if can_collaborate_with_project?
@@ -77,6 +80,7 @@ module CreatesCommit
     end
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def new_merge_request_path
     project_new_merge_request_path(
       @project_to_commit_into,
@@ -88,20 +92,28 @@ module CreatesCommit
       }
     )
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def existing_merge_request_path
-    project_merge_request_path(@project, @merge_request)
+    project_merge_request_path(@project, @merge_request) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def merge_request_exists?
-    return @merge_request if defined?(@merge_request)
-
-    @merge_request = MergeRequestsFinder.new(current_user, project_id: @project.id).execute.opened
-      .find_by(source_project_id: @project_to_commit_into, source_branch: @branch_name, target_branch: @start_branch)
+    strong_memoize(:merge_request) do
+      MergeRequestsFinder.new(current_user, project_id: @project.id)
+        .execute
+        .opened
+        .find_by(
+          source_project_id: @project_to_commit_into,
+          source_branch: @branch_name,
+          target_branch: @start_branch)
+    end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def different_project?
-    @project_to_commit_into != @project
+    @project_to_commit_into != @project # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def create_merge_request?
@@ -109,6 +121,6 @@ module CreatesCommit
     # as the target branch in the same project,
     # we don't want to create a merge request.
     params[:create_merge_request].present? &&
-      (different_project? || @start_branch != @branch_name)
+      (different_project? || @start_branch != @branch_name) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 end

app/controllers/concerns/group_tree.rb

 module GroupTree
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def render_group_tree(groups)
     @groups = if params[:filter].present?
                 Gitlab::GroupHierarchy.new(groups.search(params[:filter]))
@@ -20,5 +21,6 @@ module GroupTree
         render json: serializer.represent(@groups)
       end
     end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
   end
 end

app/controllers/concerns/issuable_actions.rb

@@ -17,7 +17,7 @@ module IssuableActions
   end
 
   def update
-    @issuable = update_service.execute(issuable)
+    @issuable = update_service.execute(issuable) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
     respond_to do |format|
       format.html do
@@ -83,7 +83,7 @@ module IssuableActions
   def render_conflict_response
     respond_to do |format|
       format.html do
-        @conflict = true
+        @conflict = true # rubocop:disable Gitlab/ModuleWithInstanceVariables
         render :edit
       end
 
@@ -98,7 +98,7 @@ module IssuableActions
   end
 
   def labels
-    @labels ||= LabelsFinder.new(current_user, project_id: @project.id).execute
+    @labels ||= LabelsFinder.new(current_user, project_id: @project.id).execute # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def authorize_destroy_issuable!
@@ -108,7 +108,7 @@ module IssuableActions
   end
 
   def authorize_admin_issuable!
-    unless can?(current_user, :"admin_#{resource_name}", @project)
+    unless can?(current_user, :"admin_#{resource_name}", @project) # rubocop:disable Gitlab/ModuleWithInstanceVariables
       return access_denied!
     end
   end
@@ -143,6 +143,7 @@ module IssuableActions
     @resource_name ||= controller_name.singularize
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def render_entity_json
     if @issuable.valid?
       render json: serializer.represent(@issuable)
@@ -150,6 +151,7 @@ module IssuableActions
       render json: { errors: @issuable.errors.full_messages }, status: :unprocessable_entity
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def serializer
     raise NotImplementedError
@@ -160,6 +162,6 @@ module IssuableActions
   end
 
   def parent
-    @project || @group
+    @project || @group # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 end

app/controllers/concerns/issuable_collections.rb

@@ -2,6 +2,7 @@ module IssuableCollections
   extend ActiveSupport::Concern
   include SortingHelper
   include Gitlab::IssuableMetadata
+  include Gitlab::Utils::StrongMemoize
 
   included do
     helper_method :finder
@@ -9,6 +10,7 @@ module IssuableCollections
 
   private
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def set_issuables_index
     @issuables          = issuables_collection
     @issuables          = @issuables.page(params[:page])
@@ -33,6 +35,7 @@ module IssuableCollections
       @users.push(author) if author
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def issuables_collection
     finder.execute.preload(preload_for_collection)
@@ -41,7 +44,7 @@ module IssuableCollections
   def redirect_out_of_range(total_pages)
     return false if total_pages.zero?
 
-    out_of_range = @issuables.current_page > total_pages
+    out_of_range = @issuables.current_page > total_pages # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
     if out_of_range
       redirect_to(url_for(params.merge(page: total_pages, only_path: true)))
@@ -51,7 +54,7 @@ module IssuableCollections
   end
 
   def issuable_page_count
-    page_count_for_relation(@issuables, finder.row_count)
+    page_count_for_relation(@issuables, finder.row_count) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def page_count_for_relation(relation, row_count)
@@ -66,6 +69,7 @@ module IssuableCollections
     finder_class.new(current_user, filter_params)
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def filter_params
     set_sort_order_from_cookie
     set_default_state
@@ -90,6 +94,7 @@ module IssuableCollections
 
     @filter_params.permit(IssuableFinder::VALID_PARAMS)
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def set_default_state
     params[:state] = 'opened' if params[:state].blank?
@@ -131,9 +136,9 @@ module IssuableCollections
   end
 
   def finder
-    return @finder if defined?(@finder)
-
-    @finder = issuable_finder_for(@finder_type)
+    strong_memoize(:finder) do
+      issuable_finder_for(@finder_type) # rubocop:disable Gitlab/ModuleWithInstanceVariables
+    end
   end
 
   def collection_type

app/controllers/concerns/issues_action.rb

@@ -2,6 +2,7 @@ module IssuesAction
   extend ActiveSupport::Concern
   include IssuableCollections
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def issues
     @finder_type = IssuesFinder
     @label = finder.labels.first
@@ -17,4 +18,5 @@ module IssuesAction
       format.atom { render layout: 'xml.atom' }
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/controllers/concerns/merge_requests_action.rb

@@ -2,6 +2,7 @@ module MergeRequestsAction
   extend ActiveSupport::Concern
   include IssuableCollections
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def merge_requests
     @finder_type = MergeRequestsFinder
     @label = finder.labels.first
@@ -10,6 +11,7 @@ module MergeRequestsAction
 
     @issuable_meta_data = issuable_meta_data(@merge_requests, collection_type)
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   private
 

app/controllers/concerns/milestone_actions.rb

@@ -6,7 +6,7 @@ module MilestoneActions
       format.html { redirect_to milestone_redirect_path }
       format.json do
         render json: tabs_json("shared/milestones/_merge_requests_tab", {
-          merge_requests: @milestone.sorted_merge_requests,
+          merge_requests: @milestone.sorted_merge_requests, # rubocop:disable Gitlab/ModuleWithInstanceVariables
           show_project_name: true
         })
       end
@@ -18,7 +18,7 @@ module MilestoneActions
       format.html { redirect_to milestone_redirect_path }
       format.json do
         render json: tabs_json("shared/milestones/_participants_tab", {
-          users: @milestone.participants
+          users: @milestone.participants # rubocop:disable Gitlab/ModuleWithInstanceVariables
         })
       end
     end
@@ -29,7 +29,7 @@ module MilestoneActions
       format.html { redirect_to milestone_redirect_path }
       format.json do
         render json: tabs_json("shared/milestones/_labels_tab", {
-          labels: @milestone.labels
+          labels: @milestone.labels # rubocop:disable Gitlab/ModuleWithInstanceVariables
         })
       end
     end
@@ -43,6 +43,7 @@ module MilestoneActions
     }
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def milestone_redirect_path
     if @project
       project_milestone_path(@project, @milestone)
@@ -52,4 +53,5 @@ module MilestoneActions
       dashboard_milestone_path(@milestone.safe_title, title: @milestone.title)
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/controllers/concerns/notes_actions.rb

 module NotesActions
   include RendersNotes
+  include Gitlab::Utils::StrongMemoize
   extend ActiveSupport::Concern
 
   included do
@@ -30,6 +31,7 @@ module NotesActions
     render json: notes_json
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def create
     create_params = note_params.merge(
       merge_request_diff_head_sha: params[:merge_request_diff_head_sha],
@@ -47,7 +49,9 @@ module NotesActions
       format.html { redirect_back_or_default }
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def update
     @note = Notes::UpdateService.new(project, current_user, note_params).execute(note)
 
@@ -60,6 +64,7 @@ module NotesActions
       format.html { redirect_back_or_default }
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def destroy
     if note.editable?
@@ -138,7 +143,7 @@ module NotesActions
         end
     else
       template = "discussions/_diff_discussion"
-      @fresh_discussion = true
+      @fresh_discussion = true # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
       locals = { discussions: [discussion], on_image: on_image }
     end
@@ -191,7 +196,7 @@ module NotesActions
   end
 
   def noteable
-    @noteable ||= notes_finder.target || @note&.noteable
+    @noteable ||= notes_finder.target || @note&.noteable # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def require_noteable!
@@ -211,20 +216,21 @@ module NotesActions
   end
 
   def note_project
-    return @note_project if defined?(@note_project)
-    return nil unless project
+    strong_memoize(:note_project) do
+      return nil unless project
 
-    note_project_id = params[:note_project_id]
+      note_project_id = params[:note_project_id]
 
-    @note_project =
-      if note_project_id.present?
-        Project.find(note_project_id)
-      else
-        project
-      end
+      the_project =
+        if note_project_id.present?
+          Project.find(note_project_id)
+        else
+          project
+        end
 
-    return access_denied! unless can?(current_user, :create_note, @note_project)
+      return access_denied! unless can?(current_user, :create_note, the_project)
 
-    @note_project
+      the_project
+    end
   end
 end

app/controllers/concerns/oauth_applications.rb

@@ -14,6 +14,6 @@ module OauthApplications
   end
 
   def load_scopes
-    @scopes = Doorkeeper.configuration.scopes
+    @scopes ||= Doorkeeper.configuration.scopes
   end
 end

app/controllers/concerns/preview_markdown.rb

 module PreviewMarkdown
   extend ActiveSupport::Concern
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def preview_markdown
     result = PreviewMarkdownService.new(@project, current_user, params).execute
 
@@ -20,4 +21,5 @@ module PreviewMarkdown
       }
     }
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/controllers/concerns/renders_commits.rb

 module RendersCommits
   def prepare_commits_for_rendering(commits)
-    Banzai::CommitRenderer.render(commits, @project, current_user)
+    Banzai::CommitRenderer.render(commits, @project, current_user) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
     commits
   end

app/controllers/concerns/renders_notes.rb

 module RendersNotes
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def prepare_notes_for_rendering(notes, noteable = nil)
     preload_noteable_for_regular_notes(notes)
     preload_max_access_for_authors(notes, @project)
@@ -7,6 +8,7 @@ module RendersNotes
 
     notes
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   private
 

app/controllers/concerns/service_params.rb

@@ -67,7 +67,7 @@ module ServiceParams
   FILTER_BLANK_PARAMS = [:password].freeze
 
   def service_params
-    dynamic_params = @service.event_channel_names + @service.event_names
+    dynamic_params = @service.event_channel_names + @service.event_names # rubocop:disable Gitlab/ModuleWithInstanceVariables
     service_params = params.permit(:id, service: allowed_service_params + dynamic_params)
 
     if service_params[:service].is_a?(Hash)

app/controllers/concerns/snippets_actions.rb

@@ -4,6 +4,7 @@ module SnippetsActions
   def edit
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def raw
     disposition = params[:inline] == 'false' ? 'attachment' : 'inline'
 
@@ -14,6 +15,7 @@ module SnippetsActions
       filename: @snippet.sanitized_file_name
     )
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   private
 

app/controllers/concerns/spammable_actions.rb

@@ -2,6 +2,7 @@ module SpammableActions
   extend ActiveSupport::Concern
 
   include Recaptcha::Verify
+  include Gitlab::Utils::StrongMemoize
 
   included do
     before_action :authorize_submit_spammable!, only: :mark_as_spam
@@ -18,9 +19,9 @@ module SpammableActions
   private
 
   def ensure_spam_config_loaded!
-    return @spam_config_loaded if defined?(@spam_config_loaded)
-
-    @spam_config_loaded = Gitlab::Recaptcha.load_configurations!
+    strong_memoize(:spam_config_loaded) do
+      Gitlab::Recaptcha.load_configurations!
+    end
   end
 
   def recaptcha_check_with_fallback(&fallback)

app/controllers/concerns/toggle_subscription_action.rb

@@ -12,7 +12,7 @@ module ToggleSubscriptionAction
   private
 
   def subscribable_project
-    @project || raise(NotImplementedError)
+    @project ||= raise(NotImplementedError)
   end
 
   def subscribable_resource

app/models/concerns/mentionable.rb

@@ -44,13 +44,11 @@ module Mentionable
   end
 
   def all_references(current_user = nil, extractor: nil)
-    @extractors ||= {}
-
     # Use custom extractor if it's passed in the function parameters.
     if extractor
-      @extractors[current_user] = extractor
+      extractors[current_user] = extractor
     else
-      extractor = @extractors[current_user] ||= Gitlab::ReferenceExtractor.new(project, current_user)
+      extractor = extractors[current_user] ||= Gitlab::ReferenceExtractor.new(project, current_user)
 
       extractor.reset_memoized_values
     end
@@ -69,6 +67,10 @@ module Mentionable
     extractor
   end
 
+  def extractors
+    @extractors ||= {}
+  end
+
   def mentioned_users(current_user = nil)
     all_references(current_user).users
   end

app/models/concerns/milestoneish.rb

@@ -103,9 +103,11 @@ module Milestoneish
   end
 
   def memoize_per_user(user, method_name)
-    @memoized ||= {}
-    @memoized[method_name] ||= {}
-    @memoized[method_name][user&.id] ||= yield
+    memoized_users[method_name][user&.id] ||= yield
+  end
+
+  def memoized_users
+    @memoized_users ||= Hash.new { |h, k| h[k] = {} }
   end
 
   # override in a class that includes this module to get a faster query

app/models/concerns/noteable.rb

@@ -46,6 +46,7 @@ module Noteable
     notes.inc_relations_for_view.grouped_diff_discussions(*args)
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def resolvable_discussions
     @resolvable_discussions ||=
       if defined?(@discussions)
@@ -54,6 +55,7 @@ module Noteable
         discussion_notes.resolvable.discussions(self)
       end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def discussions_resolvable?
     resolvable_discussions.any?(&:resolvable?)

app/models/concerns/participable.rb

@@ -56,15 +56,17 @@ module Participable
   #
   # Returns an Array of User instances.
   def participants(current_user = nil)
-    @participants ||= Hash.new do |hash, user|
-      hash[user] = raw_participants(user)
-    end
-
-    @participants[current_user]
+    all_participants[current_user]
   end
 
   private
 
+  def all_participants
+    @all_participants ||= Hash.new do |hash, user|
+      hash[user] = raw_participants(user)
+    end
+  end
+
   def raw_participants(current_user = nil)
     current_user ||= author
     ext = Gitlab::ReferenceExtractor.new(project, current_user)

app/models/concerns/relative_positioning.rb

@@ -52,7 +52,7 @@ module RelativePositioning
     # to its predecessor. This process will recursively move all the predecessors until we have a place
     if (after.relative_position - before.relative_position) < 2
       before.move_before
-      @positionable_neighbours = [before]
+      @positionable_neighbours = [before] # rubocop:disable Gitlab/ModuleWithInstanceVariables
     end
 
     self.relative_position = position_between(before.relative_position, after.relative_position)
@@ -65,7 +65,7 @@ module RelativePositioning
     if before.shift_after?
       issue_to_move = self.class.in_projects(project_ids).find_by!(relative_position: pos_after)
       issue_to_move.move_after
-      @positionable_neighbours = [issue_to_move]
+      @positionable_neighbours = [issue_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
       pos_after = issue_to_move.relative_position
     end
@@ -80,7 +80,7 @@ module RelativePositioning
     if after.shift_before?
       issue_to_move = self.class.in_projects(project_ids).find_by!(relative_position: pos_before)
       issue_to_move.move_before
-      @positionable_neighbours = [issue_to_move]
+      @positionable_neighbours = [issue_to_move] # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
       pos_before = issue_to_move.relative_position
     end
@@ -132,6 +132,7 @@ module RelativePositioning
     end
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def save_positionable_neighbours
     return unless @positionable_neighbours
 
@@ -140,4 +141,5 @@ module RelativePositioning
 
     status
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/models/concerns/resolvable_discussion.rb

@@ -31,15 +31,11 @@ module ResolvableDiscussion
   end
 
   def resolvable?
-    return @resolvable if @resolvable.present?
-
-    @resolvable = potentially_resolvable? && notes.any?(&:resolvable?)
+    @resolvable ||= potentially_resolvable? && notes.any?(&:resolvable?)
   end
 
   def resolved?
-    return @resolved if @resolved.present?
-
-    @resolved = resolvable? && notes.none?(&:to_be_resolved?)
+    @resolved ||= resolvable? && notes.none?(&:to_be_resolved?)
   end
 
   def first_note
@@ -49,13 +45,13 @@ module ResolvableDiscussion
   def first_note_to_resolve
     return unless resolvable?
 
-    @first_note_to_resolve ||= notes.find(&:to_be_resolved?)
+    @first_note_to_resolve ||= notes.find(&:to_be_resolved?) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def last_resolved_note
     return unless resolved?
 
-    @last_resolved_note ||= resolved_notes.sort_by(&:resolved_at).last
+    @last_resolved_note ||= resolved_notes.sort_by(&:resolved_at).last # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def resolved_notes
@@ -95,7 +91,7 @@ module ResolvableDiscussion
     yield(notes_relation)
 
     # Set the notes array to the updated notes
-    @notes = notes_relation.fresh.to_a
+    @notes = notes_relation.fresh.to_a # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
     self.class.memoized_values.each do |var|
       instance_variable_set(:"@#{var}", nil)

app/models/concerns/routable.rb

@@ -88,7 +88,7 @@ module Routable
 
   def full_name
     if route && route.name.present?
-      @full_name ||= route.name
+      @full_name ||= route.name # rubocop:disable Gitlab/ModuleWithInstanceVariables
     else
       update_route if persisted?
 
@@ -112,7 +112,7 @@ module Routable
 
   def expires_full_path_cache
     RequestStore.delete(full_path_key) if RequestStore.active?
-    @full_path = nil
+    @full_path = nil # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def build_full_path
@@ -127,7 +127,7 @@ module Routable
 
   def uncached_full_path
     if route && route.path.present?
-      @full_path ||= route.path
+      @full_path ||= route.path # rubocop:disable Gitlab/ModuleWithInstanceVariables
     else
       update_route if persisted?
 
@@ -166,7 +166,7 @@ module Routable
     route || build_route(source: self)
     route.path = build_full_path
     route.name = build_full_name
-    @full_path = nil
-    @full_name = nil
+    @full_path = nil # rubocop:disable Gitlab/ModuleWithInstanceVariables
+    @full_name = nil # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 end

app/models/concerns/spammable.rb

@@ -12,6 +12,7 @@ module Spammable
 
     attr_accessor :spam
     attr_accessor :spam_log
+    alias_method :spam?, :spam
 
     after_validation :check_for_spam, on: [:create, :update]
 
@@ -34,10 +35,6 @@ module Spammable
     end
   end
 
-  def spam?
-    @spam
-  end
-
   def check_for_spam
     error_msg = if Gitlab::Recaptcha.enabled?
                   "Your #{spammable_entity_type} has been recognized as spam. "\

app/models/concerns/taskable.rb

@@ -39,7 +39,7 @@ module Taskable
   def task_list_items
     return [] if description.blank?
 
-    @task_list_items ||= Taskable.get_tasks(description)
+    @task_list_items ||= Taskable.get_tasks(description) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def tasks

app/models/concerns/time_trackable.rb

@@ -21,6 +21,7 @@ module TimeTrackable
     has_many :timelogs, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def spend_time(options)
     @time_spent = options[:duration]
     @time_spent_user = options[:user]
@@ -36,6 +37,7 @@ module TimeTrackable
     end
   end
   alias_method :spend_time=, :spend_time
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def total_time_spent
     timelogs.sum(:time_spent)
@@ -52,9 +54,10 @@ module TimeTrackable
   private
 
   def reset_spent_time
-    timelogs.new(time_spent: total_time_spent * -1, user: @time_spent_user)
+    timelogs.new(time_spent: total_time_spent * -1, user: @time_spent_user) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def add_or_subtract_spent_time
     timelogs.new(
       time_spent: time_spent,
@@ -62,16 +65,19 @@ module TimeTrackable
       spent_at: @spent_at
     )
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def check_negative_time_spent
     return if time_spent.nil? || time_spent == :reset
 
-    # we need to cache the total time spent so multiple calls to #valid?
-    # doesn't give a false error
-    @original_total_time_spent ||= total_time_spent
-
-    if time_spent < 0 && (time_spent.abs > @original_total_time_spent)
+    if time_spent < 0 && (time_spent.abs > original_total_time_spent)
       errors.add(:time_spent, 'Time to subtract exceeds the total time spent')
     end
   end
+
+  # we need to cache the total time spent so multiple calls to #valid?
+  # doesn't give a false error
+  def original_total_time_spent
+    @original_total_time_spent ||= total_time_spent
+  end
 end

app/serializers/concerns/with_pagination.rb

@@ -14,7 +14,7 @@ module WithPagination
   # we shouldn't try to paginate single resources
   def represent(resource, opts = {})
     if paginated? && resource.respond_to?(:page)
-      super(@paginator.paginate(resource), opts)
+      super(paginator.paginate(resource), opts)
     else
       super(resource, opts)
     end

app/services/concerns/issues/resolve_discussions.rb

 module Issues
   module ResolveDiscussions
+    include Gitlab::Utils::StrongMemoize
+
     attr_reader :merge_request_to_resolve_discussions_of_iid, :discussion_to_resolve_id
 
+    # rubocop:disable Gitlab/ModuleWithInstanceVariables
     def filter_resolve_discussion_params
       @merge_request_to_resolve_discussions_of_iid ||= params.delete(:merge_request_to_resolve_discussions_of)
       @discussion_to_resolve_id ||= params.delete(:discussion_to_resolve)
     end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
     def merge_request_to_resolve_discussions_of
-      return @merge_request_to_resolve_discussions_of if defined?(@merge_request_to_resolve_discussions_of)
-
-      @merge_request_to_resolve_discussions_of = MergeRequestsFinder.new(current_user, project_id: project.id)
-                                                     .execute
-                                                     .find_by(iid: merge_request_to_resolve_discussions_of_iid)
+      strong_memoize(:merge_request_to_resolve_discussions_of) do
+        MergeRequestsFinder.new(current_user, project_id: project.id)
+          .execute
+          .find_by(iid: merge_request_to_resolve_discussions_of_iid)
+      end
     end
 
     def discussions_to_resolve
       return [] unless merge_request_to_resolve_discussions_of
 
-      @discussions_to_resolve ||=
+      @discussions_to_resolve ||= # rubocop:disable Gitlab/ModuleWithInstanceVariables
         if discussion_to_resolve_id
           discussion_or_nil = merge_request_to_resolve_discussions_of
                                 .find_discussion(discussion_to_resolve_id)

app/services/spam_check_service.rb

@@ -7,16 +7,19 @@
 # - params with :request
 #
 module SpamCheckService
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def filter_spam_check_params
     @request            = params.delete(:request)
     @api                = params.delete(:api)
     @recaptcha_verified = params.delete(:recaptcha_verified)
     @spam_log_id        = params.delete(:spam_log_id)
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   # In order to be proceed to the spam check process, @spammable has to be
   # a dirty instance, which means it should be already assigned with the new
   # attribute values.
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def spam_check(spammable, user)
     spam_service = SpamService.new(spammable, @request)
 
@@ -24,4 +27,5 @@ module SpamCheckService
       user.spam_logs.find_by(id: @spam_log_id)&.update!(recaptcha_verified: true)
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 end

app/workers/concerns/new_issuable.rb

@@ -9,15 +9,15 @@ module NewIssuable
   end
 
   def set_user(user_id)
-    @user = User.find_by(id: user_id)
+    @user = User.find_by(id: user_id) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-    log_error(User, user_id) unless @user
+    log_error(User, user_id) unless @user # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def set_issuable(issuable_id)
-    @issuable = issuable_class.find_by(id: issuable_id)
+    @issuable = issuable_class.find_by(id: issuable_id) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-    log_error(issuable_class, issuable_id) unless @issuable
+    log_error(issuable_class, issuable_id) unless @issuable # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   def log_error(record_class, record_id)

config/initializers/fix_local_cache_middleware.rb

@@ -6,7 +6,7 @@ module LocalCacheRegistryCleanupWithEnsure
 
   def call(env)
     LocalCacheRegistry.set_cache_for(local_cache_key, LocalStore.new)
-    response = @app.call(env)
+    response = @app.call(env) # rubocop:disable Gitlab/ModuleWithInstanceVariables
     response[2] = ::Rack::BodyProxy.new(response[2]) do
       LocalCacheRegistry.set_cache_for(local_cache_key, nil)
     end

config/initializers/rspec_profiling.rb

@@ -19,10 +19,10 @@ module RspecProfilingExt
     def example_finished(*args)
       super
     rescue => err
-      return if @already_logged_example_finished_error
+      return if @already_logged_example_finished_error # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
       $stderr.puts "rspec_profiling couldn't collect an example: #{err}. Further warnings suppressed."
-      @already_logged_example_finished_error = true
+      @already_logged_example_finished_error = true # rubocop:disable Gitlab/ModuleWithInstanceVariables
     end
 
     alias_method :example_passed, :example_finished

config/initializers/rugged_use_gitlab_git_attributes.rb

@@ -15,8 +15,11 @@ module Rugged
   class Repository
     module UseGitlabGitAttributes
       def fetch_attributes(name, *)
+        attributes.attributes(name)
+      end
+
+      def attributes
         @attributes ||= Gitlab::Git::Attributes.new(path)
-        @attributes.attributes(name)
       end
     end
 

doc/development/README.md

@@ -37,6 +37,7 @@ comments: false
 - [`Gemfile` guidelines](gemfile.md)
 - [Sidekiq debugging](sidekiq_debugging.md)
 - [Gotchas](gotchas.md) to avoid
+- [Avoid modules with instance variables](module_with_instance_variables.md) if possible
 - [Issue and merge requests state models](object_state_models.md)
 - [How to dump production data to staging](db_dump.md)
 - [Working with the GitHub importer](github_importer.md)

doc/development/module_with_instance_variables.md

+## Modules with instance variables could be considered harmful
+
+### Background
+
+Rails somehow encourages people using modules and instance variables
+everywhere. For example, using instance variables in the controllers,
+helpers, and views. They're also encouraging the use of
+`ActiveSupport::Concern`, which further strengthens the idea of
+saving everything in a giant, single object, and people could access
+everything in that one giant object.
+
+### The problems
+
+Of course this is convenient to develop, because we just have everything
+within reach. However this has a number of downsides when that chosen object
+is growing, it would later become out of control for the same reason.
+
+There are just too many things in the same context, and we don't know if
+those things are tightly coupled or not, depending on each others or not.
+It's very hard to tell when the complexity grows to a point, and it makes
+tracking the code also extremely hard. For example, a class could be using
+3 different instance variables, and all of them could be initialized and
+manipulated from 3 different modules. It's hard to track when those variables
+start giving us troubles. We don't know which module would suddenly change
+one of the variables. Everything could touch anything.
+
+### Similar concerns
+
+People are saying multiple inheritance is bad. Mixing multiple modules with
+multiple instance variables scattering everywhere suffer from the same issue.
+The same applies to `ActiveSupport::Concern`. See:
+[Consider replacing concerns with dedicated classes & composition](
+https://gitlab.com/gitlab-org/gitlab-ce/issues/23786)
+
+There's also a similar idea:
+[Use decorators and interface segregation to solve overgrowing models problem](
+https://gitlab.com/gitlab-org/gitlab-ce/issues/13484)
+
+Note that `included` doesn't solve the whole issue. They define the
+dependencies, but they still allow each modules to talk implicitly via the
+instance variables in the final giant object, and that's where the problem is.
+
+### Solutions
+
+We should split the giant object into multiple objects, and they communicate
+with each other with the API, i.e. public methods. In short, composition over
+inheritance. This way, each smaller objects would have their own respective
+limited states, i.e. instance variables. If one instance variable goes wrong,
+we would be very clear that it's from that single small object, because
+no one else could be touching it.
+
+With clearly defined API, this would make things less coupled and much easier
+to debug and track, and much more extensible for other objects to use, because
+they communicate in a clear way, rather than implicit dependencies.
+
+### Acceptable use
+
+However, it's not always bad to use instance variables in a module,
+as long as it's contained in the same module; that is, no other modules or
+objects are touching them, then it would be an acceptable use.
+
+We especially allow the case where a single instance variable is used with
+`||=` to setup the value. This would look like:
+
+``` ruby
+module M
+  def f
+    @f ||= true
+  end
+end
+```
+
+Unfortunately it's not easy to code more complex rules into the cop, so
+we rely on people's best judgement. If we could find another good pattern
+we could easily add to the cop, we should do it.
+
+### How to rewrite and avoid disabling this cop
+
+Even if we could just disable the cop, we should avoid doing so. Some code
+could be easily rewritten in simple form. Consider this acceptable method:
+
+``` ruby
+module Gitlab
+  module Emoji
+    def emoji_unicode_version(name)
+      @emoji_unicode_versions_by_name ||=
+        JSON.parse(File.read(Rails.root.join('fixtures', 'emojis', 'emoji-unicode-version-map.json')))
+      @emoji_unicode_versions_by_name[name]
+    end
+  end
+end
+```
+
+This method is totally fine because it's already self-contained. No other
+methods should be using `@emoji_unicode_versions_by_name` and we're good.
+However it's still offending the cop because it's not just `||=`, and the
+cop is not smart enough to judge that this is fine.
+
+On the other hand, we could split this method into two:
+
+``` ruby
+module Gitlab
+  module Emoji
+    def emoji_unicode_version(name)
+      emoji_unicode_versions_by_name[name]
+    end
+
+    private
+
+    def emoji_unicode_versions_by_name
+      @emoji_unicode_versions_by_name ||=
+        JSON.parse(File.read(Rails.root.join('fixtures', 'emojis', 'emoji-unicode-version-map.json')))
+    end
+  end
+end
+```
+
+Now the cop won't complain. Here's a bad example which we could rewrite:
+
+``` ruby
+module SpamCheckService
+  def filter_spam_check_params
+    @request            = params.delete(:request)
+    @api                = params.delete(:api)
+    @recaptcha_verified = params.delete(:recaptcha_verified)
+    @spam_log_id        = params.delete(:spam_log_id)
+  end
+
+  def spam_check(spammable, user)
+    spam_service = SpamService.new(spammable, @request)
+
+    spam_service.when_recaptcha_verified(@recaptcha_verified, @api) do
+      user.spam_logs.find_by(id: @spam_log_id)&.update!(recaptcha_verified: true)
+    end
+  end
+end
+```
+
+There are several implicit dependencies here. First, `params` should be
+defined before use. Second, `filter_spam_check_params` should be called
+before `spam_check`. These are all implicit and the includer could be using
+those instance variables without awareness.
+
+This should be rewritten like:
+
+``` ruby
+class SpamCheckService
+  def initialize(request:, api:, recaptcha_verified:, spam_log_id:)
+    @request            = request
+    @api                = api
+    @recaptcha_verified = recaptcha_verified
+    @spam_log_id        = spam_log_id
+  end
+
+  def spam_check(spammable, user)
+    spam_service = SpamService.new(spammable, @request)
+
+    spam_service.when_recaptcha_verified(@recaptcha_verified, @api) do
+      user.spam_logs.find_by(id: @spam_log_id)&.update!(recaptcha_verified: true)
+    end
+  end
+end
+```
+
+And use it like:
+
+``` ruby
+class UpdateSnippetService < BaseService
+  def execute
+    # ...
+    spam = SpamCheckService.new(params.slice!(:request, :api, :recaptcha_verified, :spam_log_id))
+
+    spam.check(snippet, current_user)
+    # ...
+  end
+end
+```
+
+This way, all those instance variables are isolated in `SpamCheckService`
+rather than whatever includes the module, and those modules which were also
+included, making it much easier to track down any issues,
+and reducing the chance of having name conflicts.
+
+### How to disable this cop
+
+Put the disabling comment right after your code in the same line:
+
+``` ruby
+module M
+  def violating_method
+    @f + @g # rubocop:disable Gitlab/ModuleWithInstanceVariables
+  end
+end
+```
+
+If there are multiple lines, you could also enable and disable for a section:
+
+``` ruby
+module M
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
+  def violating_method
+    @f = 0
+    @g = 1
+    @h = 2
+  end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
+end
+```
+
+Note that you need to enable it at some point, otherwise everything below
+won't be checked.
+
+### Things we might need to ignore right now
+
+Because of the way Rails helpers and mailers work, we might not be able to
+avoid the use of instance variables there. For those cases, we could ignore
+them at the moment. At least we're not going to share those modules with
+other random objects, so they're still somewhat isolated.
+
+### Instance variables in views
+
+They're bad because we can't easily tell who's using the instance variables
+(from controller's point of view) and where we set them up (from partials'
+point of view), making it extremely hard to track data dependency.
+
+We're trying to use something like this instead:
+
+``` haml
+= render 'projects/commits/commit', commit: commit, ref: ref, project: project
+```
+
+And in the partial:
+
+``` haml
+- ref = local_assigns.fetch(:ref)
+- commit = local_assigns.fetch(:commit)
+- project = local_assigns.fetch(:project)
+```
+
+This way it's clearer where those values were coming from, and we gain the
+benefit to have typo check over using instance variables. In the future,
+we should also forbid the use of instance variables in partials.

features/support/env.rb

@@ -50,11 +50,11 @@ module StdoutReporterWithScenarioLocation
   # Override the standard reporter to show filename and line number next to each
   # scenario for easy, focused re-runs
   def before_scenario_run(scenario, step_definitions = nil)
-    @max_step_name_length = scenario.steps.map(&:name).map(&:length).max if scenario.steps.any?
+    @max_step_name_length = scenario.steps.map(&:name).map(&:length).max if scenario.steps.any? # rubocop:disable Gitlab/ModuleWithInstanceVariables
     name = scenario.name
 
     # This number has no significance, it's just to line things up
-    max_length = @max_step_name_length + 19
+    max_length = @max_step_name_length + 19 # rubocop:disable Gitlab/ModuleWithInstanceVariables
     out.puts "\n  #{'Scenario:'.green} #{name.light_green.ljust(max_length)}" \
       " # #{scenario.feature.filename}:#{scenario.line}"
   end

lib/api/helpers.rb

@@ -34,6 +34,11 @@ module API
       end
     end
 
+    # rubocop:disable Gitlab/ModuleWithInstanceVariables
+    # We can't rewrite this with StrongMemoize because `sudo!` would
+    # actually write to `@current_user`, and `sudo?` would immediately
+    # call `current_user` again which reads from `@current_user`.
+    # We should rewrite this in a way that using StrongMemoize is possible
     def current_user
       return @current_user if defined?(@current_user)
 
@@ -47,6 +52,7 @@ module API
 
       @current_user
     end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
     def sudo?
       initial_current_user != current_user
@@ -432,7 +438,7 @@ module API
     end
 
     def job_token_authentication?
-      initial_current_user && @job_token_authentication
+      initial_current_user && @job_token_authentication # rubocop:disable Gitlab/ModuleWithInstanceVariables
     end
 
     def warden
@@ -450,10 +456,10 @@ module API
     end
 
     def initial_current_user
-      return @initial_current_user if defined?(@initial_current_user)
+      return @initial_current_user if defined?(@initial_current_user) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
       begin
-        @initial_current_user = Gitlab::Auth::UniqueIpsLimiter.limit_user! { find_current_user! }
+        @initial_current_user = Gitlab::Auth::UniqueIpsLimiter.limit_user! { find_current_user! } # rubocop:disable Gitlab/ModuleWithInstanceVariables
       rescue Gitlab::Auth::UnauthorizedError
         unauthorized!
       end
@@ -477,7 +483,7 @@ module API
       sudoed_user = find_user(sudo_identifier)
       not_found!("User with ID or username '#{sudo_identifier}'") unless sudoed_user
 
-      @current_user = sudoed_user
+      @current_user = sudoed_user # rubocop:disable Gitlab/ModuleWithInstanceVariables
     end
 
     def sudo_identifier

lib/api/helpers/internal_helpers.rb

@@ -6,18 +6,16 @@ module API
         'git-upload-pack' => :ssh_upload_pack
       }.freeze
 
+      attr_reader :redirected_path
+
       def wiki?
-        set_project unless defined?(@wiki)
-        @wiki
+        set_project unless defined?(@wiki) # rubocop:disable Gitlab/ModuleWithInstanceVariables
+        @wiki # rubocop:disable Gitlab/ModuleWithInstanceVariables
       end
 
       def project
-        set_project unless defined?(@project)
-        @project
-      end
-
-      def redirected_path
-        @redirected_path
+        set_project unless defined?(@project) # rubocop:disable Gitlab/ModuleWithInstanceVariables
+        @project # rubocop:disable Gitlab/ModuleWithInstanceVariables
       end
 
       def ssh_authentication_abilities
@@ -69,6 +67,7 @@ module API
 
       private
 
+      # rubocop:disable Gitlab/ModuleWithInstanceVariables
       def set_project
         if params[:gl_repository]
           @project, @wiki = Gitlab::GlRepository.parse(params[:gl_repository])
@@ -77,6 +76,7 @@ module API
           @project, @wiki, @redirected_path = Gitlab::RepoPath.parse(params[:project])
         end
       end
+      # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
       # Project id to pass between components that don't share/don't have
       # access to the same filesystem mounts

lib/extracts_path.rb

@@ -40,7 +40,7 @@ module ExtractsPath
   def extract_ref(id)
     pair = ['', '']
 
-    return pair unless @project
+    return pair unless @project # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
     if id =~ /^(\h{40})(.+)/
       # If the ref appears to be a SHA, we're done, just split the string
@@ -104,6 +104,7 @@ module ExtractsPath
   #
   # Automatically renders `not_found!` if a valid tree path could not be
   # resolved (e.g., when a user inserts an invalid path or ref).
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def assign_ref_vars
     # assign allowed options
     allowed_options = ["filter_ref"]
@@ -131,9 +132,10 @@ module ExtractsPath
   rescue RuntimeError, NoMethodError, InvalidPathError
     render_404
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def tree
-    @tree ||= @repo.tree(@commit.id, @path)
+    @tree ||= @repo.tree(@commit.id, @path) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   private
@@ -146,8 +148,8 @@ module ExtractsPath
   end
 
   def ref_names
-    return [] unless @project
+    return [] unless @project # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
-    @ref_names ||= @project.repository.ref_names
+    @ref_names ||= @project.repository.ref_names # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 end

lib/gitlab/cache/request_cache.rb

@@ -45,11 +45,13 @@ module Gitlab
         klass.prepend(extension)
       end
 
+      attr_accessor :request_cache_key_block
+
       def request_cache_key(&block)
         if block_given?
-          @request_cache_key = block
+          self.request_cache_key_block = block
         else
-          @request_cache_key
+          request_cache_key_block
         end
       end
 

lib/gitlab/ci/charts.rb

@@ -6,7 +6,7 @@ module Gitlab
           query
             .group("DATE(#{::Ci::Pipeline.table_name}.created_at)")
             .count(:created_at)
-            .transform_keys { |date| date.strftime(@format) }
+            .transform_keys { |date| date.strftime(@format) } # rubocop:disable Gitlab/ModuleWithInstanceVariables
         end
 
         def interval_step

lib/gitlab/ci/config/entry/configurable.rb

@@ -29,15 +29,15 @@ module Gitlab
 
             self.class.nodes.each do |key, factory|
               factory
-                .value(@config[key])
+                .value(config[key])
                 .with(key: key, parent: self)
 
-              @entries[key] = factory.create!
+              entries[key] = factory.create!
             end
 
             yield if block_given?
 
-            @entries.each_value do |entry|
+            entries.each_value do |entry|
               entry.compose!(deps)
             end
           end
@@ -59,13 +59,13 @@ module Gitlab
             def helpers(*nodes)
               nodes.each do |symbol|
                 define_method("#{symbol}_defined?") do
-                  @entries[symbol]&.specified?
+                  entries[symbol]&.specified?
                 end
 
                 define_method("#{symbol}_value") do
-                  return unless @entries[symbol] && @entries[symbol].valid?
+                  return unless entries[symbol] && entries[symbol].valid?
 
-                  @entries[symbol].value
+                  entries[symbol].value
                 end
               end
             end

lib/gitlab/ci/config/entry/node.rb

@@ -90,6 +90,12 @@ module Gitlab
           def self.aspects
             @aspects ||= []
           end
+
+          private
+
+          def entries
+            @entries
+          end
         end
       end
     end

lib/gitlab/ci/config/entry/validatable.rb

@@ -13,7 +13,7 @@ module Gitlab
           end
 
           def errors
-            @validator.messages + descendants.flat_map(&:errors)
+            @validator.messages + descendants.flat_map(&:errors) # rubocop:disable Gitlab/ModuleWithInstanceVariables
           end
 
           class_methods do

lib/gitlab/ci/pipeline/chain/helpers.rb

@@ -3,16 +3,18 @@ module Gitlab
     module Pipeline
       module Chain
         module Helpers
-          def branch_exists?
-            return @is_branch if defined?(@is_branch)
+          include Gitlab::Utils::StrongMemoize
 
-            @is_branch = project.repository.branch_exists?(pipeline.ref)
+          def branch_exists?
+            strong_memoize(:is_branch) do
+              project.repository.branch_exists?(pipeline.ref)
+            end
           end
 
           def tag_exists?
-            return @is_tag if defined?(@is_tag)
-
-            @is_tag = project.repository.tag_exists?(pipeline.ref)
+            strong_memoize(:is_tag) do
+              project.repository.tag_exists?(pipeline.ref)
+            end
           end
 
           def error(message)

lib/gitlab/current_settings.rb

@@ -53,7 +53,7 @@ module Gitlab
     end
 
     def in_memory_application_settings
-      @in_memory_application_settings ||= ::ApplicationSetting.new(::ApplicationSetting.defaults)
+      @in_memory_application_settings ||= ::ApplicationSetting.new(::ApplicationSetting.defaults) # rubocop:disable Gitlab/ModuleWithInstanceVariables
     rescue ActiveRecord::StatementInvalid, ActiveRecord::UnknownAttributeError
       # In case migrations the application_settings table is not created yet,
       # we fallback to a simple OpenStruct

lib/gitlab/cycle_analytics/base_event_fetcher.rb

@@ -56,7 +56,9 @@ module Gitlab
       end
 
       def allowed_ids
-        nil
+        @allowed_ids ||= allowed_ids_finder_class
+          .new(@options[:current_user], project_id: @project.id)
+          .execute.where(id: event_result_ids).pluck(:id)
       end
 
       def event_result_ids

lib/gitlab/cycle_analytics/base_query.rb

@@ -14,9 +14,9 @@ module Gitlab
       def stage_query
         query = mr_closing_issues_table.join(issue_table).on(issue_table[:id].eq(mr_closing_issues_table[:issue_id]))
           .join(issue_metrics_table).on(issue_table[:id].eq(issue_metrics_table[:issue_id]))
-          .where(issue_table[:project_id].eq(@project.id))
+          .where(issue_table[:project_id].eq(@project.id)) # rubocop:disable Gitlab/ModuleWithInstanceVariables
           .where(issue_table[:deleted_at].eq(nil))
-          .where(issue_table[:created_at].gteq(@options[:from]))
+          .where(issue_table[:created_at].gteq(@options[:from])) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
         # Load merge_requests
         query = query.join(mr_table, Arel::Nodes::OuterJoin)

lib/gitlab/cycle_analytics/code_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class CodeEventFetcher < BaseEventFetcher
-      include MergeRequestAllowed
-
       def initialize(*args)
         @projections = [mr_table[:title],
                         mr_table[:iid],
@@ -20,6 +18,10 @@ module Gitlab
       def serialize(event)
         AnalyticsMergeRequestSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids_finder_class
+        MergeRequestsFinder
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/issue_allowed.rb

-module Gitlab
-  module CycleAnalytics
-    module IssueAllowed
-      def allowed_ids
-        @allowed_ids ||= IssuesFinder.new(@options[:current_user], project_id: @project.id).execute.where(id: event_result_ids).pluck(:id)
-      end
-    end
-  end
-end

lib/gitlab/cycle_analytics/issue_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class IssueEventFetcher < BaseEventFetcher
-      include IssueAllowed
-
       def initialize(*args)
         @projections = [issue_table[:title],
                         issue_table[:iid],
@@ -18,6 +16,10 @@ module Gitlab
       def serialize(event)
         AnalyticsIssueSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids_finder_class
+        IssuesFinder
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/merge_request_allowed.rb

-module Gitlab
-  module CycleAnalytics
-    module MergeRequestAllowed
-      def allowed_ids
-        @allowed_ids ||= MergeRequestsFinder.new(@options[:current_user], project_id: @project.id).execute.where(id: event_result_ids).pluck(:id)
-      end
-    end
-  end
-end

lib/gitlab/cycle_analytics/plan_event_fetcher.rb

@@ -19,6 +19,10 @@ module Gitlab
 
       private
 
+      def allowed_ids
+        nil
+      end
+
       def merge_request_diff_commits
         @merge_request_diff_commits ||=
           MergeRequestDiffCommit

lib/gitlab/cycle_analytics/production_helper.rb

@@ -2,7 +2,9 @@ module Gitlab
   module CycleAnalytics
     module ProductionHelper
       def stage_query
-        super.where(mr_metrics_table[:first_deployed_to_production_at].gteq(@options[:from]))
+        super
+          .where(mr_metrics_table[:first_deployed_to_production_at]
+          .gteq(@options[:from])) # rubocop:disable Gitlab/ModuleWithInstanceVariables
       end
     end
   end

lib/gitlab/cycle_analytics/review_event_fetcher.rb

 module Gitlab
   module CycleAnalytics
     class ReviewEventFetcher < BaseEventFetcher
-      include MergeRequestAllowed
-
       def initialize(*args)
         @projections = [mr_table[:title],
                         mr_table[:iid],
@@ -14,9 +12,15 @@ module Gitlab
         super(*args)
       end
 
+      private
+
       def serialize(event)
         AnalyticsMergeRequestSerializer.new(project: @project).represent(event)
       end
+
+      def allowed_ids_finder_class
+        MergeRequestsFinder
+      end
     end
   end
 end

lib/gitlab/cycle_analytics/staging_event_fetcher.rb

@@ -22,6 +22,10 @@ module Gitlab
 
       private
 
+      def allowed_ids
+        nil
+      end
+
       def serialize(event)
         AnalyticsBuildSerializer.new.represent(event['build'])
       end

lib/gitlab/database/rename_reserved_paths_migration/v1/migration_classes.rb

@@ -6,7 +6,7 @@ module Gitlab
           module Routable
             def full_path
               if route && route.path.present?
-                @full_path ||= route.path
+                @full_path ||= route.path # rubocop:disable Gitlab/ModuleWithInstanceVariables
               else
                 update_route if persisted?
 
@@ -30,7 +30,7 @@ module Gitlab
             def prepare_route
               route || build_route(source: self)
               route.path = build_full_path
-              @full_path = nil
+              @full_path = nil # rubocop:disable Gitlab/ModuleWithInstanceVariables
             end
           end
 

lib/gitlab/emoji.rb

@@ -31,8 +31,7 @@ module Gitlab
     end
 
     def emoji_unicode_version(name)
-      @emoji_unicode_versions_by_name ||= JSON.parse(File.read(Rails.root.join('fixtures', 'emojis', 'emoji-unicode-version-map.json')))
-      @emoji_unicode_versions_by_name[name]
+      emoji_unicode_versions_by_name[name]
     end
 
     def normalize_emoji_name(name)
@@ -56,5 +55,12 @@ module Gitlab
 
       ActionController::Base.helpers.content_tag('gl-emoji', emoji_info['moji'], title: emoji_info['description'], data: data)
     end
+
+    private
+
+    def emoji_unicode_versions_by_name
+      @emoji_unicode_versions_by_name ||=
+        JSON.parse(File.read(Rails.root.join('fixtures', 'emojis', 'emoji-unicode-version-map.json')))
+    end
   end
 end

lib/gitlab/git/popen.rb

@@ -16,8 +16,8 @@ module Gitlab
         vars['PWD'] = path
         options = { chdir: path }
 
-        @cmd_output = ""
-        @cmd_status = 0
+        cmd_output = ""
+        cmd_status = 0
         Open3.popen3(vars, *cmd, options) do |stdin, stdout, stderr, wait_thr|
           yield(stdin) if block_given?
           stdin.close
@@ -25,14 +25,14 @@ module Gitlab
           if lazy_block
             return lazy_block.call(stdout.lazy)
           else
-            @cmd_output << stdout.read
+            cmd_output << stdout.read
           end
 
-          @cmd_output << stderr.read
-          @cmd_status = wait_thr.value.exitstatus
+          cmd_output << stderr.read
+          cmd_status = wait_thr.value.exitstatus
         end
 
-        [@cmd_output, @cmd_status]
+        [cmd_output, cmd_status]
       end
 
       def popen_with_timeout(cmd, timeout, path, vars = {})

lib/gitlab/import_export/command_line_util.rb

@@ -32,7 +32,7 @@ module Gitlab
 
       def execute(cmd)
         output, status = Gitlab::Popen.popen(cmd)
-        @shared.error(Gitlab::ImportExport::Error.new(output.to_s)) unless status.zero?
+        @shared.error(Gitlab::ImportExport::Error.new(output.to_s)) unless status.zero? # rubocop:disable Gitlab/ModuleWithInstanceVariables
         status.zero?
       end
 

lib/gitlab/metrics/influx_db.rb

@@ -156,6 +156,7 @@ module Gitlab
 
       # When enabled this should be set before being used as the usual pattern
       # "@foo ||= bar" is _not_ thread-safe.
+      # rubocop:disable Gitlab/ModuleWithInstanceVariables
       def pool
         if influx_metrics_enabled?
           if @pool.nil?
@@ -172,6 +173,7 @@ module Gitlab
           @pool
         end
       end
+      # rubocop:enable Gitlab/ModuleWithInstanceVariables
     end
   end
 end

lib/gitlab/metrics/prometheus.rb

@@ -4,6 +4,7 @@ module Gitlab
   module Metrics
     module Prometheus
       include Gitlab::CurrentSettings
+      include Gitlab::Utils::StrongMemoize
 
       REGISTRY_MUTEX = Mutex.new
       PROVIDER_MUTEX = Mutex.new
@@ -17,16 +18,18 @@ module Gitlab
       end
 
       def prometheus_metrics_enabled?
-        return @prometheus_metrics_enabled if defined?(@prometheus_metrics_enabled)
-
-        @prometheus_metrics_enabled = prometheus_metrics_enabled_unmemoized
+        strong_memoize(:prometheus_metrics_enabled) do
+          prometheus_metrics_enabled_unmemoized
+        end
       end
 
       def registry
-        return @registry if @registry
-
-        REGISTRY_MUTEX.synchronize do
-          @registry ||= ::Prometheus::Client.registry
+        strong_memoize(:registry) do
+          REGISTRY_MUTEX.synchronize do
+            strong_memoize(:registry) do
+              ::Prometheus::Client.registry
+            end
+          end
         end
       end
 

lib/gitlab/slash_commands/presenters/issue_base.rb

@@ -11,28 +11,28 @@ module Gitlab
         end
 
         def project
-          @resource.project
+          resource.project
         end
 
         def author
-          @resource.author
+          resource.author
         end
 
         def fields
           [
             {
               title: "Assignee",
-              value: @resource.assignees.any? ? @resource.assignees.first.name : "_None_",
+              value: resource.assignees.any? ? resource.assignees.first.name : "_None_",
               short: true
             },
             {
               title: "Milestone",
-              value: @resource.milestone ? @resource.milestone.title : "_None_",
+              value: resource.milestone ? resource.milestone.title : "_None_",
               short: true
             },
             {
               title: "Labels",
-              value: @resource.labels.any? ? @resource.label_names.join(', ') : "_None_",
+              value: resource.labels.any? ? resource.label_names.join(', ') : "_None_",
               short: true
             },
             {
@@ -42,6 +42,10 @@ module Gitlab
             }
           ]
         end
+
+        private
+
+        attr_reader :resource
       end
     end
   end

lib/tasks/gettext.rake

@@ -23,6 +23,7 @@ namespace :gettext do
   desc 'Lint all po files in `locale/'
   task lint: :environment do
     require 'simple_po_parser'
+    require 'gitlab/utils'
 
     FastGettext.silence_errors
     files = Dir.glob(Rails.root.join('locale/*/gitlab.po'))

lib/tasks/gitlab/task_helpers.rb

 require 'rainbow/ext/string'
+require 'gitlab/utils/strong_memoize'
 
 module Gitlab
   TaskFailedError = Class.new(StandardError)
   TaskAbortedByUserError = Class.new(StandardError)
 
   module TaskHelpers
+    include Gitlab::Utils::StrongMemoize
+
     extend self
 
     # Ask if the user wants to continue
@@ -105,16 +108,16 @@ module Gitlab
     end
 
     def gitlab_user?
-      return @is_gitlab_user unless @is_gitlab_user.nil?
-
-      current_user = run_command(%w(whoami)).chomp
-      @is_gitlab_user = current_user == gitlab_user
+      strong_memoize(:is_gitlab_user) do
+        current_user = run_command(%w(whoami)).chomp
+        current_user == gitlab_user
+      end
     end
 
     def warn_user_is_not_gitlab
-      return if @warned_user_not_gitlab
+      return if gitlab_user?
 
-      unless gitlab_user?
+      strong_memoize(:warned_user_not_gitlab) do
         current_user = run_command(%w(whoami)).chomp
 
         puts " Warning ".color(:black).background(:yellow)
@@ -122,8 +125,6 @@ module Gitlab
         puts "  Things may work\/fail for the wrong reasons."
         puts "  For correct results you should run this as user #{gitlab_user.color(:magenta)}."
         puts ""
-
-        @warned_user_not_gitlab = true
       end
     end
 

qa/qa/runtime/scenario.rb

@@ -6,13 +6,15 @@ module QA
     module Scenario
       extend self
 
-      attr_reader :attributes
+      def attributes
+        @attributes ||= {}
+      end
 
       def define(attribute, value)
-        (@attributes ||= {}).store(attribute.to_sym, value)
+        attributes.store(attribute.to_sym, value)
 
         define_singleton_method(attribute) do
-          @attributes[attribute.to_sym].tap do |value|
+          attributes[attribute.to_sym].tap do |value|
             if value.to_s.empty?
               raise ArgumentError, "Empty `#{attribute}` attribute!"
             end

rubocop/cop/gitlab/module_with_instance_variables.rb

+module RuboCop
+  module Cop
+    module Gitlab
+      class ModuleWithInstanceVariables < RuboCop::Cop::Cop
+        MSG = <<~EOL.freeze
+          Do not use instance variables in a module. Please read this
+          for the rationale behind it:
+
+          https://docs.gitlab.com/ee/development/module_with_instance_variables.html
+        EOL
+
+        def on_module(node)
+          check_method_definition(node)
+
+          # Not sure why some module would have an extra begin wrapping around
+          node.each_child_node(:begin) do |begin_node|
+            check_method_definition(begin_node)
+          end
+        end
+
+        private
+
+        def check_method_definition(node)
+          node.each_child_node(:def) do |definition|
+            # We allow this pattern:
+            #
+            #     def f
+            #       @f ||= true
+            #     end
+            if only_ivar_or_assignment?(definition)
+              # We don't allow if any other ivar is used
+              definition.each_descendant(:ivar) do |offense|
+                add_offense(offense, :expression)
+              end
+            # We allow initialize method and single ivar
+            elsif !initialize_method?(definition) && !single_ivar?(definition)
+              definition.each_descendant(:ivar, :ivasgn) do |offense|
+                add_offense(offense, :expression)
+              end
+            end
+          end
+        end
+
+        def only_ivar_or_assignment?(definition)
+          node = definition.child_nodes.last
+
+          definition.child_nodes.size == 2 &&
+            node.or_asgn_type? && node.child_nodes.first.ivasgn_type?
+        end
+
+        def single_ivar?(definition)
+          node = definition.child_nodes.last
+
+          definition.child_nodes.size == 2 && node.ivar_type?
+        end
+
+        def initialize_method?(definition)
+          definition.children.first == :initialize
+        end
+      end
+    end
+  end
+end

rubocop/rubocop.rb

@@ -7,6 +7,7 @@ require_relative 'cop/line_break_after_guard_clauses'
 require_relative 'cop/polymorphic_associations'
 require_relative 'cop/project_path_helper'
 require_relative 'cop/redirect_with_status'
+require_relative 'cop/gitlab/module_with_instance_variables'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_concurrent_foreign_key'
 require_relative 'cop/migration/add_concurrent_index'

spec/lib/gitlab/cycle_analytics/base_event_fetcher_spec.rb

@@ -23,6 +23,8 @@ describe Gitlab::CycleAnalytics::BaseEventFetcher do
     allow_any_instance_of(described_class).to receive(:serialize) do |event|
       event
     end
+    allow_any_instance_of(described_class)
+      .to receive(:allowed_ids).and_return(nil)
 
     stub_const('Gitlab::CycleAnalytics::BaseEventFetcher::MAX_EVENTS', max_events)
 

spec/rubocop/cop/gitlab/module_with_instance_variables_spec.rb

+require 'spec_helper'
+require 'rubocop'
+require 'rubocop/rspec/support'
+require_relative '../../../../rubocop/cop/gitlab/module_with_instance_variables'
+
+describe RuboCop::Cop::Gitlab::ModuleWithInstanceVariables do
+  include CopHelper
+
+  subject(:cop) { described_class.new }
+
+  shared_examples('registering offense') do |options|
+    let(:offending_lines) { options[:offending_lines] }
+
+    it 'registers an offense when instance variable is used in a module' do
+      inspect_source(cop, source)
+
+      aggregate_failures do
+        expect(cop.offenses.size).to eq(offending_lines.size)
+        expect(cop.offenses.map(&:line)).to eq(offending_lines)
+      end
+    end
+  end
+
+  shared_examples('not registering offense') do
+    it 'does not register offenses' do
+      inspect_source(cop, source)
+
+      expect(cop.offenses).to be_empty
+    end
+  end
+
+  context 'when source is a regular module' do
+    it_behaves_like 'registering offense', offending_lines: [3] do
+      let(:source) do
+        <<~RUBY
+          module M
+            def f
+              @f = true
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is a nested module' do
+    it_behaves_like 'registering offense', offending_lines: [4] do
+      let(:source) do
+        <<~RUBY
+          module N
+            module M
+              def f
+                @f = true
+              end
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is a nested module with multiple offenses' do
+    it_behaves_like 'registering offense', offending_lines: [4, 12] do
+      let(:source) do
+        <<~RUBY
+          module N
+            module M
+              def f
+                @f = true
+              end
+
+              def g
+                true
+              end
+
+              def h
+                @h = true
+              end
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is using simple or ivar assignment' do
+    it_behaves_like 'not registering offense' do
+      let(:source) do
+        <<~RUBY
+          module M
+            def f
+              @f ||= true
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is using simple ivar' do
+    it_behaves_like 'not registering offense' do
+      let(:source) do
+        <<~RUBY
+          module M
+            def f?
+              @f
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is defining initialize' do
+    it_behaves_like 'not registering offense' do
+      let(:source) do
+        <<~RUBY
+          module M
+            def initialize
+              @a = 1
+              @b = 2
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is using simple or ivar assignment with other ivar' do
+    it_behaves_like 'registering offense', offending_lines: [3] do
+      let(:source) do
+        <<~RUBY
+          module M
+            def f
+              @f ||= g(@g)
+            end
+          end
+        RUBY
+      end
+    end
+  end
+
+  context 'when source is using or ivar assignment with something else' do
+    it_behaves_like 'registering offense', offending_lines: [3, 4] do
+      let(:source) do
+        <<~RUBY
+          module M
+            def f
+              @f ||= true
+              @f.to_s
+            end
+          end
+        RUBY
+      end
+    end
+  end
+end