fs: add RangeSeeker interface

fs/fs.go

@@ -672,6 +672,19 @@ type ListRer interface {
 	ListR(dir string, callback ListRCallback) error
 }
 
+// RangeSeeker is the interface that wraps the RangeSeek method.
+//
+// Some of the returns from Object.Open() may optionally implement
+// this method for efficiency purposes.
+type RangeSeeker interface {
+	// RangeSeek behaves like a call to Seek(offset int64, whence
+	// int) with the output wrapped in an io.LimitedReader
+	// limiting the total length to limit.
+	//
+	// RangeSeek with a limit of < 0 is equivalent to a regular Seek.
+	RangeSeek(offset int64, whence int, length int64) (int64, error)
+}
+
 // ObjectsChan is a channel of Objects
 type ObjectsChan chan Object