wgengine/router: split out from wgengine.

The router implementations are logically separate, with their own API.

Signed-off-by: David Anderson <danderson@tailscale.com>

wgengine/router/router_darwin.go

@@ -0,0 +1,38 @@
+// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package router
+
+import (
+	"github.com/tailscale/wireguard-go/device"
+	"github.com/tailscale/wireguard-go/tun"
+	"tailscale.com/types/logger"
+)
+
+type darwinRouter struct {
+	tunname string
+}
+
+func newUserspaceRouter(logf logger.Logf, _ *device.Device, tundev tun.Device) (Router, error) {
+	tunname, err := tundev.Name()
+	if err != nil {
+		return nil, err
+	}
+	return &darwinRouter{tunname: tunname}, nil
+}
+
+func (r *darwinRouter) Up() error {
+	return nil
+}
+
+func (r *darwinRouter) SetRoutes(rs RouteSettings) error {
+	if SetRoutesFunc != nil {
+		return SetRoutesFunc(rs)
+	}
+	return nil
+}
+
+func (r *darwinRouter) Close() error {
+	return nil
+}