cmd/containerboot: switch to IPN bus monitoring instead of polling.

We still have to shell out to `tailscale up` because the container image's
API includes "arbitrary flags to tailscale up", unfortunately. But this
should still speed up startup a little, and also enables k8s-bound containers
to update their device information as new netmap updates come in.

Fixes #6657

Signed-off-by: David Anderson <danderson@tailscale.com>

cmd/containerboot/kube.go

@@ -22,6 +22,7 @@
 	"strings"
 	"time"
 
+	"tailscale.com/tailcfg"
 	"tailscale.com/util/multierr"
 )
 
@@ -140,9 +141,9 @@ func findKeyInKubeSecret(ctx context.Context, secretName string) (string, error)
 	return "", nil
 }
 
-// storeDeviceID writes deviceID into the "device_id" data field of
-// the kube secret secretName.
-func storeDeviceID(ctx context.Context, secretName, deviceID string) error {
+// storeDeviceInfo writes deviceID into the "device_id" data field of the kube
+// secret secretName.
+func storeDeviceInfo(ctx context.Context, secretName string, deviceID tailcfg.StableNodeID, fqdn string) error {
 	// First check if the secret exists at all. Even if running on
 	// kubernetes, we do not necessarily store state in a k8s secret.
 	req, err := http.NewRequest("GET", fmt.Sprintf("/api/v1/namespaces/%s/secrets/%s", kubeNamespace, secretName), nil)
@@ -161,7 +162,8 @@ func storeDeviceID(ctx context.Context, secretName, deviceID string) error {
 
 	m := map[string]map[string]string{
 		"stringData": {
-			"device_id": deviceID,
+			"device_id":   string(deviceID),
+			"device_fqdn": fqdn,
 		},
 	}
 	var b bytes.Buffer

cmd/containerboot/main.go

@@ -60,7 +60,8 @@
 
 	"golang.org/x/sys/unix"
 	"tailscale.com/client/tailscale"
-	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/ipn"
+	"tailscale.com/util/deephash"
 )
 
 func main() {
@@ -152,46 +153,149 @@ func main() {
 		log.Fatalf("failed to bring up tailscale: %v", err)
 	}
 
-	st, err := authTailscaled(ctx, client, cfg)
+	w, err := client.WatchIPNBus(ctx, ipn.NotifyInitialNetMap|ipn.NotifyInitialPrefs|ipn.NotifyInitialState)
 	if err != nil {
-		log.Fatalf("failed to auth tailscale: %v", err)
+		log.Fatalf("failed to watch tailscaled for updates: %v", err)
 	}
 
-	if cfg.ProxyTo != "" {
-		if err := installIPTablesRule(ctx, cfg.ProxyTo, st.TailscaleIPs); err != nil {
-			log.Fatalf("installing proxy rules: %v", err)
+	// Because we're still shelling out to `tailscale up` to get access to its
+	// flag parser, we have to stop watching the IPN bus so that we can block on
+	// the subcommand without stalling anything. Then once it's done, we resume
+	// watching the bus.
+	//
+	// Depending on the requested mode of operation, this auth step happens at
+	// different points in containerboot's lifecycle, hence the helper function.
+	didLogin := false
+	authTailscale := func() error {
+		if didLogin {
+			return nil
+		}
+		didLogin = true
+		w.Close()
+		if err := tailscaleUp(ctx, cfg); err != nil {
+			return fmt.Errorf("failed to auth tailscale: %v", err)
+		}
+		w, err = client.WatchIPNBus(ctx, ipn.NotifyInitialNetMap|ipn.NotifyInitialState)
+		if err != nil {
+			return fmt.Errorf("rewatching tailscaled for updates after auth: %v", err)
+		}
+		return nil
+	}
+
+	if !cfg.AuthOnce {
+		if err := authTailscale(); err != nil {
+			log.Fatalf("failed to auth tailscale: %v", err)
 		}
 	}
-	if cfg.InKubernetes && cfg.KubernetesCanPatch && cfg.KubeSecret != "" {
-		if err := storeDeviceID(ctx, cfg.KubeSecret, string(st.Self.ID)); err != nil {
-			log.Fatalf("storing device ID in kube secret: %v", err)
+
+authLoop:
+	for {
+		n, err := w.Next()
+		if err != nil {
+			log.Fatalf("failed to read from tailscaled: %v", err)
 		}
-		if cfg.AuthOnce {
-			// We were told to only auth once, so any secret-bound
-			// authkey is no longer needed. We don't strictly need to
-			// wipe it, but it's good hygiene.
-			log.Printf("Deleting authkey from kube secret")
-			if err := deleteAuthKey(ctx, cfg.KubeSecret); err != nil {
-				log.Fatalf("deleting authkey from kube secret: %v", err)
+
+		if n.State != nil {
+			switch *n.State {
+			case ipn.NeedsLogin:
+				if err := authTailscale(); err != nil {
+					log.Fatalf("failed to auth tailscale: %v", err)
+				}
+			case ipn.NeedsMachineAuth:
+				log.Printf("machine authorization required, please visit the admin panel")
+			case ipn.Running:
+				// Technically, all we want is to keep monitoring the bus for
+				// netmap updates. However, in order to make the container crash
+				// if tailscale doesn't initially come up, the watch has a
+				// startup deadline on it. So, we have to break out of this
+				// watch loop, cancel the watch, and watch again with no
+				// deadline to continue monitoring for changes.
+				break authLoop
+			default:
+				log.Printf("tailscaled in state %q, waiting", *n.State)
 			}
 		}
 	}
 
-	log.Println("Startup complete, waiting for shutdown signal")
-	// Reap all processes, since we are PID1 and need to collect
-	// zombies.
+	w.Close()
+
+	if cfg.InKubernetes && cfg.KubeSecret != "" && cfg.KubernetesCanPatch && cfg.AuthOnce {
+		// We were told to only auth once, so any secret-bound
+		// authkey is no longer needed. We don't strictly need to
+		// wipe it, but it's good hygiene.
+		log.Printf("Deleting authkey from kube secret")
+		if err := deleteAuthKey(ctx, cfg.KubeSecret); err != nil {
+			log.Fatalf("deleting authkey from kube secret: %v", err)
+		}
+	}
+
+	w, err = client.WatchIPNBus(context.Background(), ipn.NotifyInitialNetMap|ipn.NotifyInitialState)
+	if err != nil {
+		log.Fatalf("rewatching tailscaled for updates after auth: %v", err)
+	}
+
+	var (
+		wantProxy         = cfg.ProxyTo != ""
+		wantDeviceInfo    = cfg.InKubernetes && cfg.KubeSecret != "" && cfg.KubernetesCanPatch
+		startupTasksDone  = false
+		currentIPs        deephash.Sum // tailscale IPs assigned to device
+		currentDeviceInfo deephash.Sum // device ID and fqdn
+	)
 	for {
-		var status unix.WaitStatus
-		pid, err := unix.Wait4(-1, &status, 0, nil)
-		if errors.Is(err, unix.EINTR) {
-			continue
-		}
+		n, err := w.Next()
 		if err != nil {
-			log.Fatalf("Waiting for exited processes: %v", err)
+			log.Fatalf("failed to read from tailscaled: %v", err)
 		}
-		if pid == daemonPid {
-			log.Printf("Tailscaled exited")
-			os.Exit(0)
+
+		if n.State != nil && *n.State != ipn.Running {
+			// Something's gone wrong and we've left the authenticated state.
+			// Our container image never recovered gracefully from this, and the
+			// control flow required to make it work now is hard. So, just crash
+			// the container and rely on the container runtime to restart us,
+			// whereupon we'll go through initial auth again.
+			log.Fatalf("tailscaled left running state (now in state %q), exiting", *n.State)
+		}
+		if n.NetMap != nil {
+			if cfg.ProxyTo != "" && len(n.NetMap.Addresses) > 0 && deephash.Update(&currentIPs, &n.NetMap.Addresses) {
+				if err := installIPTablesRule(ctx, cfg.ProxyTo, n.NetMap.Addresses); err != nil {
+					log.Fatalf("installing proxy rules: %v", err)
+				}
+			}
+			deviceInfo := []any{n.NetMap.SelfNode.StableID, n.NetMap.SelfNode.Name}
+			if cfg.InKubernetes && cfg.KubernetesCanPatch && cfg.KubeSecret != "" && deephash.Update(&currentDeviceInfo, &deviceInfo) {
+				if err := storeDeviceInfo(ctx, cfg.KubeSecret, n.NetMap.SelfNode.StableID, n.NetMap.SelfNode.Name); err != nil {
+					log.Fatalf("storing device ID in kube secret: %v", err)
+				}
+			}
+		}
+		if !startupTasksDone {
+			if (!wantProxy || currentIPs != deephash.Sum{}) && (!wantDeviceInfo || currentDeviceInfo != deephash.Sum{}) {
+				// This log message is used in tests to detect when all
+				// post-auth configuration is done.
+				log.Println("Startup complete, waiting for shutdown signal")
+				startupTasksDone = true
+
+				// Reap all processes, since we are PID1 and need to collect zombies. We can
+				// only start doing this once we've stopped shelling out to things
+				// `tailscale up`, otherwise this goroutine can reap the CLI subprocesses
+				// and wedge bringup.
+				go func() {
+					for {
+						var status unix.WaitStatus
+						pid, err := unix.Wait4(-1, &status, 0, nil)
+						if errors.Is(err, unix.EINTR) {
+							continue
+						}
+						if err != nil {
+							log.Fatalf("Waiting for exited processes: %v", err)
+						}
+						if pid == daemonPid {
+							log.Printf("Tailscaled exited")
+							os.Exit(0)
+						}
+					}
+				}()
+			}
 		}
 	}
 }
@@ -242,58 +346,6 @@ func startTailscaled(ctx context.Context, cfg *settings) (*tailscale.LocalClient
 	return tsClient, cmd.Process.Pid, nil
 }
 
-// startAndAuthTailscaled starts the tailscale daemon and attempts to
-// auth it, according to the settings in cfg. If successful, returns
-// tailscaled's Status and pid.
-func authTailscaled(ctx context.Context, client *tailscale.LocalClient, cfg *settings) (*ipnstate.Status, error) {
-	didLogin := false
-	if !cfg.AuthOnce {
-		if err := tailscaleUp(ctx, cfg); err != nil {
-			return nil, fmt.Errorf("couldn't log in: %v", err)
-		}
-		didLogin = true
-	}
-
-	// Poll for daemon state until it goes to either Running or
-	// NeedsLogin. The latter only happens if cfg.AuthOnce is true,
-	// because in that case we only try to auth when it's necessary to
-	// reach the running state.
-	for {
-		if ctx.Err() != nil {
-			return nil, ctx.Err()
-		}
-
-		loopCtx, cancel := context.WithTimeout(ctx, time.Second)
-		st, err := client.Status(loopCtx)
-		cancel()
-		if err != nil {
-			return nil, fmt.Errorf("Getting tailscaled state: %w", err)
-		}
-
-		switch st.BackendState {
-		case "Running":
-			if len(st.TailscaleIPs) > 0 {
-				return st, nil
-			}
-			log.Printf("No Tailscale IPs assigned yet")
-		case "NeedsLogin":
-			if !didLogin {
-				// Alas, we cannot currently trigger an authkey login from
-				// LocalAPI, so we still have to shell out to the
-				// tailscale CLI for this bit.
-				if err := tailscaleUp(ctx, cfg); err != nil {
-					return nil, fmt.Errorf("couldn't log in: %v", err)
-				}
-				didLogin = true
-			}
-		default:
-			log.Printf("tailscaled in state %q, waiting", st.BackendState)
-		}
-
-		time.Sleep(100 * time.Millisecond)
-	}
-}
-
 // tailscaledArgs uses cfg to construct the argv for tailscaled.
 func tailscaledArgs(cfg *settings) []string {
 	args := []string{"--socket=" + cfg.Socket}
@@ -428,7 +480,7 @@ func ensureIPForwarding(root, proxyTo, routes string) error {
 	return nil
 }
 
-func installIPTablesRule(ctx context.Context, dstStr string, tsIPs []netip.Addr) error {
+func installIPTablesRule(ctx context.Context, dstStr string, tsIPs []netip.Prefix) error {
 	dst, err := netip.ParseAddr(dstStr)
 	if err != nil {
 		return err
@@ -438,16 +490,22 @@ func installIPTablesRule(ctx context.Context, dstStr string, tsIPs []netip.Addr)
 		argv0 = "ip6tables"
 	}
 	var local string
-	for _, ip := range tsIPs {
-		if ip.Is4() != dst.Is4() {
+	for _, pfx := range tsIPs {
+		if !pfx.IsSingleIP() {
 			continue
 		}
-		local = ip.String()
+		if pfx.Addr().Is4() != dst.Is4() {
+			continue
+		}
+		local = pfx.Addr().String()
 		break
 	}
 	if local == "" {
 		return fmt.Errorf("no tailscale IP matching family of %s found in %v", dstStr, tsIPs)
 	}
+	// Technically, if the control server ever changes the IPs assigned to this
+	// node, we'll slowly accumulate iptables rules. This shouldn't happen, so
+	// for now we'll live with it.
 	cmd := exec.CommandContext(ctx, argv0, "-t", "nat", "-I", "PREROUTING", "1", "-d", local, "-j", "DNAT", "--to-destination", dstStr)
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr

cmd/containerboot/main_test.go

@@ -31,9 +31,11 @@
 
 	"github.com/google/go-cmp/cmp"
 	"golang.org/x/sys/unix"
-	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/ipn"
 	"tailscale.com/tailcfg"
 	"tailscale.com/tstest"
+	"tailscale.com/types/netmap"
+	"tailscale.com/types/ptr"
 )
 
 func TestContainerBoot(t *testing.T) {
@@ -91,18 +93,13 @@ func TestContainerBoot(t *testing.T) {
 	}
 
 	argFile := filepath.Join(d, "args")
-	tsIPs := []netip.Addr{netip.MustParseAddr("100.64.0.1")}
 	runningSockPath := filepath.Join(d, "tmp/tailscaled.sock")
 
-	// TODO: refactor this 1-2 stuff if we ever need a third
-	// step. Right now all of containerboot's modes either converge
-	// with no further interaction needed, or with one extra step
-	// only.
 	type phase struct {
-		// Make LocalAPI report this status, then wait for the Wants below to be
-		// satisfied. A zero Status is a valid state for a just-started
-		// tailscaled.
-		Status ipnstate.Status
+		// If non-nil, send this IPN bus notification (and remember it as the
+		// initial update for any future new watchers, then wait for all the
+		// Waits below to be true before proceeding to the next phase.
+		Notify *ipn.Notify
 
 		// WantCmds is the commands that containerboot should run in this phase.
 		WantCmds []string
@@ -113,6 +110,16 @@ type phase struct {
 		// contents.
 		WantFiles map[string]string
 	}
+	runningNotify := &ipn.Notify{
+		State: ptr.To(ipn.Running),
+		NetMap: &netmap.NetworkMap{
+			SelfNode: &tailcfg.Node{
+				StableID: tailcfg.StableNodeID("myID"),
+				Name:     "test-node.test.ts.net",
+			},
+			Addresses: []netip.Prefix{netip.MustParsePrefix("100.64.0.1/32")},
+		},
+	}
 	tests := []struct {
 		Name          string
 		Env           map[string]string
@@ -132,10 +139,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -153,10 +157,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -174,10 +175,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -195,10 +193,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 					WantFiles: map[string]string{
 						"proc/sys/net/ipv4/ip_forward":          "0",
 						"proc/sys/net/ipv6/conf/all/forwarding": "0",
@@ -221,10 +216,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 					WantFiles: map[string]string{
 						"proc/sys/net/ipv4/ip_forward":          "1",
 						"proc/sys/net/ipv6/conf/all/forwarding": "0",
@@ -247,10 +239,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 					WantFiles: map[string]string{
 						"proc/sys/net/ipv4/ip_forward":          "0",
 						"proc/sys/net/ipv6/conf/all/forwarding": "1",
@@ -273,10 +262,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 					WantFiles: map[string]string{
 						"proc/sys/net/ipv4/ip_forward":          "1",
 						"proc/sys/net/ipv6/conf/all/forwarding": "1",
@@ -299,10 +285,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 					WantCmds: []string{
 						"/usr/bin/iptables -t nat -I PREROUTING 1 -d 100.64.0.1 -j DNAT --to-destination 1.2.3.4",
 					},
@@ -322,18 +305,15 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "NeedsLogin",
+					Notify: &ipn.Notify{
+						State: ptr.To(ipn.NeedsLogin),
 					},
 					WantCmds: []string{
 						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --authkey=tskey-key",
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -357,16 +337,11 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-						Self: &ipnstate.PeerStatus{
-							ID: tailcfg.StableNodeID("myID"),
-						},
-					},
+					Notify: runningNotify,
 					WantKubeSecret: map[string]string{
-						"authkey":   "tskey-key",
-						"device_id": "myID",
+						"authkey":     "tskey-key",
+						"device_fqdn": "test-node.test.ts.net",
+						"device_id":   "myID",
 					},
 				},
 			},
@@ -389,13 +364,7 @@ type phase struct {
 					WantKubeSecret: map[string]string{},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-						Self: &ipnstate.PeerStatus{
-							ID: tailcfg.StableNodeID("myID"),
-						},
-					},
+					Notify:         runningNotify,
 					WantKubeSecret: map[string]string{},
 				},
 			},
@@ -421,8 +390,8 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "NeedsLogin",
+					Notify: &ipn.Notify{
+						State: ptr.To(ipn.NeedsLogin),
 					},
 					WantCmds: []string{
 						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --authkey=tskey-key",
@@ -432,15 +401,56 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-						Self: &ipnstate.PeerStatus{
-							ID: tailcfg.StableNodeID("myID"),
+					Notify: runningNotify,
+					WantKubeSecret: map[string]string{
+						"device_fqdn": "test-node.test.ts.net",
+						"device_id":   "myID",
+					},
+				},
+			},
+		},
+		{
+			Name: "kube_storage_updates",
+			Env: map[string]string{
+				"KUBERNETES_SERVICE_HOST":       kube.Host,
+				"KUBERNETES_SERVICE_PORT_HTTPS": kube.Port,
+			},
+			KubeSecret: map[string]string{
+				"authkey": "tskey-key",
+			},
+			Phases: []phase{
+				{
+					WantCmds: []string{
+						"/usr/bin/tailscaled --socket=/tmp/tailscaled.sock --state=kube:tailscale --statedir=/tmp --tun=userspace-networking",
+						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --authkey=tskey-key",
+					},
+					WantKubeSecret: map[string]string{
+						"authkey": "tskey-key",
+					},
+				},
+				{
+					Notify: runningNotify,
+					WantKubeSecret: map[string]string{
+						"authkey":     "tskey-key",
+						"device_fqdn": "test-node.test.ts.net",
+						"device_id":   "myID",
+					},
+				},
+				{
+					Notify: &ipn.Notify{
+						State: ptr.To(ipn.Running),
+						NetMap: &netmap.NetworkMap{
+							SelfNode: &tailcfg.Node{
+								StableID: tailcfg.StableNodeID("newID"),
+								Name:     "new-name.test.ts.net",
+							},
+							Addresses: []netip.Prefix{netip.MustParsePrefix("100.64.0.1/32")},
 						},
 					},
 					WantKubeSecret: map[string]string{
-						"device_id": "myID",
+						"authkey":     "tskey-key",
+						"device_fqdn": "new-name.test.ts.net",
+						"device_id":   "newID",
 					},
 				},
 			},
@@ -459,13 +469,7 @@ type phase struct {
 					},
 				},
 				{
-					// The tailscale up call blocks until auth is complete, so
-					// by the time it returns the next converged state is
-					// Running.
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -482,10 +486,7 @@ type phase struct {
 					},
 				},
 				{
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -502,10 +503,7 @@ type phase struct {
 						"/usr/bin/tailscale --socket=/tmp/tailscaled.sock up --accept-dns=false --widget=rotated",
 					},
 				}, {
-					Status: ipnstate.Status{
-						BackendState: "Running",
-						TailscaleIPs: tsIPs,
-					},
+					Notify: runningNotify,
 				},
 			},
 		},
@@ -552,7 +550,7 @@ type phase struct {
 
 			var wantCmds []string
 			for _, p := range test.Phases {
-				lapi.SetStatus(p.Status)
+				lapi.Notify(p.Notify)
 				wantCmds = append(wantCmds, p.WantCmds...)
 				waitArgs(t, 2*time.Second, d, argFile, strings.Join(wantCmds, "\n"))
 				err := tstest.WaitFor(2*time.Second, func() error {
@@ -688,7 +686,8 @@ type localAPI struct {
 	srv *http.Server
 
 	sync.Mutex
-	status ipnstate.Status
+	cond   *sync.Cond
+	notify *ipn.Notify
 }
 
 func (l *localAPI) Start() error {
@@ -706,6 +705,7 @@ func (l *localAPI) Start() error {
 		Handler: l,
 	}
 	l.Path = path
+	l.cond = sync.NewCond(&l.Mutex)
 	go l.srv.Serve(ln)
 	return nil
 }
@@ -715,29 +715,49 @@ func (l *localAPI) Close() {
 }
 
 func (l *localAPI) Reset() {
-	l.SetStatus(ipnstate.Status{
-		BackendState: "NoState",
-	})
-}
-
-func (l *localAPI) SetStatus(st ipnstate.Status) {
 	l.Lock()
 	defer l.Unlock()
-	l.status = st
+	l.notify = nil
+	l.cond.Broadcast()
+}
+
+func (l *localAPI) Notify(n *ipn.Notify) {
+	if n == nil {
+		return
+	}
+	l.Lock()
+	defer l.Unlock()
+	l.notify = n
+	l.cond.Broadcast()
 }
 
 func (l *localAPI) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if r.Method != "GET" {
 		panic(fmt.Sprintf("unsupported method %q", r.Method))
 	}
-	if r.URL.Path != "/localapi/v0/status" {
-		panic(fmt.Sprintf("unsupported localAPI path %q", r.URL.Path))
+	if r.URL.Path != "/localapi/v0/watch-ipn-bus" {
+		panic(fmt.Sprintf("unsupported path %q", r.URL.Path))
 	}
+
 	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	if f, ok := w.(http.Flusher); ok {
+		f.Flush()
+	}
+	enc := json.NewEncoder(w)
 	l.Lock()
 	defer l.Unlock()
-	if err := json.NewEncoder(w).Encode(l.status); err != nil {
-		panic("json encode failed")
+	for {
+		if l.notify != nil {
+			if err := enc.Encode(l.notify); err != nil {
+				// Usually broken pipe as the test client disconnects.
+				return
+			}
+			if f, ok := w.(http.Flusher); ok {
+				f.Flush()
+			}
+		}
+		l.cond.Wait()
 	}
 }