Enabled feature flag for fast deletions

Fast destroy is used only if the feature flag is enabled, otherwise
uploads are still deleted using carrier wave. It's disabled by default.

app/models/concerns/with_uploads.rb

@@ -18,6 +18,7 @@
 module WithUploads
   extend ActiveSupport::Concern
   include FastDestroyAll::Helpers
+  include FeatureGate
   # Currently there is no simple way how to select only not-mounted
   # uploads, it should be all FileUploaders so we select them by
@@ -26,12 +27,44 @@ module WithUploads
   included do
     has_many :uploads, as: :model
-    has_many :file_uploads, -> { where(uploader: FILE_UPLOADERS) }, class_name: 'Upload', as: :model, dependent: :delete_all # rubocop:disable Cop/ActiveRecordDependent
+    has_many :file_uploads, -> { where(uploader: FILE_UPLOADERS) }, class_name: 'Upload', as: :model
+
+    # TODO: when feature flag is removed, we can use just dependent: destroy
+    # option on :file_uploads
+    before_destroy :remove_file_uploads
     use_fast_destroy :file_uploads
   end
+  def perform_fast_destroy(subject)
+    super if fast_destroy_enabled?
+  end
+
   def retrieve_upload(_identifier, paths)
     uploads.find_by(path: paths)
   end
+
+  private
+
+  # mounted uploads are deleted in carrierwave's after_commit hook,
+  # but FileUploaders which are not mounted must be deleted explicitly and
+  # it can not be done in after_commit because FileUploader requires loads
+  # associated model on destroy (which is already deleted in after_commit)
+  def remove_file_uploads
+    fast_destroy_enabled? ? delete_uploads : destroy_uploads
+  end
+
+  def delete_uploads
+    file_uploads.delete_all(:delete_all)
+  end
+
+  def destroy_uploads
+    file_uploads.find_each do |upload|
+      upload.destroy
+    end
+  end
+
+  def fast_destroy_enabled?
+    Feature.enabled?(:fast_destroy_uploads, self)
+  end
 end

app/models/uploads/local.rb

@@ -3,7 +3,7 @@
 module Uploads
   class Local < Base
     def keys(relation)
-      relation.includes(:model).find_each.map {|u| u.absolute_path }
+      relation.includes(:model).find_each.map(&:absolute_path)
     end
     def delete_keys(keys)

changelogs/unreleased/fast-upload-delete.yml

----
-title: Delete file uploads asynchronously when deleting a project or other resources.
-merge_request: 20977
-author:
-type: added

spec/support/shared_examples/models/with_uploads_shared_examples.rb

@@ -44,6 +44,26 @@ shared_examples_for 'model with uploads' do |supports_fileuploads|
           model_object.destroy
         end
       end
+
+      describe 'destroy strategy depending on feature flag' do
+        let!(:upload) { create(:upload, uploader: FileUploader, model: model_object) }
+
+        it 'does not destroy uploads by default' do
+          expect(model_object).to receive(:delete_uploads)
+          expect(model_object).not_to receive(:destroy_uploads)
+
+          model_object.destroy
+        end
+
+        it 'uses before destroy callback if feature flag is disabled' do
+          stub_feature_flags(fast_destroy_uploads: false)
+
+          expect(model_object).to receive(:destroy_uploads)
+          expect(model_object).not_to receive(:delete_uploads)
+
+          model_object.destroy
+        end
+      end
     end
   end
 end