Add a new I18n.isolation_scope= option to explicitly use threads or fibers

lib/i18n.rb

@@ -55,13 +55,30 @@ def self.reserved_keys_pattern # :nodoc:
   module Base
     # Gets I18n configuration object.
     def config
-      Thread.current.thread_variable_get(:i18n_config) ||
-        Thread.current.thread_variable_set(:i18n_config, I18n::Config.new)
+      case isolation_scope
+      when :thread
+        Thread.current.thread_variable_get(:i18n_config) || self.config = I18n::Config.new
+      when :fiber
+        Fiber[:i18n_config] || self.config = I18n::Config.new
+      end
     end
 
     # Sets I18n configuration object.
     def config=(value)
-      Thread.current.thread_variable_set(:i18n_config, value)
+      case isolation_scope
+      when :thread
+        Thread.current.thread_variable_set(:i18n_config, value)
+      when :fiber
+        Fiber[:i18n_config] = value
+      end
+    end
+
+    def isolation_scope
+      @@isolation_scope ||= :thread
+    end
+
+    def isolation_scope=(scope)
+      @@isolation_scope = scope
     end
 
     # Write methods which delegates to the configuration object

test/i18n_test.rb

@@ -63,6 +63,20 @@ def setup
     I18n.locale = :en
   end
 
+  test "isolation_scope= can set the current isolation scope to Fiber[]" do
+    begin
+      assert_nothing_raised {
+        I18n.isolation_scope = :fiber
+        I18n.locale = 'de'
+      }
+      assert_equal :de, I18n.locale
+      assert_equal :de, Fiber[:i18n_config].locale
+      I18n.locale = :en
+    ensure
+      I18n.isolation_scope = :thread
+    end
+  end
+
   test "locale= doesn't ignore junk" do
     assert_raises(NoMethodError) { I18n.locale = Class }
   end
@@ -549,7 +563,7 @@ def call(exception, locale, key, options); key; end
     end
   end
 
-  test "I18n.locale is preserved in Fiber context" do
+  test "I18n.locale is preserved in Fiber context when using Thread locals" do
     I18n.available_locales = [:en, :ja]
     I18n.default_locale = :ja
     I18n.locale = :en
@@ -561,4 +575,22 @@ def call(exception, locale, key, options); key; end
 
     assert_equal :en, fiber_locale
   end
+
+  test "I18n.locale is preserved in Fiber context when using Fiber locals" do
+    begin
+      I18n.isolation_scope = :fiber
+      I18n.available_locales = [:en, :ja]
+      I18n.default_locale = :ja
+      I18n.locale = :en
+
+      fiber_locale = nil
+      Fiber.new do
+        fiber_locale = I18n.locale
+      end.resume
+
+      assert_equal :en, fiber_locale
+    ensure
+      I18n.isolation_scope = :thread
+    end
+  end
 end