A safe way to create class variables in a multisite environment.

This should allow plugins to set class variables that will not
stomp on other plugins.

app/models/group.rb

@@ -1,13 +1,14 @@
 # frozen_string_literal: true
 
 require_dependency 'enum'
+require_dependency 'multisite_class_var'
 
 class Group < ActiveRecord::Base
   include HasCustomFields
   include AnonCacheInvalidator
+  include MultisiteClassVar
 
-  cattr_accessor :preloaded_custom_field_names
-  self.preloaded_custom_field_names = Set.new
+  multisite_class_var(:preloaded_custom_field_names) { Set.new }
 
   has_many :category_groups, dependent: :destroy
   has_many :group_users, dependent: :destroy

app/models/post.rb

@@ -6,6 +6,7 @@ require_dependency 'post_analyzer'
 require_dependency 'validators/post_validator'
 require_dependency 'plugin/filter'
 require_dependency 'email_cook'
+require_dependency 'multisite_class_var'
 
 require 'archetype'
 require 'digest/sha1'
@@ -16,9 +17,9 @@ class Post < ActiveRecord::Base
   include Searchable
   include HasCustomFields
   include LimitedEdit
+  include MultisiteClassVar
 
-  cattr_accessor :permitted_create_params
-  self.permitted_create_params = Set.new
+  multisite_class_var(:permitted_create_params) { Set.new }
 
   # increase this number to force a system wide post rebake
   BAKED_VERSION = 1

app/models/topic_list.rb

@@ -1,29 +1,24 @@
 require_dependency 'avatar_lookup'
 require_dependency 'primary_group_lookup'
+require_dependency 'multisite_class_var'
 
 class TopicList
   include ActiveModel::Serialization
+  include MultisiteClassVar
 
-  cattr_accessor :preloaded_custom_fields
-  self.preloaded_custom_fields = Set.new
+  multisite_class_var(:preloaded_custom_fields) { Set.new }
+  multisite_class_var(:preload_callbacks) { Set.new }
 
   def self.on_preload(&blk)
-    (@preload ||= Set.new) << blk
+    preload_callbacks << blk
   end
 
   def self.cancel_preload(&blk)
-    if @preload
-      @preload.delete blk
-      if @preload.length == 0
-        @preload = nil
-      end
-    end
+    preload_callbacks.delete(blk)
   end
 
   def self.preload(topics, object)
-    if @preload
-      @preload.each { |preload| preload.call(topics, object) }
-    end
+    preload_callbacks.each { |p| p.call(topics, object) }
   end
 
   attr_accessor :more_topics_url,
@@ -117,8 +112,8 @@ class TopicList
       ft.topic_list = self
     end
 
-    if preloaded_custom_fields.present?
-      Topic.preload_custom_fields(@topics, preloaded_custom_fields)
+    if self.class.preloaded_custom_fields.present?
+      Topic.preload_custom_fields(@topics, self.class.preloaded_custom_fields)
     end
 
     TopicList.preload(@topics, self)

lib/multisite_class_var.rb

@@ -0,0 +1,20 @@
+# Support for a class variable that is multisite aware.
+
+module MultisiteClassVar
+
+  def self.included(base)
+    base.extend ClassMethods
+  end
+
+  module ClassMethods
+    def multisite_class_var(name, &default)
+      @multisite_class_vars ||= {}
+      @multisite_class_vars[name] = {}
+
+      define_singleton_method(name) do
+        @multisite_class_vars[name][RailsMultisite::ConnectionManagement.current_db] ||= default.call
+      end
+    end
+  end
+
+end

lib/topic_view.rb

@@ -2,8 +2,10 @@ require_dependency 'guardian'
 require_dependency 'topic_query'
 require_dependency 'filter_best_posts'
 require_dependency 'gaps'
+require_dependency 'multisite_class_var'
 
 class TopicView
+  include MultisiteClassVar
 
   attr_reader :topic, :posts, :guardian, :filtered_posts, :chunk_size, :print, :message_bus_last_id
   attr_accessor :draft, :draft_key, :draft_sequence, :user_custom_fields, :post_custom_fields
@@ -24,9 +26,7 @@ class TopicView
     @default_post_custom_fields ||= ["action_code_who"]
   end
 
-  def self.post_custom_fields_whitelisters
-    @post_custom_fields_whitelisters ||= Set.new
-  end
+  multisite_class_var(:post_custom_fields_whitelisters) { Set.new }
 
   def self.add_post_custom_fields_whitelister(&block)
     post_custom_fields_whitelisters << block

spec/multisite/multisite_class_var_spec.rb

@@ -0,0 +1,65 @@
+require 'rails_helper'
+require 'multisite_class_var'
+
+RSpec.describe MultisiteClassVar do
+
+  it "will add the class variables" do
+    class_with_set = Class.new do
+      include MultisiteClassVar
+      multisite_class_var(:class_set) { Set.new }
+      multisite_class_var(:class_array) { Array.new }
+    end
+
+    class_with_set.class_set << 'a'
+    class_with_set.class_array << 'c'
+
+    expect(class_with_set.class_set).to contain_exactly('a')
+    expect(class_with_set.class_array).to contain_exactly('c')
+  end
+
+  context "multisite environment" do
+    let(:conn) { RailsMultisite::ConnectionManagement }
+
+    before do
+      @original_provider = SiteSetting.provider
+      SiteSetting.provider = SiteSettings::DbProvider.new(SiteSetting)
+      conn.config_filename = "spec/fixtures/multisite/two_dbs.yml"
+      conn.load_settings!
+      conn.remove_class_variable(:@@current_db)
+    end
+
+    after do
+      [:@@db_spec_cache, :@@host_spec_cache, :@@default_spec].each do |class_variable|
+        conn.remove_class_variable(class_variable)
+      end
+      conn.set_current_db
+    end
+
+    it "keeps the variable specific to the current site" do
+      class_with_set = Class.new do
+        include MultisiteClassVar
+        multisite_class_var(:class_set) { Set.new }
+      end
+
+      conn.with_connection('default') do
+        expect(class_with_set.class_set).to be_blank
+        class_with_set.class_set << 'item0'
+      end
+
+      conn.with_connection('second') do
+        expect(class_with_set.class_set).to be_blank
+        class_with_set.class_set << 'item1'
+      end
+
+      conn.with_connection('default') do
+        expect(class_with_set.class_set).to contain_exactly('item0')
+      end
+
+      conn.with_connection('second') do
+        expect(class_with_set.class_set).to contain_exactly('item1')
+      end
+
+    end
+  end
+
+end