libtailscale: use b.netmon and sys.Bus consistently

libtailscale/backend.go

@@ -249,6 +249,11 @@ func (a *App) runBackend(ctx context.Context, hardwareAttestation bool) error {
 				vpnService.service = nil
 			}
 		case i := <-onDNSConfigChanged:
+			// TODO (barnstar): Consider using [dns.Manager.RecompileDNSConfig] here.
+			// NetworkChanged injects a netmon event that has the side effect
+			// regenerating the DNS config but have the means to do
+			// that independently of userspace engine network changes which may
+			// eliminate some unnecessary work.
 			go b.NetworkChanged(i)
 		}
 	}
@@ -265,7 +270,7 @@ func (a *App) newBackend(dataDir string, appCtx AppContext, store *stateStore,
 		devices:  newTUNDevices(),
 		settings: settings,
 		appCtx:   appCtx,
-		bus:      eventbus.New(),
+		bus:      sys.Bus.Get(),
 	}
 
 	var logID logid.PrivateID

libtailscale/net.go

@@ -275,10 +275,10 @@ func (b *backend) NetworkChanged(ifname string) {
 
 	// Set the interface name and alert the monitor.
 	netmon.UpdateLastKnownDefaultRouteInterface(ifname)
-	if b.sys != nil {
-		if nm, ok := b.sys.NetMon.GetOK(); ok {
-			nm.InjectEvent()
-		}
+	if b.netMon != nil {
+		b.netMon.InjectEvent()
+	} else {
+		log.Printf("NetworkChanged: netMon is nil")
 	}
 }