face: avoid multiple onFail events

refs #1497

Change-Id: I8fda3fea5cd8a314b30eef45da104109e9748afc
diff --git a/daemon/face/face.cpp b/daemon/face/face.cpp
index 75600b4..9d2368b 100644
--- a/daemon/face/face.cpp
+++ b/daemon/face/face.cpp
@@ -1,11 +1,12 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
 /**
- * Copyright (c) 2014  Regents of the University of California,
- *                     Arizona Board of Regents,
- *                     Colorado State University,
- *                     University Pierre & Marie Curie, Sorbonne University,
- *                     Washington University in St. Louis,
- *                     Beijing Institute of Technology
+ * Copyright (c) 2014,  Regents of the University of California,
+ *                      Arizona Board of Regents,
+ *                      Colorado State University,
+ *                      University Pierre & Marie Curie, Sorbonne University,
+ *                      Washington University in St. Louis,
+ *                      Beijing Institute of Technology,
+ *                      The University of Memphis
  *
  * This file is part of NFD (Named Data Networking Forwarding Daemon).
  * See AUTHORS.md for complete list of NFD authors and contributors.
@@ -39,6 +40,7 @@
   , m_remoteUri(remoteUri)
   , m_localUri(localUri)
   , m_isOnDemand(false)
+  , m_isFailed(false)
 {
   onReceiveInterest += bind(&increaseCounter, ref(m_counters.getNInInterests()));
   onReceiveData     += bind(&increaseCounter, ref(m_counters.getNInDatas()));
@@ -115,4 +117,15 @@
   }
 }
 
+void
+Face::fail(const std::string& reason)
+{
+  if (m_isFailed) {
+    return;
+  }
+
+  m_isFailed = true;
+  this->onFail(reason);
+}
+
 } //namespace nfd