cmd/tailscaled: rename variables to be more descriptive

renamed from `useNetstack` to `onlyNetstack` which is 1 letter more but
more descriptive because we always have netstack enabled and `useNetstack`
doesn't convey what it is supposed to be used for. e.g. we always use
netstack for Tailscale SSH.

Also renamed shouldWrapNetstack to handleSubnetsInNetstack as it was only used
to configure subnet routing via netstack.

Updates tailscale/corp#8020

Signed-off-by: Maisem Ali <maisem@tailscale.com>

cmd/tailscaled/tailscaled.go

@@ -462,7 +462,7 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logid string) (_ *ip
 	socksListener, httpProxyListener := mustStartProxyListeners(args.socksAddr, args.httpProxyAddr)
 
 	dialer := &tsdial.Dialer{Logf: logf} // mutated below (before used)
-	e, useNetstack, err := createEngine(logf, linkMon, dialer)
+	e, onlyNetstack, err := createEngine(logf, linkMon, dialer)
 	if err != nil {
 		return nil, fmt.Errorf("createEngine: %w", err)
 	}
@@ -482,10 +482,10 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logid string) (_ *ip
 	if err != nil {
 		return nil, fmt.Errorf("newNetstack: %w", err)
 	}
-	ns.ProcessLocalIPs = useNetstack
-	ns.ProcessSubnets = useNetstack || shouldWrapNetstack()
+	ns.ProcessLocalIPs = onlyNetstack
+	ns.ProcessSubnets = onlyNetstack || handleSubnetsInNetstack()
 
-	if useNetstack {
+	if onlyNetstack {
 		dialer.UseNetstackForIP = func(ip netip.Addr) bool {
 			_, ok := e.PeerForIP(ip)
 			return ok
@@ -540,16 +540,21 @@ func getLocalBackend(ctx context.Context, logf logger.Logf, logid string) (_ *ip
 	return lb, nil
 }
 
-func createEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer) (e wgengine.Engine, useNetstack bool, err error) {
+// createEngine tries to the wgengine.Engine based on the order of tunnels
+// specified in the command line flags.
+//
+// onlyNetstack is true if the user has explicitly requested that we use netstack
+// for all networking.
+func createEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer) (e wgengine.Engine, onlyNetstack bool, err error) {
 	if args.tunname == "" {
 		return nil, false, errors.New("no --tun value specified")
 	}
 	var errs []error
 	for _, name := range strings.Split(args.tunname, ",") {
 		logf("wgengine.NewUserspaceEngine(tun %q) ...", name)
-		e, useNetstack, err = tryEngine(logf, linkMon, dialer, name)
+		e, onlyNetstack, err = tryEngine(logf, linkMon, dialer, name)
 		if err == nil {
-			return e, useNetstack, nil
+			return e, onlyNetstack, nil
 		}
 		logf("wgengine.NewUserspaceEngine(tun %q) error: %v", name, err)
 		errs = append(errs, err)
@@ -557,8 +562,12 @@ func createEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer)
 	return nil, false, multierr.New(errs...)
 }
 
-func shouldWrapNetstack() bool {
-	if v, ok := envknob.LookupBool("TS_DEBUG_WRAP_NETSTACK"); ok {
+// handleSubnetsInNetstack reports whether netstack should handle subnet routers
+// as opposed to the OS. We do this if the OS doesn't support subnet routers
+// (e.g. Windows) or if the user has explicitly requested it (e.g.
+// --tun=userspace-networking).
+func handleSubnetsInNetstack() bool {
+	if v, ok := envknob.LookupBool("TS_DEBUG_NETSTACK_SUBNETS"); ok {
 		return v
 	}
 	if distro.Get() == distro.Synology {
@@ -575,15 +584,15 @@ func shouldWrapNetstack() bool {
 
 var tstunNew = tstun.New
 
-func tryEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer, name string) (e wgengine.Engine, useNetstack bool, err error) {
+func tryEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer, name string) (e wgengine.Engine, onlyNetsack bool, err error) {
 	conf := wgengine.Config{
 		ListenPort:  args.port,
 		LinkMonitor: linkMon,
 		Dialer:      dialer,
 	}
 
-	useNetstack = name == "userspace-networking"
-	netns.SetEnabled(!useNetstack)
+	onlyNetsack = name == "userspace-networking"
+	netns.SetEnabled(!onlyNetsack)
 
 	if args.birdSocketPath != "" && createBIRDClient != nil {
 		log.Printf("Connecting to BIRD at %s ...", args.birdSocketPath)
@@ -592,7 +601,7 @@ func tryEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer, na
 			return nil, false, fmt.Errorf("createBIRDClient: %w", err)
 		}
 	}
-	if useNetstack {
+	if onlyNetsack {
 		if runtime.GOOS == "linux" && distro.Get() == distro.Synology {
 			// On Synology in netstack mode, still init a DNS
 			// manager (directManager) to avoid the health check
@@ -631,15 +640,15 @@ func tryEngine(logf logger.Logf, linkMon *monitor.Mon, dialer *tsdial.Dialer, na
 		}
 		conf.DNS = d
 		conf.Router = r
-		if shouldWrapNetstack() {
+		if handleSubnetsInNetstack() {
 			conf.Router = netstack.NewSubnetRouterWrapper(conf.Router)
 		}
 	}
 	e, err = wgengine.NewUserspaceEngine(logf, conf)
 	if err != nil {
-		return nil, useNetstack, err
+		return nil, onlyNetsack, err
 	}
-	return e, useNetstack, nil
+	return e, onlyNetsack, nil
 }
 
 func newDebugMux() *http.ServeMux {