derp: add per-reason packet drop counter.

In parallel with the summed counter for now, for dashboard migration.

Signed-off-by: David Anderson <dave@natulte.net>

derp/derp_server.go

@@ -50,6 +50,12 @@ type Server struct {
 	packetsSent, bytesSent  expvar.Int
 	packetsRecv, bytesRecv  expvar.Int
 	packetsDropped          expvar.Int
+	packetsDroppedReason    metrics.LabelMap
+	packetsDroppedUnknown   *expvar.Int
+	packetsDroppedGone      *expvar.Int
+	packetsDroppedQueueHead *expvar.Int
+	packetsDroppedQueueTail *expvar.Int
+	packetsDroppedWrite     *expvar.Int
 	accepts                 expvar.Int
 	curClients              expvar.Int
 	curHomeClients          expvar.Int // ones with preferred
@@ -84,10 +90,16 @@ func NewServer(privateKey key.Private, logf logger.Logf) *Server {
 		privateKey:           privateKey,
 		publicKey:            privateKey.Public(),
 		logf:                 logf,
+		packetsDroppedReason: metrics.LabelMap{Label: "reason"},
 		clients:              make(map[key.Public]*sclient),
 		clientsEver:          make(map[key.Public]bool),
 		netConns:             make(map[Conn]chan struct{}),
 	}
+	s.packetsDroppedUnknown = s.packetsDroppedReason.Get("unknown_dest")
+	s.packetsDroppedGone = s.packetsDroppedReason.Get("gone")
+	s.packetsDroppedQueueHead = s.packetsDroppedReason.Get("queue_head")
+	s.packetsDroppedQueueTail = s.packetsDroppedReason.Get("queue_tail")
+	s.packetsDroppedWrite = s.packetsDroppedReason.Get("write_error")
 	return s
 }
 
@@ -293,6 +305,7 @@ func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl ui
 
 	if dst == nil {
 		s.packetsDropped.Add(1)
+		s.packetsDroppedUnknown.Add(1)
 		if debug {
 			c.logf("dropping packet for unknown %x", dstKey)
 		}
@@ -303,6 +316,7 @@ func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl ui
 	defer dst.mu.RUnlock()
 	if dst.sendQueue == nil {
 		s.packetsDropped.Add(1)
+		s.packetsDroppedGone.Add(1)
 		if debug {
 			c.logf("dropping packet for shutdown client %x", dstKey)
 		}
@@ -327,6 +341,7 @@ func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl ui
 		select {
 		case <-dst.sendQueue:
 			s.packetsDropped.Add(1)
+			s.packetsDroppedQueueHead.Add(1)
 			if debug {
 				c.logf("dropping packet from client %x queue head", dstKey)
 			}
@@ -337,6 +352,7 @@ func (c *sclient) handleFrameSendPacket(ctx context.Context, ft frameType, fl ui
 	// contended queue with racing writers. Give up and tail-drop in
 	// this case to keep reader unblocked.
 	s.packetsDropped.Add(1)
+	s.packetsDroppedQueueTail.Add(1)
 	if debug {
 		c.logf("dropping packet from client %x queue tail", dstKey)
 	}
@@ -527,6 +543,7 @@ func (c *sclient) sendLoop(ctx context.Context) error {
 				break
 			}
 			c.s.packetsDropped.Add(1)
+			c.s.packetsDroppedGone.Add(1)
 			if debug {
 				c.logf("dropping packet for shutdown %x", c.key)
 			}
@@ -575,6 +592,7 @@ func (c *sclient) sendPacket(srcKey key.Public, contents []byte) (err error) {
 		// Stats update.
 		if err != nil {
 			c.s.packetsDropped.Add(1)
+			c.s.packetsDroppedWrite.Add(1)
 			if debug {
 				c.logf("dropping packet to %x: %v", c.key, err)
 			}
@@ -624,6 +642,7 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("bytes_received", &s.bytesRecv)
 	m.Set("bytes_sent", &s.bytesSent)
 	m.Set("packets_dropped", &s.packetsDropped)
+	m.Set("packets_dropped_reason", &s.packetsDroppedReason)
 	m.Set("packets_sent", &s.packetsSent)
 	m.Set("packets_received", &s.packetsRecv)
 	m.Set("unknown_frames", &s.unknownFrames)