chore: add setCallback to nodeUpdater (#11635)

we need to be able to (re-)set the node callback when we lose and regain connection to a coordinator over the network.

tailnet/node.go

@@ -55,14 +55,20 @@ func (u *nodeUpdater) updateLoop() {
 			u.logger.Debug(context.Background(), "closing nodeUpdater updateLoop")
 			return
 		}
-		node := u.nodeLocked()
 		u.dirty = false
 		u.phase = configuring
 		u.Broadcast()
 
+		callback := u.callback
+		if callback == nil {
+			u.logger.Debug(context.Background(), "skipped sending node; no node callback")
+			continue
+		}
+
 		// We cannot reach nodes without DERP for discovery. Therefore, there is no point in sending
 		// the node without this, and we can save ourselves from churn in the tailscale/wireguard
 		// layer.
+		node := u.nodeLocked()
 		if node.PreferredDERP == 0 {
 			u.logger.Debug(context.Background(), "skipped sending node; no PreferredDERP", slog.F("node", node))
 			continue
@@ -70,7 +76,7 @@ func (u *nodeUpdater) updateLoop() {
 
 		u.L.Unlock()
 		u.logger.Debug(context.Background(), "calling nodeUpdater callback", slog.F("node", node))
-		u.callback(node)
+		callback(node)
 		u.L.Lock()
 	}
 }
@@ -155,7 +161,7 @@ func (u *nodeUpdater) setDERPForcedWebsocket(region int, reason string) {
 }
 
 // setStatus handles the status callback from the wireguard engine to learn about new endpoints
-// (e.g. discovered by STUN)
+// (e.g. discovered by STUN).  u.L MUST NOT be held
 func (u *nodeUpdater) setStatus(s *wgengine.Status, err error) {
 	u.logger.Debug(context.Background(), "wireguard status", slog.F("status", s), slog.Error(err))
 	if err != nil {
@@ -181,6 +187,7 @@ func (u *nodeUpdater) setStatus(s *wgengine.Status, err error) {
 	u.Broadcast()
 }
 
+// setAddresses sets the local addresses for the node. u.L MUST NOT be held.
 func (u *nodeUpdater) setAddresses(ips []netip.Prefix) {
 	u.L.Lock()
 	defer u.L.Unlock()
@@ -192,3 +199,13 @@ func (u *nodeUpdater) setAddresses(ips []netip.Prefix) {
 	u.dirty = true
 	u.Broadcast()
 }
+
+// setCallback sets the callback for node changes. It also triggers a call
+// for the current node immediately. u.L MUST NOT be held.
+func (u *nodeUpdater) setCallback(callback func(node *Node)) {
+	u.L.Lock()
+	defer u.L.Unlock()
+	u.callback = callback
+	u.dirty = true
+	u.Broadcast()
+}

tailnet/node_internal_test.go

@@ -441,3 +441,44 @@ func TestNodeUpdater_setAddresses_same(t *testing.T) {
 	}()
 	_ = testutil.RequireRecvCtx(ctx, t, done)
 }
+
+func TestNodeUpdater_setCallback(t *testing.T) {
+	t.Parallel()
+	ctx := testutil.Context(t, testutil.WaitShort)
+	logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
+	id := tailcfg.NodeID(1)
+	nodeKey := key.NewNode().Public()
+	discoKey := key.NewDisco().Public()
+	uut := newNodeUpdater(
+		logger,
+		nil,
+		id, nodeKey, discoKey,
+	)
+	defer uut.close()
+
+	// Given: preferred DERP is 1
+	addrs := []netip.Prefix{netip.MustParsePrefix("192.168.0.200/32")}
+	uut.L.Lock()
+	uut.preferredDERP = 1
+	uut.addresses = slices.Clone(addrs)
+	uut.L.Unlock()
+
+	// When: we set callback
+	nodeCh := make(chan *Node)
+	uut.setCallback(func(n *Node) {
+		nodeCh <- n
+	})
+
+	// Then: we get a node update
+	node := testutil.RequireRecvCtx(ctx, t, nodeCh)
+	require.Equal(t, nodeKey, node.Key)
+	require.Equal(t, discoKey, node.DiscoKey)
+	require.Equal(t, 1, node.PreferredDERP)
+
+	done := make(chan struct{})
+	go func() {
+		defer close(done)
+		uut.close()
+	}()
+	_ = testutil.RequireRecvCtx(ctx, t, done)
+}