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/inmemory/inmemory.go

@@ -10,14 +10,31 @@ import (
 	"sync"
 
 	"github.com/docker/docker-registry/storagedriver"
+	"github.com/docker/docker-registry/storagedriver/factory"
 )
 
+const DriverName = "inmemory"
+
+func init() {
+	factory.Register(DriverName, &inMemoryDriverFactory{})
+}
+
+// Implements the factory.StorageDriverFactory interface
+type inMemoryDriverFactory struct{}
+
+func (factory *inMemoryDriverFactory) Create(parameters map[string]string) (storagedriver.StorageDriver, error) {
+	return New(), nil
+}
+
+// InMemory Storage Driver backed by a map
+// Intended solely for example and testing purposes
 type InMemoryDriver struct {
 	storage map[string][]byte
 	mutex   sync.RWMutex
 }
 
-func NewDriver() *InMemoryDriver {
+// Constructs a new InMemoryDriver
+func New() *InMemoryDriver {
 	return &InMemoryDriver{storage: make(map[string][]byte)}
 }
 

storagedriver/inmemory/inmemory_test.go

@@ -13,8 +13,8 @@ func Test(t *testing.T) { TestingT(t) }
 
 func init() {
 	inmemoryDriverConstructor := func() (storagedriver.StorageDriver, error) {
-		return NewDriver(), nil
+		return New(), nil
 	}
 	testsuites.RegisterInProcessSuite(inmemoryDriverConstructor, testsuites.NeverSkip)
-	testsuites.RegisterIPCSuite("inmemory", nil, testsuites.NeverSkip)
+	testsuites.RegisterIPCSuite(DriverName, nil, testsuites.NeverSkip)
 }