fix(caldav): render a resource's content once per request

A multistatus response asks for a resource's iCal content once per
requested property. It is now rendered once per resource and request.

pkg/routes/caldav/listStorageProvider.go

@@ -1110,6 +1110,9 @@ type VikunjaProjectResourceAdapter struct {
 
 	isPrincipal  bool
 	isCollection bool
+
+	// Rendered once per request: a multistatus response asks for it per requested property.
+	content *string
 }
 
 // IsCollection checks if the resoure in the adapter is a collection
@@ -1153,6 +1156,14 @@ func (vlra *VikunjaProjectResourceAdapter) CalculateEtag() string {
 
 // GetContent returns the content string of a resource (a task in our case)
 func (vlra *VikunjaProjectResourceAdapter) GetContent() string {
+	if vlra.content == nil {
+		content := vlra.renderContent()
+		vlra.content = &content
+	}
+	return *vlra.content
+}
+
+func (vlra *VikunjaProjectResourceAdapter) renderContent() string {
 	if vlra.project != nil && vlra.projectTasks != nil {
 		return caldav.GetCaldavTodosForTasks(vlra.project, vlra.projectTasks)
 	}

pkg/routes/caldav/requested_props_test.go

@@ -0,0 +1,39 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package caldav
+
+import (
+	"testing"
+
+	"code.vikunja.io/api/pkg/models"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestResourceContentIsRenderedOnce(t *testing.T) {
+	task := &models.Task{UID: "uid-rendered-once", Title: "first"}
+	rr := &VikunjaProjectResourceAdapter{task: task}
+
+	content := rr.GetContent()
+	require.Contains(t, content, "SUMMARY:first")
+
+	// A second render would pick up the new title.
+	task.Title = "second"
+	assert.Equal(t, content, rr.GetContent())
+	assert.Equal(t, int64(len(content)), rr.GetContentSize())
+}