Make ActionController::Caching::Sweeper class thread safe

Gemfile

@@ -2,6 +2,7 @@ source 'https://rubygems.org'
 
 # Specify your gem's dependencies in active_record-observers.gemspec
 gemspec
+gem "minitest", "~> 5.8.4"
 gem 'rails', github: 'rails/rails', branch: '5-1-stable'
 gem 'activeresource', github: 'rails/activeresource'
 

gemfiles/rails_4_2.gemfile

@@ -1,6 +1,7 @@
 source "https://rubygems.org"
 
 gem "mocha", require: false
+gem "minitest", "~> 5.8.4"
 gem "rails", "~> 4.2.0"
 
 gemspec path: "../"

gemfiles/rails_5_0.gemfile

@@ -2,6 +2,7 @@ source "https://rubygems.org"
 
 # Specify your gem"s dependencies in active_record-observers.gemspec
 gem "rails", github: "rails/rails", branch: "5-0-stable"
+gem "minitest", "~> 5.8.4"
 gem "activeresource", github: "rails/activeresource"
 
 gem "mocha", require: false

gemfiles/rails_master.gemfile

@@ -2,6 +2,7 @@ source "https://rubygems.org"
 
 # Specify your gem"s dependencies in active_record-observers.gemspec
 gem "rails", github: "rails/rails"
+gem "minitest", "~> 5.8.4"
 gem "activeresource", github: "rails/activeresource"
 
 gem "mocha", require: false

lib/rails/observers/action_controller/caching/sweeper.rb

@@ -1,11 +1,17 @@
 module ActionController #:nodoc:
   module Caching
     class Sweeper < ActiveRecord::Observer #:nodoc:
-      attr_accessor :controller
-
       def initialize(*args)
         super
-        @controller = nil
+        self.controller = nil
+      end
+
+      def controller
+        Thread.current["observer:#{self.class.name}_controller"]
+      end
+
+      def controller=(controller)
+        Thread.current["observer:#{self.class.name}_controller"] = controller
       end
 
       def before(controller)
@@ -53,8 +59,12 @@ def callback(timing)
       end
 
       def method_missing(method, *arguments, &block)
-        return super unless @controller
-        @controller.__send__(method, *arguments, &block)
+        return super if controller.nil?
+        controller.__send__(method, *arguments, &block)
+      end
+
+      def respond_to_missing?(method, include_private = false)
+        (controller.present? && controller.respond_to?(method)) || super
       end
     end
   end

test/helper.rb

@@ -1,5 +1,5 @@
 require 'minitest/autorun'
-require 'mocha/mini_test'
+require 'mocha/minitest'
 require 'active_record'
 require 'rails'
 require 'rails/observers/activerecord/active_record'