Add basic metric for recording use after N seconds

Signed-off-by: julianknodt <julian@julians-MacBook-Air.local>

derp/derp_server.go

@@ -120,6 +120,8 @@ type Server struct {
 	multiForwarderCreated    expvar.Int
 	multiForwarderDeleted    expvar.Int
 	removePktForwardOther    expvar.Int
+	clientsInUse5Sec         expvar.Int // Number of clients using Derp after 5 seconds.
+	clientsInUse10Sec        expvar.Int
 
 	mu          sync.Mutex
 	closed      bool
@@ -457,6 +459,7 @@ func (s *Server) accept(nc Conn, brw *bufio.ReadWriter, remoteAddr string, connN
 		done:        ctx.Done(),
 		remoteAddr:  remoteAddr,
 		connectedAt: time.Now(),
+		lastPktAt:   time.Now(),
 		sendQueue:   make(chan pkt, perClientSendQueueDepth),
 		peerGone:    make(chan key.Public),
 		canMesh:     clientInfo.MeshKey != "" && clientInfo.MeshKey == s.meshKey,
@@ -663,6 +666,7 @@ func (c *sclient) handleFrameSendPacket(ft frameType, fl uint32) error {
 		}
 		return nil
 	}
+	c.markLastPktAt()
 
 	p := pkt{
 		bs:  contents,
@@ -901,6 +905,7 @@ type sclient struct {
 	// Owned by run, not thread-safe.
 	br          *bufio.Reader
 	connectedAt time.Time
+	lastPktAt   time.Time
 	preferred   bool
 
 	// Owned by sender, not thread-safe.
@@ -1062,6 +1067,21 @@ func (c *sclient) sendPeerPresent(peer key.Public) error {
 	return err
 }
 
+func (c *sclient) markLastPktAt() {
+	old := c.lastPktAt
+	curr := time.Now()
+	c.lastPktAt = curr
+	// If we've been connected for over 5 seconds and haven't previously
+	since_old := old.Sub(c.connectedAt)
+	since_now := curr.Sub(c.connectedAt)
+	if since_old <= 5*time.Second && since_now > 5*time.Second {
+		c.s.clientsInUse5Sec.Add(1)
+	}
+	if since_old <= 10*time.Second && since_now > 10*time.Second {
+		c.s.clientsInUse10Sec.Add(1)
+	}
+}
+
 // sendMeshUpdates drains as many mesh peerStateChange entries as
 // possible into the write buffer WITHOUT flushing or otherwise
 // blocking (as it holds c.s.mu while working). If it can't drain them
@@ -1290,6 +1310,8 @@ func (s *Server) ExpVar() expvar.Var {
 	m.Set("multiforwarder_created", &s.multiForwarderCreated)
 	m.Set("multiforwarder_deleted", &s.multiForwarderDeleted)
 	m.Set("packet_forwarder_delete_other_value", &s.removePktForwardOther)
+	m.Set("clients_inuse_after_5_sec", &s.clientsInUse5Sec)
+	m.Set("clients_inuse_after_10_sec", &s.clientsInUse10Sec)
 	var expvarVersion expvar.String
 	expvarVersion.Set(version.Long)
 	m.Set("version", &expvarVersion)

derp/derp_test.go

@@ -774,6 +774,25 @@ func TestForwarderRegistration(t *testing.T) {
 	})
 }
 
+func TestLastAliveCounter(t *testing.T) {
+	ts := newTestServer(t)
+	defer ts.close(t)
+	wantCounter := func(c *expvar.Int, want int) {
+		t.Helper()
+		if got := c.Value(); got != int64(want) {
+			t.Errorf("counter = %v; want %v", got, want)
+		}
+	}
+	wantCounter(&ts.s.clientsInUse5Sec, 0)
+	tc0 := newRegularClient(t, ts, "c0")
+	time.Sleep(6 * time.Second)
+	for _, sc := range ts.s.clients {
+		sc.markLastPktAt()
+	}
+	wantCounter(&ts.s.clientsInUse5Sec, 1)
+	tc0.close(t)
+}
+
 func TestMetaCert(t *testing.T) {
 	priv := newPrivateKey(t)
 	pub := priv.Public()