cmd/tailscale/cli: add debug --ipn mode

To watch the IPN message bus.

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

cmd/tailscale/cli/debug.go

@@ -6,12 +6,15 @@ package cli
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"flag"
+	"fmt"
 	"os"
 
 	"github.com/peterbourgon/ff/v2/ffcli"
 	"tailscale.com/client/tailscale"
+	"tailscale.com/ipn"
 )
 
 var debugCmd = &ffcli.Command{
@@ -20,12 +23,16 @@ var debugCmd = &ffcli.Command{
 	FlagSet: (func() *flag.FlagSet {
 		fs := flag.NewFlagSet("debug", flag.ExitOnError)
 		fs.BoolVar(&debugArgs.goroutines, "daemon-goroutines", false, "If true, dump the tailscaled daemon's goroutines")
+		fs.BoolVar(&debugArgs.ipn, "ipn", false, "If true, subscribe to IPN notifications")
+		fs.BoolVar(&debugArgs.netMap, "netmap", true, "whether to include netmap in --ipn mode")
 		return fs
 	})(),
 }
 
 var debugArgs struct {
 	goroutines bool
+	ipn        bool
+	netMap     bool
 }
 
 func runDebug(ctx context.Context, args []string) error {
@@ -38,6 +45,22 @@ func runDebug(ctx context.Context, args []string) error {
 			return err
 		}
 		os.Stdout.Write(goroutines)
+		return nil
+	}
+	if debugArgs.ipn {
+		c, bc, ctx, cancel := connect(ctx)
+		defer cancel()
+
+		bc.SetNotifyCallback(func(n ipn.Notify) {
+			if !debugArgs.netMap {
+				n.NetMap = nil
+			}
+			j, _ := json.MarshalIndent(n, "", "\t")
+			fmt.Printf("%s\n", j)
+		})
+		bc.RequestEngineStatus()
+		pump(ctx, bc, c)
+		return errors.New("exit")
 	}
 	return nil
 }