Move file duplication code to concern to share

I chose `has_one_migrating` as method name for two reasons:

 1. It reflects Active Storage's method `has_one_attached`.
 2. And it has the same length as Paperclip's `has_attached_file`.
    Therefore the commits don't need any whitespace changes.

When we change it to `has_one_attached`, we will also remove the
Paperclip options which then don't need whitespace changes either.

app/models/concerns/has_migrating_file.rb

@@ -0,0 +1,57 @@
+# frozen_string_literal: true
+
+module HasMigratingFile
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def has_one_migrating(name, paperclip_options = {})
+      # Active Storage declaration
+      has_one_attached name
+
+      # Backup Active Storage methods before they get overridden by Paperclip.
+      alias_method "active_storage_#{name}", name
+      alias_method "active_storage_#{name}=", "#{name}="
+
+      # Paperclip declaration
+      #
+      # This will define the `name` and `name=` methods as well.
+      has_attached_file name, paperclip_options
+
+      # Paperclip callback to duplicate file with Active Storage
+      #
+      # We store files with Paperclip *and* Active Storage while we migrate
+      # old Paperclip files to Active Storage. This enables availability
+      # during the migration.
+      after_post_process do
+        if public_send(name).errors.blank?
+          file = File.open(processed_local_file_path)
+          attach_file(name, file)
+        end
+      end
+    end
+  end
+
+  def attach_file(name, io)
+    attachable = {
+      io: io,
+      filename: public_send("#{name}_file_name"),
+      content_type: public_send("#{name}_content_type"),
+      identify: false,
+    }
+    public_send("active_storage_#{name}=", attachable)
+  end
+
+  private
+
+  def processed_local_file_path(name)
+    attachment = public_send(name)
+
+    temporary = attachment.queued_for_write[:original]
+
+    if temporary&.path.present?
+      temporary.path
+    else
+      attachment.path
+    end
+  end
+end

app/models/spree/image.rb

@@ -4,27 +4,18 @@ require 'spree/core/s3_support'
 
 module Spree
   class Image < Asset
+    include HasMigratingFile
+
     validates_attachment_presence :attachment
     validate :no_attachment_errors
 
-    # Active Storage declaration
-    has_one_attached :attachment
-
-    # Backup Active Storage methods before they get overridden by Paperclip.
-    alias_method :active_storage_attachment, :attachment
-    alias_method :active_storage_attachment=, :attachment=
-
-    # Paperclip declaration
-    #
-    # This will define the `name` and `name=` methods as well.
-    #
     # This is where the styles are used in the app:
     # - mini: used in the BackOffice: Bulk Product Edit page and Order Cycle edit page
     # - small: used in the FrontOffice: Product List page
     # - product: used in the BackOffice: Product Image upload modal in the Bulk Product Edit page
     #                                      and Product image edit page
     # - large: used in the FrontOffice: product modal
-    has_attached_file :attachment,
+    has_one_migrating :attachment,
                       styles: { mini: "48x48#", small: "227x227#",
                                 product: "240x240>", large: "600x600>" },
                       default_style: :product,
@@ -32,18 +23,6 @@ module Spree
                       path: ':rails_root/public/spree/products/:id/:style/:basename.:extension',
                       convert_options: { all: '-strip -auto-orient -colorspace sRGB' }
 
-    after_post_process do
-      if attachment.errors.blank?
-        attachable = {
-          io: File.open(local_filename_of_original),
-          filename: attachment_file_name,
-          content_type: attachment_content_type,
-          identify: false,
-        }
-        self.active_storage_attachment = attachable
-      end
-    end
-
     # save the w,h of the original image (from which others can be calculated)
     # we need to look at the write-queue for images which have not been saved yet
     after_post_process :find_dimensions