wgengine: make SetRoutesFunc less appealing to use.

It exists as a way to glue the mac-specific xcode logic with the OSS
logic, and shouldn't be used by anything but the xcode glue.

wgengine/router_darwin_support.go

@@ -0,0 +1,25 @@
+// 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.
+
+// +build linux darwin
+
+package wgengine
+
+// SetRoutesFunc applies the given route settings to the OS network
+// stack.
+//
+// This is logically part of the router_darwin.go implementation, and
+// should not be used on other platforms.
+//
+// The code to reconfigure the network stack on MacOS and iOS is in
+// the non-open `ipn-go-bridge` package, which bridges between the Go
+// and Swift pieces of the application. The ipn-go-bridge sets
+// SetRoutesFunc at startup.
+//
+// So why isn't this in router_darwin.go? Because in the non-oss
+// repository, we build ipn-go-bridge when developing on Linux as well
+// as MacOS, so that we don't have to wait until the Mac CI to
+// discover that we broke it. So this one definition needs to exist in
+// both the darwin and linux builds. Hence this file and build tag.
+var SetRoutesFunc func(rs RouteSettings) error

wgengine/wgengine.go

@@ -36,10 +36,6 @@ type RouteSettings struct {
 	Cfg        wgcfg.Config
 }
 
-// Only used on darwin for now
-// TODO(apenwarr): This probably belongs in the darwinRouter struct.
-var SetRoutesFunc func(rs RouteSettings) error
-
 func (rs *RouteSettings) OnlyRelevantParts() string {
 	var peers [][]wgcfg.CIDR
 	for _, p := range rs.Cfg.Peers {