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

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

cmd/tailscaled/tailscaled.go

@@ -29,6 +29,7 @@ import (
 	"time"
 
 	"github.com/go-multierror/multierror"
+	"tailscale.com/ipn/ipnlocal"
 	"tailscale.com/ipn/ipnserver"
 	"tailscale.com/logpolicy"
 	"tailscale.com/net/socks5"
@@ -227,6 +228,8 @@ func run() error {
 		return err
 	}
 
+	localBEFuture := ipnlocal.NewLocalBackendFuture()
+
 	var ns *netstack.Impl
 	if useNetstack {
 		tunDev, magicConn := e.(wgengine.InternalsGetter).GetInternals()
@@ -298,6 +301,7 @@ func run() error {
 		LegacyConfigPath:   paths.LegacyConfigPath(),
 		SurviveDisconnects: true,
 		DebugMux:           debugMux,
+		OnBackendCreated:   localBEFuture.Set,
 	}
 	err = ipnserver.Run(ctx, logf, pol.PublicID.String(), ipnserver.FixedEngine(e), opts)
 	// Cancelation is not an error: it is the only way to stop ipnserver.

ipn/ipnlocal/future.go

@@ -0,0 +1,28 @@
+// 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 ipnlocal
+
+import "sync"
+
+// LocalBackendFuture is a Future that returns a *LocalBackend.
+type LocalBackendFuture struct {
+	getOnce sync.Once
+	ch      chan *LocalBackend
+	v       *LocalBackend
+}
+
+func (f *LocalBackendFuture) Get() *LocalBackend {
+	f.getOnce.Do(f.get)
+	return f.v
+}
+
+func (f *LocalBackendFuture) get()                { f.v = <-f.ch }
+func (f *LocalBackendFuture) Set(v *LocalBackend) { f.ch <- v }
+
+func NewLocalBackendFuture() *LocalBackendFuture {
+	return &LocalBackendFuture{
+		ch: make(chan *LocalBackend, 1),
+	}
+}

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)