cmd/tailscaled, ipn/{ipnlocal,ipnserver}: let netstack get access to LocalBackend

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/ipnserver/server.go

@@ -92,6 +92,10 @@ type Options struct {
 	// DebugMux, if non-nil, specifies an HTTP ServeMux in which
 	// to register a debug handler.
 	DebugMux *http.ServeMux
+
+	// OnBackendCreated, if non-nil, is called once when the LocalBackend
+	// is created.
+	OnBackendCreated func(*ipnlocal.LocalBackend)
 }
 
 // server is an IPN backend and its set of 0 or more active connections
@@ -739,6 +743,10 @@ func Run(ctx context.Context, logf logger.Logf, logid string, getEngine func() (
 		return smallzstd.NewDecoder(nil)
 	})
 
+	if opts.OnBackendCreated != nil {
+		opts.OnBackendCreated(b)
+	}
+
 	if opts.DebugMux != nil {
 		opts.DebugMux.HandleFunc("/debug/ipn", func(w http.ResponseWriter, r *http.Request) {
 			serveHTMLStatus(w, b)