tsnet: block rather than poll in setup for TestListenService

TestListenService needs to setup state (capabilities, advertised routes,
ACL tags, etc.). It is imperative that this state propagates to all nodes
in the test tailnet before proceeding with the test. To achieve this,
TestListenService currently polls each node's local backend in a loop.
Using local.Client.WatchIPNBus improves the situation by blocking until
a new netmap comes in.

Fixes tailscale/corp#36244

Signed-off-by: Harry Harpham <harry@tailscale.com>

tsnet/tsnet_test.go

@@ -58,6 +58,7 @@ import (
 	"tailscale.com/types/ipproto"
 	"tailscale.com/types/key"
 	"tailscale.com/types/logger"
+	"tailscale.com/types/netmap"
 	"tailscale.com/types/views"
 	"tailscale.com/util/mak"
 	"tailscale.com/util/must"
@@ -1280,30 +1281,20 @@ func TestListenService(t *testing.T) {
 
 				// Wait until both nodes have up-to-date netmaps before
 				// proceeding with the test.
-				netmapUpToDate := func(s *Server) bool {
-					nm := s.lb.NetMap()
-					return slices.ContainsFunc(nm.DNS.ExtraRecords, func(r tailcfg.DNSRecord) bool {
+				netmapUpToDate := func(nm *netmap.NetworkMap) bool {
+					return nm != nil && slices.ContainsFunc(nm.DNS.ExtraRecords, func(r tailcfg.DNSRecord) bool {
 						return r.Value == serviceVIP
 					})
 				}
-				for !netmapUpToDate(serviceClient) {
-					time.Sleep(10 * time.Millisecond)
-				}
-				for !netmapUpToDate(serviceHost) {
-					time.Sleep(10 * time.Millisecond)
-				}
-
-				// == Done setting up mock state ==
-
-				// Start the Service listeners.
-				listeners := make([]*ServiceListener, 0, len(tt.modes))
-				for _, input := range tt.modes {
-					ln := must.Get(serviceHost.ListenService(serviceName.String(), input))
-					defer ln.Close()
-					listeners = append(listeners, ln)
+				waitForLatestNetmap := func(t *testing.T, s *Server) {
+					t.Helper()
+					w := must.Get(s.localClient.WatchIPNBus(t.Context(), ipn.NotifyInitialNetMap))
+					defer w.Close()
+					for n := must.Get(w.Next()); !netmapUpToDate(n.NetMap); n = must.Get(w.Next()) {
+					}
 				}
-
-				tt.run(t, listeners, serviceClient)
+				waitForLatestNetmap(t, serviceClient)
+				waitForLatestNetmap(t, serviceHost)
 			}
 
 			t.Run("TUN", func(t *testing.T) { doTest(t, true) })