testcontrol: ensure Server.UpdateNode triggers netmap updates

Updating a node on a testcontrol server should trigger netmap updates to
all connected streaming clients. This was not the case previous to this
change and consequently caused race conditions in tests. It was possible
for a test to call UpdateNode and for connected nodes to never see the
update propagate.

Updates #16340
Fixes #18703

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

tstest/integration/testcontrol/testcontrol.go

@@ -1075,9 +1075,7 @@ func sendUpdate(dst chan<- updateType, updateType updateType) bool {
 	}
 }
 
-func (s *Server) UpdateNode(n *tailcfg.Node) (peersToUpdate []tailcfg.NodeID) {
-	s.mu.Lock()
-	defer s.mu.Unlock()
+func (s *Server) updateNodeLocked(n *tailcfg.Node) (peersToUpdate []tailcfg.NodeID) {
 	if n.Key.IsZero() {
 		panic("zero nodekey")
 	}
@@ -1085,6 +1083,15 @@ func (s *Server) UpdateNode(n *tailcfg.Node) (peersToUpdate []tailcfg.NodeID) {
 	return s.nodeIDsLocked(n.ID)
 }
 
+// UpdateNode updates or adds the input node, then triggers a netmap update for
+// all attached streaming clients.
+func (s *Server) UpdateNode(n *tailcfg.Node) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.updateNodeLocked(n)
+	s.updateLocked("UpdateNode", s.nodeIDsLocked(0))
+}
+
 func (s *Server) incrInServeMap(delta int) {
 	s.mu.Lock()
 	defer s.mu.Unlock()
@@ -1143,7 +1150,9 @@ func (s *Server) serveMap(w http.ResponseWriter, r *http.Request, mkey key.Machi
 				}
 			}
 		}
-		peersToUpdate = s.UpdateNode(node)
+		s.mu.Lock()
+		peersToUpdate = s.updateNodeLocked(node)
+		s.mu.Unlock()
 	}
 
 	nodeID := node.ID