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.

cmd/tsshd/tsshd.go

@@ -32,7 +32,7 @@ import (
 	"github.com/gliderlabs/ssh"
 	"github.com/kr/pty"
 	gossh "golang.org/x/crypto/ssh"
-	"tailscale.com/interfaces"
+	"tailscale.com/net/interfaces"
 )
 
 var (

net/interfaces/interfaces.go

@@ -137,6 +137,36 @@ func LocalAddresses() (regular, loopback []string, err error) {
 	return regular, loopback, nil
 }
 
+// Interface is a wrapper around Go's net.Interface with some extra methods.
+type Interface struct {
+	*net.Interface
+}
+
+func (i Interface) IsLoopback() bool { return isLoopback(i.Interface) }
+func (i Interface) IsUp() bool       { return isUp(i.Interface) }
+
+// ForeachInterfaceAddress calls fn for each interface's address on the machine.
+func ForeachInterfaceAddress(fn func(Interface, net.IP)) error {
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return err
+	}
+	for i := range ifaces {
+		iface := &ifaces[i]
+		addrs, err := iface.Addrs()
+		if err != nil {
+			return err
+		}
+		for _, a := range addrs {
+			switch v := a.(type) {
+			case *net.IPNet:
+				fn(Interface{iface}, v.IP)
+			}
+		}
+	}
+	return nil
+}
+
 var cgNAT = func() *net.IPNet {
 	_, ipNet, err := net.ParseCIDR("100.64.0.0/10")
 	if err != nil {

netcheck/netcheck.go

@@ -18,8 +18,8 @@ import (
 
 	"golang.org/x/sync/errgroup"
 	"tailscale.com/derp/derpmap"
-	"tailscale.com/interfaces"
 	"tailscale.com/net/dnscache"
+	"tailscale.com/net/interfaces"
 	"tailscale.com/stun"
 	"tailscale.com/stunner"
 	"tailscale.com/types/logger"

tsweb/tsweb.go

@@ -20,8 +20,8 @@ import (
 	"strings"
 	"time"
 
-	"tailscale.com/interfaces"
 	"tailscale.com/metrics"
+	"tailscale.com/net/interfaces"
 )
 
 // DevMode controls whether extra output in shown, for when the binary is being run in dev mode.

wgengine/magicsock/magicsock.go

@@ -31,8 +31,8 @@ import (
 	"tailscale.com/derp"
 	"tailscale.com/derp/derphttp"
 	"tailscale.com/derp/derpmap"
-	"tailscale.com/interfaces"
 	"tailscale.com/net/dnscache"
+	"tailscale.com/net/interfaces"
 	"tailscale.com/netcheck"
 	"tailscale.com/stun"
 	"tailscale.com/tailcfg"

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())
+}