Define a new Features() method for Fs

Optional interfaces are becoming more important in rclone,
--track-renames and --backup-dir both rely on them.

Up to this point rclone has used interface upgrades to define optional
behaviour on Fs objects.  However when one Fs object wraps another it
is very difficult for this scheme to work accurately.  rclone has
relied on specific error messages being returned when the interface
isn't supported - this is unsatisfactory because it means you have to
call the interface to see whether it is supported.

This change enables accurate detection of optional interfaces by use
of a Features struct as returned by an obligatory Fs.Features()
method.  The Features struct contains flags and function pointers
which can be tested against nil to see whether they can be used.

As a result crypt and hubic can accurately reflect the capabilities of
the underlying Fs they are wrapping.

yandex/yandex.go

@@ -65,8 +65,9 @@ func init() {
 // Fs represents a remote yandex
 type Fs struct {
 	name       string
-	yd         *yandex.Client // client for rest api
 	root       string         //root path
+	features   *fs.Features   // optional features
+	yd         *yandex.Client // client for rest api
 	diskRoot   string         //root path with "disk:/" container name
 	mkdircache map[string]int
 }
@@ -98,6 +99,11 @@ func (f *Fs) String() string {
 	return fmt.Sprintf("Yandex %s", f.root)
 }
 
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
+}
+
 // read access token from ConfigFile string
 func getAccessToken(name string) (*oauth2.Token, error) {
 	// Read the token from the config file
@@ -126,7 +132,7 @@ func NewFs(name, root string) (fs.Fs, error) {
 	f := &Fs{
 		yd: yandexDisk,
 	}
-
+	f.features = (&fs.Features{ReadMimeType: true, WriteMimeType: true}).Fill(f)
 	f.setRoot(root)
 
 	// Check to see if the object exists and is a file