Add LengthDeferrerDataStore interface

datastore.go

@@ -114,3 +114,11 @@ type ConcaterDataStore interface {
 	// must be respected during concatenation.
 	ConcatUploads(destination string, partialUploads []string) error
 }
+
+// LengthDeferrerDataStore is the interface that must be implemented if the
+// creation-defer-length extension should be enabled. The extension enables a
+// client to upload files when their total size is not yet known. Instead, the
+// client must send the total size as soon as it becomes known.
+type LengthDeferrerDataStore interface {
+	DeclareLength(id string, length int64) error
+}