serve http: add Last-Modified headers to files and directories

This means that using `rclone serve http` preserves modification times
when used with the http backend.

Fixes #4201

cmd/serve/http/http.go

@@ -138,6 +138,9 @@ func (s *server) serveDir(w http.ResponseWriter, r *http.Request, dirRemote stri
 	orderParm := r.URL.Query().Get("order")
 	directory.ProcessQueryParams(sortParm, orderParm)
 
+	// Set the Last-Modified header to the timestamp
+	w.Header().Set("Last-Modified", dir.ModTime().UTC().Format(http.TimeFormat))
+
 	directory.Serve(w, r)
 }
 
@@ -175,6 +178,9 @@ func (s *server) serveFile(w http.ResponseWriter, r *http.Request, remote string
 		w.Header().Set("Content-Type", mimeType)
 	}
 
+	// Set the Last-Modified header to the timestamp
+	w.Header().Set("Last-Modified", file.ModTime().UTC().Format(http.TimeFormat))
+
 	// If HEAD no need to read the object since we have set the headers
 	if r.Method == "HEAD" {
 		return

cmd/serve/http/http_test.go

@@ -1,6 +1,7 @@
 package http
 
 import (
+	"context"
 	"flag"
 	"io/ioutil"
 	"net/http"
@@ -50,6 +51,11 @@ func startServer(t *testing.T, f fs.Fs) {
 
 }
 
+var (
+	datedObject  = "two.txt"
+	expectedTime = time.Date(2000, 1, 2, 3, 4, 5, 0, time.UTC)
+)
+
 func TestInit(t *testing.T) {
 	// Configure the remote
 	config.LoadConfig()
@@ -65,6 +71,11 @@ func TestInit(t *testing.T) {
 	f, err := fs.NewFs("testdata/files")
 	require.NoError(t, err)
 
+	// set date of datedObject to expectedTime
+	obj, err := f.NewObject(context.Background(), datedObject)
+	require.NoError(t, err)
+	require.NoError(t, obj.SetModTime(context.Background(), expectedTime))
+
 	startServer(t, f)
 }
 
@@ -195,6 +206,18 @@ func TestGET(t *testing.T) {
 		body, err := ioutil.ReadAll(resp.Body)
 		require.NoError(t, err)
 
+		// Check we got a Last-Modifed header and that it is a valid date
+		if test.Status == http.StatusOK || test.Status == http.StatusPartialContent {
+			lastModified := resp.Header.Get("Last-Modified")
+			assert.NotEqual(t, "", lastModified, test.Golden)
+			modTime, err := http.ParseTime(lastModified)
+			assert.NoError(t, err, test.Golden)
+			// check the actual date on our special file
+			if test.URL == datedObject {
+				assert.Equal(t, expectedTime, modTime, test.Golden)
+			}
+		}
+
 		checkGolden(t, test.Golden, body)
 	}
 }