Restore support for the -layers transformation

activestorage/app/models/active_storage/variation.rb

@@ -46,15 +46,15 @@ def initialize(transformations)
   # Accepts an open MiniMagick image instance, like what's returned by <tt>MiniMagick::Image.read(io)</tt>,
   # and performs the +transformations+ against it. The transformed image instance is then returned.
   def transform(image)
-    transformations.each do |(transformation_method, transformation_argument)|
-      if transformation_method.to_s == "combine_options"
-        image.combine_options do |combination|
-          transformation_argument.each do |(method, argument)|
-            pass_transform_argument(combination, method, argument)
+    transformations.each do |name, argument_or_subtransformations|
+      image.mogrify do |command|
+        if name.to_s == "combine_options"
+          argument_or_subtransformations.each do |subtransformation_name, subtransformation_argument|
+            pass_transform_argument(command, subtransformation_name, subtransformation_argument)
           end
+        else
+          pass_transform_argument(command, name, argument_or_subtransformations)
         end
-      else
-        pass_transform_argument(image, transformation_method, transformation_argument)
       end
     end
   end
@@ -65,15 +65,15 @@ def key
   end
 
   private
-    def eligible_argument?(argument)
-      argument.present? && argument != true
-    end
-
-    def pass_transform_argument(instance, method, argument)
+    def pass_transform_argument(command, method, argument)
       if eligible_argument?(argument)
-        instance.public_send(method, argument)
+        command.public_send(method, argument)
       else
-        instance.public_send(method)
+        command.public_send(method)
       end
     end
+
+    def eligible_argument?(argument)
+      argument.present? && argument != true
+    end
 end

activestorage/test/models/variant_test.rb

@@ -50,6 +50,14 @@ class ActiveStorage::VariantTest < ActiveSupport::TestCase
     assert_equal 20, image.height
   end
 
+  test "optimized variation of GIF blob" do
+    blob = create_file_blob(filename: "image.gif", content_type: "image/gif")
+
+    assert_nothing_raised do
+      blob.variant(layers: "Optimize").processed
+    end
+  end
+
   test "variation of invariable blob" do
     assert_raises ActiveStorage::Blob::InvariableError do
       create_file_blob(filename: "report.pdf", content_type: "application/pdf").variant(resize: "100x100")