Refactoring cloudfactory layer handler into a more generic storage

middleware concept.

This also breaks the dependency the storage package had on goamz
Signed-off-by: David Lawrence <david.lawrence@docker.com> (github: endophage)

docs/storage/driver/middleware/storagemiddleware.go

@@ -0,0 +1,40 @@
+package storagemiddleware
+
+import (
+	"fmt"
+
+	storagedriver "github.com/docker/distribution/registry/storage/driver"
+)
+
+// InitFunc is the type of a StorageMiddleware factory function and is
+// used to register the contsructor for different StorageMiddleware backends.
+type InitFunc func(storageDriver storagedriver.StorageDriver, options map[string]interface{}) (storagedriver.StorageDriver, error)
+
+var storageMiddlewares map[string]InitFunc
+
+// RegisterStorageMiddleware is used to register an StorageMiddlewareInitFunc for
+// a StorageMiddleware backend with the given name.
+func RegisterStorageMiddleware(name string, initFunc InitFunc) error {
+	if storageMiddlewares == nil {
+		storageMiddlewares = make(map[string]InitFunc)
+	}
+	if _, exists := storageMiddlewares[name]; exists {
+		return fmt.Errorf("name already registered: %s", name)
+	}
+
+	storageMiddlewares[name] = initFunc
+
+	return nil
+}
+
+// GetStorageMiddleware constructs a StorageMiddleware
+// with the given options using the named backend.
+func GetStorageMiddleware(name string, options map[string]interface{}, storageDriver storagedriver.StorageDriver) (storagedriver.StorageDriver, error) {
+	if storageMiddlewares != nil {
+		if initFunc, exists := storageMiddlewares[name]; exists {
+			return initFunc(storageDriver, options)
+		}
+	}
+
+	return nil, fmt.Errorf("no storage middleware registered with name: %s", name)
+}