wgengine/magicsock: export packet drop metric for outbound errors

This required sharing the dropped packet metric between two packages
(tstun and magicsock), so I've moved its definition to util/usermetric.

Updates tailscale/corp#22075

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

wgengine/magicsock/derp.go

@@ -674,6 +674,9 @@ func (c *Conn) runDerpWriter(ctx context.Context, dc *derphttp.Client, ch <-chan
 			if err != nil {
 				c.logf("magicsock: derp.Send(%v): %v", wr.addr, err)
 				metricSendDERPError.Add(1)
+				if !wr.isDisco {
+					c.metrics.outboundPacketsDroppedErrors.Add(1)
+				}
 			} else if !wr.isDisco {
 				c.metrics.outboundPacketsDERPTotal.Add(1)
 				c.metrics.outboundBytesDERPTotal.Add(int64(len(wr.b)))

wgengine/magicsock/magicsock.go

@@ -127,6 +127,10 @@ type metrics struct {
 	outboundBytesIPv4Total expvar.Int
 	outboundBytesIPv6Total expvar.Int
 	outboundBytesDERPTotal expvar.Int
+
+	// outboundPacketsDroppedErrors is the total number of outbound packets
+	// dropped due to errors.
+	outboundPacketsDroppedErrors expvar.Int
 }
 
 // A Conn routes UDP packets and actively manages a list of its endpoints.
@@ -605,6 +609,8 @@ func registerMetrics(reg *usermetric.Registry) *metrics {
 		"counter",
 		"Counts the number of bytes sent to other peers",
 	)
+	outboundPacketsDroppedErrors := reg.DroppedPacketsOutbound()
+
 	m := new(metrics)
 
 	// Map clientmetrics to the usermetric counters.
@@ -631,6 +637,8 @@ func registerMetrics(reg *usermetric.Registry) *metrics {
 	outboundBytesTotal.Set(pathDirectV6, &m.outboundBytesIPv6Total)
 	outboundBytesTotal.Set(pathDERP, &m.outboundBytesDERPTotal)
 
+	outboundPacketsDroppedErrors.Set(usermetric.DropLabels{Reason: usermetric.ReasonError}, &m.outboundPacketsDroppedErrors)
+
 	return m
 }
 
@@ -1202,8 +1210,13 @@ func (c *Conn) networkDown() bool { return !c.networkUp.Load() }
 // Send implements conn.Bind.
 //
 // See https://pkg.go.dev/golang.zx2c4.com/wireguard/conn#Bind.Send
-func (c *Conn) Send(buffs [][]byte, ep conn.Endpoint) error {
+func (c *Conn) Send(buffs [][]byte, ep conn.Endpoint) (err error) {
 	n := int64(len(buffs))
+	defer func() {
+		if err != nil {
+			c.metrics.outboundPacketsDroppedErrors.Add(n)
+		}
+	}()
 	metricSendData.Add(n)
 	if c.networkDown() {
 		metricSendDataNetworkDown.Add(n)

wgengine/magicsock/magicsock_test.go

@@ -63,6 +63,7 @@ import (
 	"tailscale.com/types/nettype"
 	"tailscale.com/types/ptr"
 	"tailscale.com/util/cibuild"
+	"tailscale.com/util/must"
 	"tailscale.com/util/racebuild"
 	"tailscale.com/util/set"
 	"tailscale.com/util/usermetric"
@@ -3083,3 +3084,27 @@ func TestMaybeRebindOnError(t *testing.T) {
 		}
 	})
 }
+
+func TestNetworkDownSendErrors(t *testing.T) {
+	netMon := must.Get(netmon.New(t.Logf))
+	defer netMon.Close()
+
+	reg := new(usermetric.Registry)
+	conn := must.Get(NewConn(Options{
+		DisablePortMapper: true,
+		Logf:              t.Logf,
+		NetMon:            netMon,
+		Metrics:           reg,
+	}))
+	defer conn.Close()
+
+	conn.SetNetworkUp(false)
+	if err := conn.Send([][]byte{{00}}, &lazyEndpoint{}); err == nil {
+		t.Error("expected error, got nil")
+	}
+	resp := httptest.NewRecorder()
+	reg.Handler(resp, new(http.Request))
+	if !strings.Contains(resp.Body.String(), `tailscaled_outbound_dropped_packets_total{reason="error"} 1`) {
+		t.Errorf("expected NetworkDown to increment packet dropped metric; got %q", resp.Body.String())
+	}
+}