Keep new and removed state of non-working days on error state

app/controllers/admin/settings/working_days_and_hours_settings_controller.rb

@@ -96,13 +96,46 @@ def removed_non_working_day_date(non_working_day_params)
     end
 
     def modified_non_working_days_for(result)
-      return if result.nil?
+      records = non_working_day_records_from(result)
+      return if records.blank?
 
-      result.map do |record|
+      records.map do |record|
         json_attributes = record.as_json(only: %i[id name date])
+        json_attributes["date"] = json_attributes["date"].iso8601 if json_attributes["date"].respond_to?(:iso8601)
         json_attributes["_destroy"] = true if record.marked_for_destruction?
         json_attributes
       end
     end
+
+    # If we fails to save the new NonWorkingDay records, we will return the state
+    # as the user submitted it.
+    # That allows them to correct the mistake or wait for the unprocessed job to finish.
+    def non_working_day_records_from(result)
+      if result.is_a?(Enumerable) && result.all?(NonWorkingDay)
+        result.to_a
+      else
+        non_working_day_records_from_params
+      end
+    end
+
+    def non_working_day_records_from_params
+      non_working_days_params.filter_map do |attrs|
+        attrs = attrs.to_h.with_indifferent_access
+        record = find_or_build_non_working_day(attrs)
+        next unless record
+
+        record.assign_attributes(attrs.slice(:name, :date))
+        record.mark_for_destruction if attrs[:_destroy].present?
+        record
+      end
+    end
+
+    def find_or_build_non_working_day(attrs)
+      if attrs[:id].present?
+        NonWorkingDay.find_by(id: attrs[:id])
+      else
+        NonWorkingDay.new
+      end
+    end
   end
 end

spec/controllers/admin/settings/working_days_and_hours_settings_controller_spec.rb

@@ -104,5 +104,35 @@
         expect { nwd_to_delete.reload }.not_to raise_error
       end
     end
+
+    context "when fails because a previous apply job is still unprocessed",
+            with_good_job: WorkPackages::ApplyWorkingDaysChangeJob do
+      let(:non_working_days_attributes) do
+        {
+          "0" => { "name" => "Boxing Day", "date" => "2022-12-26" },
+          "1" => { "name" => "New Year", "date" => "2023-01-01" }
+        }
+      end
+
+      before do
+        WorkPackages::ApplyWorkingDaysChangeJob
+          .set(wait: 10.minutes)
+          .perform_later(user_id: user.id,
+                         previous_non_working_days: [],
+                         previous_working_days: Setting.working_days)
+      end
+
+      it "displays the error and keeps the submitted non-working days" do
+        subject
+
+        expect(response).to render_template :show
+        expect(flash[:error]).to include("have not been applied yet")
+        expect(assigns(:modified_non_working_days)).to contain_exactly(
+          hash_including("name" => "Boxing Day", "date" => "2022-12-26"),
+          hash_including("name" => "New Year", "date" => "2023-01-01")
+        )
+        expect(NonWorkingDay.where(date: %w[2022-12-26 2023-01-01])).to be_empty
+      end
+    end
   end
 end