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.

1 files changed, 29 insertions, 9 deletions

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'