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.

swift/swift.go

@@ -88,10 +88,11 @@ func init() {
 // Fs represents a remote swift server
 type Fs struct {
 	name              string            // name of this remote
+	root              string            // the path we are working on if any
+	features          *fs.Features      // optional features
 	c                 *swift.Connection // the connection to the swift server
 	container         string            // the container we are working on
 	segmentsContainer string            // container to store the segments (if any) in
-	root              string            // the path we are working on if any
 }
 
 // Object describes a swift object
@@ -127,6 +128,11 @@ func (f *Fs) String() string {
 	return fmt.Sprintf("Swift container %s path %s", f.container, f.root)
 }
 
+// Features returns the optional features of this Fs
+func (f *Fs) Features() *fs.Features {
+	return f.features
+}
+
 // Pattern to match a swift path
 var matcher = regexp.MustCompile(`^([^/]*)(.*)$`)
 
@@ -190,6 +196,7 @@ func NewFsWithConnection(name, root string, c *swift.Connection) (fs.Fs, error)
 		segmentsContainer: container + "_segments",
 		root:              directory,
 	}
+	f.features = (&fs.Features{ReadMimeType: true, WriteMimeType: true}).Fill(f)
 	// StorageURL overloading
 	storageURL := fs.ConfigFileGet(name, "storage_url")
 	if storageURL != "" {