webdav: add support for ETag to client & server

client.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"strconv"
 	"time"
 
 	"github.com/emersion/go-webdav/internal"
@@ -45,6 +46,14 @@ func (c *Client) FindCurrentUserPrincipal() (string, error) {
 	return prop.Href.Path, nil
 }
 
+var fileInfoPropfind = internal.NewPropNamePropfind(
+	internal.ResourceTypeName,
+	internal.GetContentLengthName,
+	internal.GetLastModifiedName,
+	internal.GetContentTypeName,
+	internal.GetETagName,
+)
+
 func fileInfoFromResponse(resp *internal.Response) (*FileInfo, error) {
 	path, err := resp.Path()
 	if err != nil {
@@ -75,22 +84,24 @@ func fileInfoFromResponse(resp *internal.Response) (*FileInfo, error) {
 			return nil, err
 		}
 
+		var getETag internal.GetETag
+		if err := resp.DecodeProp(&getETag); err != nil && !internal.IsNotFound(err) {
+			return nil, err
+		}
+		etag, err := strconv.Unquote(getETag.ETag)
+		if err != nil {
+			return nil, fmt.Errorf("webdav: failed to unquote ETag: %v", err)
+		}
+
 		fi.Size = getLen.Length
 		fi.ModTime = time.Time(getMod.LastModified)
 		fi.MIMEType = getType.Type
+		fi.ETag = etag
 	}
 
 	return fi, nil
 }
 
-// TODO: getetag
-var fileInfoPropfind = internal.NewPropNamePropfind(
-	internal.ResourceTypeName,
-	internal.GetContentLengthName,
-	internal.GetLastModifiedName,
-	internal.GetContentTypeName,
-)
-
 func (c *Client) Stat(name string) (*FileInfo, error) {
 	resp, err := c.ic.PropfindFlat(name, fileInfoPropfind)
 	if err != nil {

fs_local.go

@@ -1,6 +1,7 @@
 package webdav
 
 import (
+	"fmt"
 	"io"
 	"mime"
 	"net/http"
@@ -49,6 +50,12 @@ func fileInfoFromOS(p string, fi os.FileInfo) *FileInfo {
 		IsDir:   fi.IsDir(),
 		// TODO: fallback to http.DetectContentType?
 		MIMEType: mime.TypeByExtension(path.Ext(p)),
+		// RFC 2616 section 13.3.3 describes strong ETags. Ideally these would
+		// be checksums or sequence numbers, however these are expensive to
+		// compute. The modification time with nanosecond granularity is good
+		// enough, as it's very unlikely for the same file to be modified twice
+		// during a single nanosecond.
+		ETag: fmt.Sprintf("%x%x", fi.ModTime().UnixNano(), fi.Size()),
 	}
 }
 

internal/elements.go

@@ -339,6 +339,12 @@ type GetLastModified struct {
 	LastModified Time     `xml:",chardata"`
 }
 
+// https://tools.ietf.org/html/rfc4918#section-15.6
+type GetETag struct {
+	XMLName xml.Name `xml:"DAV: getetag"`
+	ETag    string   `xml:",chardata"`
+}
+
 // https://tools.ietf.org/html/rfc4918#section-14.5
 type Error struct {
 	XMLName xml.Name      `xml:"DAV: error"`

server.go

@@ -2,6 +2,7 @@ package webdav
 
 import (
 	"encoding/xml"
+	"fmt"
 	"io"
 	"net/http"
 	"os"
@@ -87,13 +88,16 @@ func (b *backend) HeadGet(w http.ResponseWriter, r *http.Request) error {
 	}
 	defer f.Close()
 
+	w.Header().Set("Content-Length", strconv.FormatInt(fi.Size, 10))
 	if fi.MIMEType != "" {
 		w.Header().Set("Content-Type", fi.MIMEType)
 	}
 	if !fi.ModTime.IsZero() {
 		w.Header().Set("Last-Modified", fi.ModTime.UTC().Format(http.TimeFormat))
 	}
-	w.Header().Set("Content-Length", strconv.FormatInt(fi.Size, 10))
+	if fi.ETag != "" {
+		w.Header().Set("ETag", fmt.Sprintf("%q", fi.ETag))
+	}
 
 	if rs, ok := f.(io.ReadSeeker); ok {
 		// If it's an io.Seeker, use http.ServeContent which supports ranges
@@ -171,7 +175,11 @@ func (b *backend) propfindFile(propfind *internal.Propfind, fi *FileInfo) (*inte
 			}
 		}
 
-		// TODO: getetag
+		if fi.ETag != "" {
+			props[internal.GetETagName] = func(*internal.RawXMLValue) (interface{}, error) {
+				return &internal.GetETag{ETag: fmt.Sprintf("%q", fi.ETag)}, nil
+			}
+		}
 	}
 
 	return internal.NewPropfindResponse(fi.Path, propfind, props)

webdav.go

@@ -7,12 +7,11 @@ import (
 	"time"
 )
 
-// TODO: add ETag to FileInfo
-
 type FileInfo struct {
 	Path     string
 	Size     int64
 	ModTime  time.Time
 	IsDir    bool
 	MIMEType string
+	ETag     string
 }