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

Add cop to make sure we don't use ivar in a module

See merge request gitlab-org/gitlab-ce!12800

.rubocop.yml

@@ -1185,7 +1185,20 @@ 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

@@ -1,6 +1,8 @@
 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

@@ -1,4 +1,5 @@
 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
@@ -81,7 +81,7 @@ module IssuableActions
   private
 
   def recaptcha_check_if_spammable(should_redirect = true, &block)
-    return yield unless @issuable.is_a? Spammable
+    return yield unless issuable.is_a? Spammable
 
     recaptcha_check_with_fallback(should_redirect, &block)
   end
@@ -89,7 +89,7 @@ module IssuableActions
   def render_conflict_response
     respond_to do |format|
       format.html do
-        @conflict = true
+        @conflict = true # rubocop:disable Gitlab/ModuleWithInstanceVariables
         render :edit
       end
 
@@ -104,7 +104,7 @@ 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!
@@ -114,7 +114,7 @@ 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
@@ -148,6 +148,7 @@ module IssuableActions
     @resource_name ||= controller_name.singularize
   end
 
+  # rubocop:disable Gitlab/ModuleWithInstanceVariables
   def render_entity_json
     if @issuable.valid?
       render json: serializer.represent(@issuable)
@@ -155,6 +156,7 @@ module IssuableActions
       render json: { errors: @issuable.errors.full_messages }, status: :unprocessable_entity
     end
   end
+  # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
   def serializer
     raise NotImplementedError
@@ -165,6 +167,6 @@ 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?
@@ -129,9 +134,9 @@ 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

@@ -1,5 +1,6 @@
 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

@@ -1,6 +1,7 @@
 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

@@ -1,6 +1,6 @@
 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

@@ -1,4 +1,5 @@
 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

@@ -66,7 +66,7 @@ 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_PARAMS_CE + 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(should_redirect = true, &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

@@ -1,24 +1,28 @@
 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

@@ -0,0 +1,242 @@
+## 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

@@ -42,11 +42,11 @@ 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

@@ -32,6 +32,11 @@ 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)
 
@@ -45,6 +50,7 @@ module API
 
       @current_user
     end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
     def sudo?
       initial_current_user != current_user
@@ -415,6 +421,7 @@ module API
 
     private
 
+    # rubocop:disable Gitlab/ModuleWithInstanceVariables
     def initial_current_user
       return @initial_current_user if defined?(@initial_current_user)
 
@@ -424,6 +431,7 @@ module API
         unauthorized!
       end
     end
+    # rubocop:enable Gitlab/ModuleWithInstanceVariables
 
     def sudo!
       return unless sudo_identifier
@@ -443,7 +451,7 @@ 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, Gitlab::GitalyClient::MigrationStatus::OPT_OUT]
       }.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"]
@@ -130,14 +131,15 @@ 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
 
   def lfs_blob_ids
     blob_ids = tree.blobs.map(&:id)
-    @lfs_blob_ids = Gitlab::Git::Blob.batch_lfs_pointers(@project.repository, blob_ids).map(&:id)
+    @lfs_blob_ids = Gitlab::Git::Blob.batch_lfs_pointers(@project.repository, blob_ids).map(&:id) # rubocop:disable Gitlab/ModuleWithInstanceVariables
   end
 
   private
@@ -150,8 +152,8 @@ 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/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

@@ -1,8 +1,6 @@
 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

@@ -1,9 +0,0 @@
-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

@@ -1,8 +1,6 @@
 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

@@ -1,9 +0,0 @@
-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

@@ -18,6 +18,10 @@ 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

@@ -1,8 +1,6 @@
 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

@@ -154,6 +154,7 @@ 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?
@@ -170,6 +171,7 @@ 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

@@ -11,32 +11,36 @@ 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
             }
           ]
         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

@@ -1,10 +1,13 @@
 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

@@ -0,0 +1,63 @@
+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

@@ -8,6 +8,7 @@ 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/sidekiq_options_queue'
 require_relative 'cop/migration/add_column'
 require_relative 'cop/migration/add_concurrent_foreign_key'

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

@@ -0,0 +1,157 @@
+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