client/systray: add startup script generator for systemd (#16801)

Updates #1708

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

cmd/tailscale/cli/systray.go

@@ -7,17 +7,41 @@ package cli
 
 import (
 	"context"
+	"flag"
+	"fmt"
 
 	"github.com/peterbourgon/ff/v3/ffcli"
 	"tailscale.com/client/systray"
 )
 
+var systrayArgs struct {
+	initSystem     string
+	installStartup bool
+}
+
 var systrayCmd = &ffcli.Command{
 	Name:       "systray",
 	ShortUsage: "tailscale systray",
 	ShortHelp:  "Run a systray application to manage Tailscale",
-	Exec: func(_ context.Context, _ []string) error {
-		new(systray.Menu).Run(&localClient)
-		return nil
-	},
+	LongHelp: `Run a systray application to manage Tailscale.
+To have the application run on startup, use the --enable-startup flag.`,
+	Exec: runSystray,
+	FlagSet: (func() *flag.FlagSet {
+		fs := newFlagSet("systray")
+		fs.StringVar(&systrayArgs.initSystem, "enable-startup", "",
+			"Install startup script for init system. Currently supported systems are [systemd].")
+		return fs
+	})(),
+}
+
+func runSystray(ctx context.Context, _ []string) error {
+	if systrayArgs.initSystem != "" {
+		if err := systray.InstallStartupScript(systrayArgs.initSystem); err != nil {
+			fmt.Printf("%s\n\n", err.Error())
+			return flag.ErrHelp
+		}
+		return nil
+	}
+	new(systray.Menu).Run(&localClient)
+	return nil
 }