Introduce #with_formats_and_variants to prevent problems with mutating finder object

actionview/lib/action_view/digestor.rb

@@ -122,11 +122,8 @@ module ActionView
 
       def template
         @template ||= begin
-          finder.formats  = [format]
-          finder.variants = [variant]
-
-          finder.disable_cache do
-            finder.find(logical_name, [], partial?)
+          finder.with_formats_and_variants([format], [variant]) do
+            finder.disable_cache { finder.find(logical_name, [], partial?) }
           end
         end
       end

actionview/lib/action_view/lookup_context.rb

@@ -246,5 +246,13 @@ module ActionView
         end
       end
     end
+
+    def with_formats_and_variants(new_formats, new_variants)
+      old_formats, old_variants = formats, variants
+      self.formats, self.variants = new_formats, new_variants
+      yield
+    ensure
+      self.formats, self.variants = old_formats, old_variants
+    end
   end
 end

actionview/test/template/digestor_test.rb

@@ -40,6 +40,14 @@ class FixtureFinder
   def disable_cache(&block)
     yield
   end
+
+  def with_formats_and_variants(new_formats, new_variants)
+    old_formats, old_variants = formats, variants
+    self.formats, self.variants = new_formats, new_variants
+    yield
+  ensure
+    self.formats, self.variants = old_formats, old_variants
+  end
 end
 
 class TemplateDigestorTest < ActionView::TestCase

actionview/test/template/lookup_context_test.rb

@@ -205,6 +205,19 @@ class LookupContextTest < ActiveSupport::TestCase
     @lookup_context.prefixes = ["foo"]
     assert_equal ["foo"], @lookup_context.prefixes
   end
+
+  test "with_formats_and_variants preserves original values after execution" do
+    @lookup_context.formats = [:html]
+    @lookup_context.variants = [:phone]
+
+    @lookup_context.with_formats_and_variants([:xml], [:tablet]) do
+      assert_equal [:xml], @lookup_context.formats
+      assert_equal [:tablet], @lookup_context.variants
+    end
+
+    assert_equal [:html], @lookup_context.formats
+    assert_equal [:phone], @lookup_context.variants
+  end
 end
 
 class LookupContextWithFalseCaching < ActiveSupport::TestCase