refactor keeping multiple copies of a specific cache key into a helper class

Change-Id: I7fd54391837e70fcb21b16573e2eb73cf533eb03
Reviewed-on: https://gerrit.instructure.com/42490
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

app/models/account.rb

@@ -736,17 +736,13 @@ class Account < ActiveRecord::Base
     state :deleted
   end
 
-  def self.all_site_admin_account_users_copies
-    [Setting.get('all_site_admin_account_users_copies', 1).to_i, 1].max
-  end
-
   def account_users_for(user)
     return [] unless user
     @account_users_cache ||= {}
     if self == Account.site_admin
       shard.activate do
         @account_users_cache[user.global_id] ||= begin
-          all_site_admin_account_users_hash = Rails.cache.fetch("all_site_admin_account_users2:#{rand(Account.all_site_admin_account_users_copies)}") do
+          all_site_admin_account_users_hash = MultiCache.fetch("all_site_admin_account_users2") do
             # this is a plain ruby hash to keep the cached portion as small as possible
             self.account_users.inject({}) { |result, au| result[au.user_id] ||= []; result[au.user_id] << [au.id, au.membership_type]; result }
           end

app/observers/cacher.rb

@@ -28,9 +28,7 @@ class Cacher < ActiveRecord::Observer
     when AccountUser
       if obj.account_id == Account.site_admin.id
         Shard.default.activate do
-          (0...Account.all_site_admin_account_users_copies).each do |i|
-            Rails.cache.delete("all_site_admin_account_users2:#{i}")
-          end
+          MultiCache.delete("all_site_admin_account_users2")
         end
       end
     end

lib/multi_cache.rb

@@ -0,0 +1,33 @@
+#
+# Copyright (C) 2014 Instructure, Inc.
+#
+# This file is part of Canvas.
+#
+# Canvas is free software: you can redistribute it and/or modify it under
+# the terms of the GNU Affero General Public License as published by the Free
+# Software Foundation, version 3 of the License.
+#
+# Canvas is distributed in the hope that it will be useful, but WITHOUT ANY
+# WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+# A PARTICULAR PURPOSE. See the GNU Affero General Public License for more
+# details.
+#
+# You should have received a copy of the GNU Affero General Public License along
+# with this program. If not, see <http://www.gnu.org/licenses/>.
+#
+
+module MultiCache
+  def self.copies(key)
+    [Setting.get("#{key}_copies", 1).to_i, 1].max
+  end
+
+  def self.fetch(key, &block)
+    Rails.cache.fetch("#{key}:#{rand(copies(key))}", &block)
+  end
+
+  def self.delete(key)
+    (0...copies(key)).each do |i|
+      Rails.cache.delete("#{key}:#{i}")
+    end
+  end
+end