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.

fstest/fstests/fstests.go

@@ -384,8 +384,8 @@ func TestFsCopy(t *testing.T) {
 	skipIfNotOk(t)
 
 	// Check have Copy
-	_, ok := remote.(fs.Copier)
-	if !ok {
+	doCopy := remote.Features().Copy
+	if doCopy == nil {
 		t.Skip("FS has no Copier interface")
 	}
 
@@ -394,7 +394,7 @@ func TestFsCopy(t *testing.T) {
 
 	// do the copy
 	src := findObject(t, file1.Path)
-	dst, err := remote.(fs.Copier).Copy(src, file1Copy.Path)
+	dst, err := doCopy(src, file1Copy.Path)
 	if err == fs.ErrorCantCopy {
 		t.Skip("FS can't copy")
 	}
@@ -417,8 +417,8 @@ func TestFsMove(t *testing.T) {
 	skipIfNotOk(t)
 
 	// Check have Move
-	_, ok := remote.(fs.Mover)
-	if !ok {
+	doMove := remote.Features().Move
+	if doMove == nil {
 		t.Skip("FS has no Mover interface")
 	}
 
@@ -433,7 +433,7 @@ func TestFsMove(t *testing.T) {
 	// separate operations
 	file2Move.Path = "other.txt"
 	src := findObject(t, file2.Path)
-	dst, err := remote.(fs.Mover).Move(src, file2Move.Path)
+	dst, err := doMove(src, file2Move.Path)
 	if err == fs.ErrorCantMove {
 		t.Skip("FS can't move")
 	}
@@ -448,7 +448,7 @@ func TestFsMove(t *testing.T) {
 	// Check conflict on "rename, then move"
 	file1Move.Path = "moveTest/other.txt"
 	src = findObject(t, file1.Path)
-	_, err = remote.(fs.Mover).Move(src, file1Move.Path)
+	_, err = doMove(src, file1Move.Path)
 	require.NoError(t, err)
 	fstest.CheckListing(t, remote, []fstest.Item{file1Move, file2Move})
 	// 1: moveTest/other.txt
@@ -456,14 +456,14 @@ func TestFsMove(t *testing.T) {
 
 	// Check conflict on "move, then rename"
 	src = findObject(t, file1Move.Path)
-	_, err = remote.(fs.Mover).Move(src, file1.Path)
+	_, err = doMove(src, file1.Path)
 	require.NoError(t, err)
 	fstest.CheckListing(t, remote, []fstest.Item{file1, file2Move})
 	// 1: file name.txt
 	// 2: other.txt
 
 	src = findObject(t, file2Move.Path)
-	_, err = remote.(fs.Mover).Move(src, file2.Path)
+	_, err = doMove(src, file2.Path)
 	require.NoError(t, err)
 	fstest.CheckListing(t, remote, []fstest.Item{file1, file2})
 	// 1: file name.txt
@@ -483,13 +483,13 @@ func TestFsDirMove(t *testing.T) {
 	skipIfNotOk(t)
 
 	// Check have DirMove
-	_, ok := remote.(fs.DirMover)
-	if !ok {
+	doDirMove := remote.Features().DirMove
+	if doDirMove == nil {
 		t.Skip("FS has no DirMover interface")
 	}
 
 	// Check it can't move onto itself
-	err := remote.(fs.DirMover).DirMove(remote)
+	err := doDirMove(remote)
 	require.Equal(t, fs.ErrorDirExists, err)
 
 	// new remote
@@ -498,7 +498,7 @@ func TestFsDirMove(t *testing.T) {
 	defer removeNewRemote()
 
 	// try the move
-	err = newRemote.(fs.DirMover).DirMove(remote)
+	err = newRemote.Features().DirMove(remote)
 	require.NoError(t, err)
 
 	// check remotes
@@ -507,7 +507,7 @@ func TestFsDirMove(t *testing.T) {
 	fstest.CheckListing(t, newRemote, []fstest.Item{file2, file1})
 
 	// move it back
-	err = remote.(fs.DirMover).DirMove(newRemote)
+	err = doDirMove(newRemote)
 	require.NoError(t, err)
 
 	// check remotes
@@ -550,8 +550,8 @@ func TestObjectFs(t *testing.T) {
 	testRemote := remote
 	if obj.Fs() != testRemote {
 		// Check to see if this wraps something else
-		if unwrap, ok := testRemote.(fs.UnWrapper); ok {
-			testRemote = unwrap.UnWrap()
+		if doUnWrap := testRemote.Features().UnWrap; doUnWrap != nil {
+			testRemote = doUnWrap()
 		}
 	}
 	assert.Equal(t, obj.Fs(), testRemote)