backend: add a Ping method to backend

backend/noop/driver.go

@@ -14,6 +14,8 @@ import (
 
 type driver struct{}
 
+// NoOp is a backend driver for Chihaya that does nothing. This is used by
+// public trackers.
 type NoOp struct{}
 
 // New returns a new Chihaya backend driver that does nothing.
@@ -26,6 +28,11 @@ func (n *NoOp) Close() error {
 	return nil
 }
 
+// Ping returns nil.
+func (n *NoOp) Ping() error {
+	return nil
+}
+
 // RecordAnnounce returns nil.
 func (n *NoOp) RecordAnnounce(delta *models.AnnounceDelta) error {
 	return nil