ftp: fix incorrect modtime after uploading a file or setting its modtime

SetModTime (and the no_check_upload Update path) kept the caller's
full precision modtime in memory while the server stores second
precision, so until the object was re-read the modtime did not match
what a fresh listing would report. This broke wrappers which compare
exact modtimes, such as the hasher backend's fingerprint and the VFS
cache. Truncate the modtime to second precision to match what the
server stores.

backend/ftp/ftp.go

@@ -1235,7 +1235,10 @@ func (o *Object) SetModTime(ctx context.Context, modTime time.Time) error {
 	path = o.fs.opt.Enc.FromStandardPath(path)
 	err = c.SetTime(path, modTime.In(time.UTC))
 	if err == nil && o.info != nil {
-		o.info.ModTime = modTime
+		// The server stores modtimes with second precision so
+		// truncate here too to keep the in-memory modtime identical
+		// to the one a fresh listing returns.
+		o.info.ModTime = modTime.Truncate(time.Second)
 		o.info.precise = true
 	}
 	o.fs.putFtpConnection(&c, err)
@@ -1390,9 +1393,12 @@ func (o *Object) Update(ctx context.Context, in io.Reader, src fs.ObjectInfo, op
 	o.fs.putFtpConnection(&c, nil)
 	if o.fs.opt.NoCheckUpload {
 		o.info = &FileInfo{
-			Name:    o.remote,
-			Size:    uint64(src.Size()),
-			ModTime: src.ModTime(ctx),
+			Name: o.remote,
+			Size: uint64(src.Size()),
+			// The server stores modtimes with second precision so
+			// truncate here too to keep the in-memory modtime
+			// identical to the one a fresh listing returns.
+			ModTime: src.ModTime(ctx).Truncate(time.Second),
 			precise: true,
 			IsDir:   false,
 		}