Adds StorageDriverFactory, unifying creation of StorageDrivers

Custom storage drivers can register a factory to create the driver by
name, similar to the database/sql package's Register and Open
factory.Create returns an in-process driver if registered or an IPC
driver if one can be found, erroring otherwise
This standardizes parameter passing for creation of storage drivers

Also adds documentation for storagedriver package and children

storagedriver/filesystem/filesystem.go

@@ -8,13 +8,45 @@ import (
 	"strings"
 
 	"github.com/docker/docker-registry/storagedriver"
+	"github.com/docker/docker-registry/storagedriver/factory"
 )
 
+const DriverName = "filesystem"
+const DefaultRootDirectory = "/tmp/registry/storage"
+
+func init() {
+	factory.Register(DriverName, &filesystemDriverFactory{})
+}
+
+// Implements the factory.StorageDriverFactory interface
+type filesystemDriverFactory struct{}
+
+func (factory *filesystemDriverFactory) Create(parameters map[string]string) (storagedriver.StorageDriver, error) {
+	return FromParameters(parameters), nil
+}
+
+// Storage Driver backed by a local filesystem
+// All provided paths will be subpaths of the RootDirectory
 type FilesystemDriver struct {
 	rootDirectory string
 }
 
-func NewDriver(rootDirectory string) *FilesystemDriver {
+// Constructs a new FilesystemDriver with a given parameters map
+// Optional Parameters:
+// - rootdirectory
+func FromParameters(parameters map[string]string) *FilesystemDriver {
+	var rootDirectory = DefaultRootDirectory
+	if parameters != nil {
+		rootDir, ok := parameters["rootdirectory"]
+		if ok {
+			rootDirectory = rootDir
+		}
+	}
+	return New(rootDirectory)
+}
+
+// Constructs a new FilesystemDriver with a given rootDirectory
+func New(rootDirectory string) *FilesystemDriver {
 	return &FilesystemDriver{rootDirectory}
 }
 
@@ -22,6 +54,8 @@ func (d *FilesystemDriver) subPath(subPath string) string {
 	return path.Join(d.rootDirectory, subPath)
 }
 
+// Implement the storagedriver.StorageDriver interface
+
 func (d *FilesystemDriver) GetContent(path string) ([]byte, error) {
 	contents, err := ioutil.ReadFile(d.subPath(path))
 	if err != nil {

storagedriver/filesystem/filesystem_test.go

@@ -17,8 +17,8 @@ func init() {
 	os.RemoveAll(rootDirectory)
 
 	filesystemDriverConstructor := func() (storagedriver.StorageDriver, error) {
-		return NewDriver(rootDirectory), nil
+		return New(rootDirectory), nil
 	}
 	testsuites.RegisterInProcessSuite(filesystemDriverConstructor, testsuites.NeverSkip)
-	testsuites.RegisterIPCSuite("filesystem", map[string]string{"RootDirectory": rootDirectory}, testsuites.NeverSkip)
+	testsuites.RegisterIPCSuite(DriverName, map[string]string{"rootdirectory": rootDirectory}, testsuites.NeverSkip)
 }