🧑‍💻 Kernel serve CalDAV service on path /caldav/ (#13321)

* 🎨 define the interface of CalDAV

* 🎨 Adjust iCalendar files directory structure

* 🎨 Implement CalDAV Calendar manage functions

* 🎨 Implement CalDAV calendar object manage functions

* 🎨 improve ETag scheme

kernel/model/dav.go

@@ -0,0 +1,77 @@
+// SiYuan - Refactor your thinking
+// Copyright (c) 2020-present, b3log.org
+//
+// 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 model
+
+import (
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+	"time"
+
+	"github.com/88250/gulu"
+	"github.com/emersion/go-webdav/caldav"
+	"github.com/emersion/go-webdav/carddav"
+	"github.com/siyuan-note/logging"
+	"github.com/siyuan-note/siyuan/kernel/util"
+)
+
+// PathJoinWithSlash joins the elements to a path with slash ('/') character
+func PathJoinWithSlash(elems ...string) string {
+	return filepath.ToSlash(filepath.Join(elems...))
+}
+
+// PathCleanWithSlash cleans the path
+func PathCleanWithSlash(p string) string {
+	return filepath.ToSlash(filepath.Clean(p))
+}
+
+// DavPath2DirectoryPath converts CalDAV/CardDAV path to absolute path of the file system
+func DavPath2DirectoryPath(davPath string) string {
+	return PathJoinWithSlash(util.DataDir, "storage", davPath)
+}
+
+func SaveMetaData[T []*caldav.Calendar | []*carddav.AddressBook](metaData T, metaDataFilePath string) error {
+	data, err := gulu.JSON.MarshalIndentJSON(metaData, "", "  ")
+	if err != nil {
+		logging.LogErrorf("marshal address books meta data failed: %s", err)
+		return err
+	}
+
+	dirPath := path.Dir(metaDataFilePath)
+	if err := os.MkdirAll(dirPath, 0755); err != nil {
+		logging.LogErrorf("create directory [%s] failed: %s", dirPath, err)
+		return err
+	}
+
+	if err := os.WriteFile(metaDataFilePath, data, 0755); err != nil {
+		logging.LogErrorf("write file [%s] failed: %s", metaDataFilePath, err)
+		return err
+	}
+
+	return nil
+}
+
+// FileETag generates an ETag for a file
+func FileETag(fileInfo fs.FileInfo) string {
+	return fmt.Sprintf(
+		"%s-%x",
+		fileInfo.ModTime().Format(time.RFC3339),
+		fileInfo.Size(),
+	)
+}