wgengine/router: add a CallbackRouter shim.

The shim implements both network and DNS configurators,
and feeds both into a single callback that receives
both configs.

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

wgengine/router/callback.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2021 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 (
+	"sync"
+
+	"tailscale.com/net/dns"
+)
+
+// CallbackRouter is an implementation of both Router and dns.OSConfigurator.
+// When either network or DNS settings are changed, SetBoth is called with both configs.
+// Mainly used as a shim for OSes that want to set both network and
+// DNS configuration simultaneously (iOS, android).
+type CallbackRouter struct {
+	SetBoth func(rcfg *Config, dcfg *dns.OSConfig) error
+	DNSMode dns.RoutingMode
+
+	mu   sync.Mutex    // protects all the following
+	rcfg *Config       // last applied router config
+	dcfg *dns.OSConfig // last applied DNS config
+}
+
+// Up implements Router.
+func (r *CallbackRouter) Up() error {
+	return nil // TODO: check that all callers have no need for initialization
+}
+
+// Set implements Router.
+func (r *CallbackRouter) Set(rcfg *Config) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.rcfg = rcfg
+	return r.SetBoth(r.rcfg, r.dcfg)
+}
+
+// SetDNS implements dns.OSConfigurator.
+func (r *CallbackRouter) SetDNS(dcfg dns.OSConfig) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.dcfg = &dcfg
+	return r.SetBoth(r.rcfg, r.dcfg)
+}
+
+// RoutingMode implements dns.OSConfigurator.
+func (r *CallbackRouter) RoutingMode() dns.RoutingMode {
+	return r.DNSMode
+}
+
+func (r *CallbackRouter) Close() error {
+	return r.SetBoth(nil, nil) // TODO: check if makes sense
+}