Added NewOsFsBallback.

- Creates a new OS file system with a fallback on an afero.Fs.
Exposed the file system used as source for the CopyDir.

vfs/copydir.go

@@ -9,7 +9,9 @@ import (
 	"github.com/spf13/afero"
 )
 
+// CopyDir represents a directory that is mimicked on hard drive.
 type CopyDir interface {
+	Fs() afero.Fs
 	Retrieve(name string) (string, error)
 	Path(name string) string
 	Open(name string) (*os.File, error)
@@ -31,6 +33,8 @@ func NewCopyDir(fs afero.Fs) (CopyDir, error) {
 	return &copyDir{fs, dir}, nil
 }
 
+func (d *copyDir) Fs() afero.Fs { return d.fs }
+
 func (d *copyDir) Retrieve(name string) (string, error) {
 	var path = d.Path(name)
 	if _, err := os.Stat(path); os.IsNotExist(err) {

vfs/fallbackfs.go

@@ -20,6 +20,12 @@ func NewFallbackFs(source, backup afero.Fs) *FallbackFs {
 	return &FallbackFs{source, backup}
 }
 
+// NewOsFsFallback creates a new Fs based on the os package, restricts access on the supplied path and uses backup as fallback.
+func NewOsFsFallback(path string, backup afero.Fs) *FallbackFs {
+	osFs := afero.NewOsFs()
+	return NewFallbackFs(afero.NewBasePathFs(osFs, path), backup)
+}
+
 func (f *FallbackFs) do(action func(afero.Fs) error) error {
 	err := action(f.source)
 	if err == nil {