face: put the pcap handle in non-blocking mode

This prevents NFD from getting stuck in pcap_next_ex()
when the network interface goes down

Change-Id: I5760fcc7c950b46aecb7619bd889508ee24133aa

daemon/face/ethernet-transport.cpp

@@ -167,7 +167,7 @@ EthernetTransport::handleRead(const boost::system::error_code& error)
 
   auto [pkt, readErr] = m_pcap.readNextPacket();
   if (pkt.empty()) {
-    NFD_LOG_FACE_WARN("Read error: " << readErr);
+    NFD_LOG_FACE_DEBUG("Read error: " << readErr);
   }
   else {
     auto [eh, frameErr] = ethernet::checkFrameHeader(pkt, m_srcAddress,

daemon/face/pcap-helper.cpp

@@ -69,6 +69,10 @@ PcapHelper::activate(int dlt)
       NDN_THROW(Error("pcap_activate: " + std::string(pcap_statustostr(ret))));
   });
 
+  char errbuf[PCAP_ERRBUF_SIZE] = {};
+  if (pcap_setnonblock(m_pcap, 1, errbuf) < 0)
+    NDN_THROW(Error("pcap_setnonblock: " + std::string(errbuf)));
+
   if (pcap_set_datalink(m_pcap, dlt) < 0)
     NDN_THROW(Error("pcap_set_datalink: " + getLastError()));
 
@@ -136,7 +140,7 @@ PcapHelper::readNextPacket() const noexcept
   if (ret < 0)
     return {span<uint8_t>{}, getLastError()};
   else if (ret == 0)
-    return {span<uint8_t>{}, "timed out"};
+    return {span<uint8_t>{}, "Nothing to read"};
   else
     return {{packet, header->caplen}, ""};
 }