wgengine/monitor: don't call LinkChange when interfaces look unchanged

Basically, don't trust the OS-level link monitor to only tell you
interesting things. Sanity check it.

Also, move the interfaces package into the net directory now that we
have it.

wgengine/monitor/monitor.go

@@ -7,9 +7,14 @@
 package monitor
 
 import (
+	"fmt"
+	"net"
+	"runtime"
+	"strings"
 	"sync"
 	"time"
 
+	"tailscale.com/net/interfaces"
 	"tailscale.com/types/logger"
 )
 
@@ -99,6 +104,7 @@ func (m *Mon) Close() error {
 // the change channel of changes, and stopping when a stop is issued.
 func (m *Mon) pump() {
 	defer m.goroutines.Done()
+	last := interfaceSummary()
 	for {
 		_, err := m.om.Receive()
 		if err != nil {
@@ -113,9 +119,17 @@ func (m *Mon) pump() {
 			continue
 		}
 
+		cur := interfaceSummary()
+		if cur == last {
+			continue
+		}
+		m.logf("wgengine/monitor: now %v (was %v)", cur, last)
+		last = cur
+
 		select {
 		case m.change <- struct{}{}:
-		default:
+		case <-m.stop:
+			return
 		}
 	}
 }
@@ -140,3 +154,17 @@ func (m *Mon) debounce() {
 		}
 	}
 }
+
+func interfaceSummary() string {
+	var sb strings.Builder
+	_ = interfaces.ForeachInterfaceAddress(func(ni interfaces.Interface, addr net.IP) {
+		if runtime.GOOS == "linux" && strings.HasPrefix(ni.Name, "tailscale") {
+			// Skip tailscale0, etc on Linux.
+			return
+		}
+		if ni.IsUp() {
+			fmt.Fprintf(&sb, "%s=%s ", ni.Name, addr)
+		}
+	})
+	return strings.TrimSpace(sb.String())
+}