ipn/ipnlocal: disallow exit nodes from using exit nodes

Nodes which have both -advertise-exit-node and -exit-node in prefs
should continue have them until the next invocation of `tailscale up`.

Updates #3569.

Signed-off-by: salman <salman@tailscale.com>

ipn/ipnlocal/local.go

@@ -2362,6 +2362,9 @@ func (b *LocalBackend) checkPrefsLocked(p *ipn.Prefs) error {
 	if err := b.checkSSHPrefsLocked(p); err != nil {
 		errs = append(errs, err)
 	}
+	if err := b.checkExitNodePrefsLocked(p); err != nil {
+		errs = append(errs, err)
+	}
 	return multierr.New(errs...)
 }
 
@@ -2441,6 +2444,13 @@ func (b *LocalBackend) isDefaultServerLocked() bool {
 	return prefs.ControlURLOrDefault() == ipn.DefaultControlURL
 }
 
+func (b *LocalBackend) checkExitNodePrefsLocked(p *ipn.Prefs) error {
+	if (p.ExitNodeIP.IsValid() || p.ExitNodeID != "") && p.AdvertisesExitNode() {
+		return errors.New("Cannot advertise an exit node and use an exit node at the same time.")
+	}
+	return nil
+}
+
 func (b *LocalBackend) EditPrefs(mp *ipn.MaskedPrefs) (ipn.PrefsView, error) {
 	b.mu.Lock()
 	if mp.EggSet {