From 1fab2002e3385c40ef48008b649d78ce5e16a868 Mon Sep 17 00:00:00 2001
From: Nikita Afanasenko <nikita@afanasenko.name>
Date: Wed, 14 Nov 2012 09:58:33 +0400
Subject: Make XmlMini.with_backend usable with threads

`XmlMini.with_backend` now may be safely used with threads:

  Thread.new do
    XmlMini.with_backend("REXML") { rexml_power }
  end
  Thread.new do
    XmlMini.with_backend("LibXML") { libxml_power }
  end

Each thread will use it's own backend.
---
 activesupport/CHANGELOG.md                   | 13 ++++++
 activesupport/lib/active_support/xml_mini.rb | 38 +++++++++++++----
 activesupport/test/xml_mini_test.rb          | 62 ++++++++++++++++++++++++++++
 3 files changed, 104 insertions(+), 9 deletions(-)

diff --git a/activesupport/CHANGELOG.md b/activesupport/CHANGELOG.md
index 7daa0d5bca..b4c49918d1 100644
--- a/activesupport/CHANGELOG.md
+++ b/activesupport/CHANGELOG.md
@@ -1,5 +1,18 @@
 ## Rails 4.0.0 (unreleased) ##
 
+*   `XmlMini.with_backend` now may be safely used with threads:
+
+        Thread.new do
+          XmlMini.with_backend("REXML") { rexml_power }
+        end
+        Thread.new do
+          XmlMini.with_backend("LibXML") { libxml_power }
+        end
+
+    Each thread will use it's own backend.
+
+    *Nikita Afanasenko*
+
 *   Dependencies no longer trigger Kernel#autoload in remove_const [fixes #8213]. *Xavier Noria*
 
 *   Simplify mocha integration and remove monkey-patches, bumping mocha to 0.13.0. *James Mead*
diff --git a/activesupport/lib/active_support/xml_mini.rb b/activesupport/lib/active_support/xml_mini.rb
index 88f9acb588..d082a0a499 100644
--- a/activesupport/lib/active_support/xml_mini.rb
+++ b/activesupport/lib/active_support/xml_mini.rb
@@ -76,23 +76,24 @@ module ActiveSupport
       )
     end
 
-    attr_reader :backend
     delegate :parse, :to => :backend
 
+    def backend
+      current_thread_backend || @backend
+    end
+
     def backend=(name)
-      if name.is_a?(Module)
-        @backend = name
-      else
-        require "active_support/xml_mini/#{name.downcase}"
-        @backend = ActiveSupport.const_get("XmlMini_#{name}")
-      end
+      backend = name && cast_backend_name_to_module(name)
+      self.current_thread_backend = backend if current_thread_backend
+      @backend = backend
     end
 
     def with_backend(name)
-      old_backend, self.backend = backend, name
+      old_backend = current_thread_backend
+      self.current_thread_backend = name && cast_backend_name_to_module(name)
       yield
     ensure
-      self.backend = old_backend
+      self.current_thread_backend = old_backend
     end
 
     def to_tag(key, value, options)
@@ -163,6 +164,25 @@ module ActiveSupport
       f.content_type = entity['content_type']
       f
     end
+
+    private
+
+      def current_thread_backend
+        Thread.current[:xml_mini_backend]
+      end
+
+      def current_thread_backend=(name)
+        Thread.current[:xml_mini_backend] = name && cast_backend_name_to_module(name)
+      end
+
+      def cast_backend_name_to_module(name)
+        if name.is_a?(Module)
+          name
+        else
+          require "active_support/xml_mini/#{name.downcase}"
+          ActiveSupport.const_get("XmlMini_#{name}")
+        end
+      end
   end
 
   XmlMini.backend = 'REXML'
diff --git a/activesupport/test/xml_mini_test.rb b/activesupport/test/xml_mini_test.rb
index 504fc96493..a025279e16 100644
--- a/activesupport/test/xml_mini_test.rb
+++ b/activesupport/test/xml_mini_test.rb
@@ -99,4 +99,66 @@ module XmlMiniTest
     end
     # TODO: test the remaining functions hidden in #to_tag.
   end
+
+  class WithBackendTest < ActiveSupport::TestCase
+    module REXML end
+    module LibXML end
+    module Nokogiri end
+
+    setup do
+      @xml = ActiveSupport::XmlMini
+    end
+
+    test "#with_backend should switch backend and then switch back" do
+      @xml.backend = REXML
+      @xml.with_backend(LibXML) do
+        assert_equal LibXML, @xml.backend
+        @xml.with_backend(Nokogiri) do
+          assert_equal Nokogiri, @xml.backend
+        end
+        assert_equal LibXML, @xml.backend
+      end
+      assert_equal REXML, @xml.backend
+    end
+
+    test "backend switch inside #with_backend block" do
+      @xml.with_backend(LibXML) do
+        @xml.backend = REXML
+        assert_equal REXML, @xml.backend
+      end
+      assert_equal REXML, @xml.backend
+    end
+  end
+
+  class ThreadSafetyTest < ActiveSupport::TestCase
+    module REXML end
+    module LibXML end
+
+    setup do
+      @xml = ActiveSupport::XmlMini
+    end
+
+    test "#with_backend should be thread-safe" do
+      @xml.backend = REXML
+      t = Thread.new do
+        @xml.with_backend(LibXML) { sleep 1 }
+      end
+      sleep 0.1 while t.status != "sleep"
+
+      # We should get `old_backend` here even while another
+      # thread is using `new_backend`.
+      assert_equal REXML, @xml.backend
+    end
+
+    test "nested #with_backend should be thread-safe" do
+      @xml.with_backend(REXML) do
+        t = Thread.new do
+          @xml.with_backend(LibXML) { sleep 1 }
+        end
+        sleep 0.1 while t.status != "sleep"
+
+        assert_equal REXML, @xml.backend
+      end
+    end
+  end
 end
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