cmd/tailscale/cli: add hidden debug subcommand

cmd/tailscale/cli/cli.go

@@ -32,6 +32,7 @@ func ActLikeCLI() bool {
 	}
 	switch os.Args[1] {
 	case "up", "status", "netcheck", "ping", "version",
+		"debug",
 		"-V", "--version", "-h", "--help":
 		return true
 	}
@@ -66,6 +67,11 @@ change in the future.
 		Exec:    func(context.Context, []string) error { return flag.ErrHelp },
 	}
 
+	// Don't advertise the debug command, but it exists.
+	if strSliceContains(args, "debug") {
+		rootCmd.Subcommands = append(rootCmd.Subcommands, debugCmd)
+	}
+
 	if err := rootCmd.Parse(args); err != nil {
 		return err
 	}
@@ -127,3 +133,12 @@ func pump(ctx context.Context, bc *ipn.BackendClient, conn net.Conn) {
 		bc.GotNotifyMsg(msg)
 	}
 }
+
+func strSliceContains(ss []string, s string) bool {
+	for _, v := range ss {
+		if v == s {
+			return true
+		}
+	}
+	return false
+}

cmd/tailscale/cli/debug.go

@@ -0,0 +1,67 @@
+// 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 cli
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"flag"
+	"log"
+	"os"
+
+	"github.com/peterbourgon/ff/v2/ffcli"
+	"tailscale.com/net/interfaces"
+	"tailscale.com/wgengine/monitor"
+)
+
+var debugCmd = &ffcli.Command{
+	Name: "debug",
+	Exec: runDebug,
+	FlagSet: (func() *flag.FlagSet {
+		fs := flag.NewFlagSet("debug", flag.ExitOnError)
+		fs.BoolVar(&debugArgs.monitor, "monitor", false, "")
+		return fs
+	})(),
+}
+
+var debugArgs struct {
+	monitor bool
+}
+
+func runDebug(ctx context.Context, args []string) error {
+	if len(args) > 0 {
+		return errors.New("unknown arguments")
+	}
+	if debugArgs.monitor {
+		return runMonitor(ctx)
+	}
+	return errors.New("only --monitor is available at the moment")
+}
+
+func runMonitor(ctx context.Context) error {
+	dump := func() {
+		st, err := interfaces.GetState()
+		if err != nil {
+			log.Printf("error getting state: %v", err)
+			return
+		}
+		j, _ := json.MarshalIndent(st, "", "    ")
+		os.Stderr.Write(j)
+	}
+	mon, err := monitor.New(log.Printf, func() {
+		log.Printf("Link monitor fired. State:")
+		dump()
+	})
+	if err != nil {
+		return err
+	}
+	log.Printf("Starting link change monitor; initial state:")
+	dump()
+	mon.Start()
+	log.Printf("Started link change monitor; waiting...")
+	select {}
+	return nil
+}