feature/*,net/tstun: add tundev_txq_drops clientmetric on Linux

By polling RTM_GETSTATS via netlink. RTM_GETSTATS is a relatively efficient and targeted (single device) polling method available since Linux v4.7. The tundevstats "feature" can be extended to other platforms in the future, and it's trivial to add new rtnl_link_stats64 counters on Linux. Updates tailscale/corp#38181 Signed-off-by: Jordan Whited <jordan@tailscale.com>
2026-03-19 15:23:17 -07:00
parent bdcf976477
commit 9c36a71a90
8 changed files with 604 additions and 0 deletions
@@ -0,0 +1,105 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+package tundevstats
+
+import (
+	"encoding/binary"
+	"os"
+	"sync/atomic"
+	"testing"
+	"testing/synctest"
+	"time"
+	"unsafe"
+
+	"github.com/mdlayher/netlink"
+	"github.com/mdlayher/netlink/nltest"
+	"github.com/tailscale/wireguard-go/tun"
+	"golang.org/x/sys/unix"
+)
+
+func Test_getIfIndex(t *testing.T) {
+	ifIndex, err := getIfIndex("lo")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if ifIndex != 1 {
+		// loopback ifIndex is effectively always 1 on Linux, see
+		// LOOPBACK_IFINDEX in the kernel (net/flow.h).
+		t.Fatalf("expected ifIndex of 1 for loopback, got: %d", ifIndex)
+	}
+}
+
+type fakeDevice struct {
+	name string
+}
+
+func (f *fakeDevice) File() *os.File                                                 { return nil }
+func (f *fakeDevice) Read(bufs [][]byte, sizes []int, offset int) (n int, err error) { return 0, nil }
+func (f *fakeDevice) Write(bufs [][]byte, offset int) (int, error)                   { return 0, nil }
+func (f *fakeDevice) MTU() (int, error)                                              { return 0, nil }
+func (f *fakeDevice) Name() (string, error)                                          { return f.name, nil }
+func (f *fakeDevice) Events() <-chan tun.Event                                       { return nil }
+func (f *fakeDevice) Close() error                                                   { return nil }
+func (f *fakeDevice) BatchSize() int                                                 { return 0 }
+
+func Test_poller(t *testing.T) {
+	getTXQDropsMetric().Set(0) // reset for test count > 1
+
+	var drops atomic.Uint64
+	// dial is a [nltest.Func] that returns an RTM_NEWSTATS response with [drops]
+	// at the txDropped offset within the [rtnlLinkStats64] attribute payload.
+	dial := func(req []netlink.Message) ([]netlink.Message, error) {
+		if len(req) != 1 {
+			t.Fatalf("unexpected number of netlink request messages: %d", len(req))
+		}
+		if req[0].Header.Type != unix.RTM_GETSTATS {
+			t.Fatalf("unexpected netlink request message type: %d want: %d", req[0].Header.Type, unix.RTM_GETSTATS)
+		}
+		data := make([]byte, unsafe.Sizeof(ifStatsMsg{}))
+		ae := netlink.NewAttributeEncoder()
+		ae.Do(iflaStatsLink64, func() ([]byte, error) {
+			ret := make([]byte, unsafe.Sizeof(rtnlLinkStats64{}))
+			binary.NativeEndian.PutUint64(ret[56:], drops.Load())
+			return ret, nil
+		})
+		attrs, err := ae.Encode()
+		if err != nil {
+			t.Fatal(err)
+		}
+		data = append(data, attrs...)
+		return []netlink.Message{
+			{
+				Header: netlink.Header{
+					Type:     unix.RTM_NEWSTATS,
+					Sequence: req[0].Header.Sequence,
+				},
+				Data: data,
+			},
+		}, nil
+	}
+
+	lo := &fakeDevice{name: "lo"}
+	drops.Store(1)
+	synctest.Test(t, func(t *testing.T) {
+		closer, err := newPollerWithNetlinkDialer(lo, func(family int, config *netlink.Config) (*netlink.Conn, error) {
+			return nltest.Dial(dial), nil
+		})
+		if err != nil {
+			t.Fatal(err)
+		}
+		synctest.Wait() // first poll complete, poller.run() durably blocked in select
+		if got := getTXQDropsMetric().Value(); got != 1 {
+			t.Errorf("got drops: %d want: %d", got, 1)
+		}
+		drops.Store(2) // increment drops to 2
+		time.Sleep(pollInterval)
+		synctest.Wait() // second poll complete, poller.run() durably blocked in select again
+		if got := getTXQDropsMetric().Value(); got != 2 {
+			t.Errorf("got drops: %d want: %d", got, 2)
+		}
+		closer.Close()
+		closer.Close() // multiple calls to Close() shouldn't panic
+	})
+
+}