cmd/tailscale/cli: add 'debug stat' subcommand

For debugging what's visible inside the macOS sandbox.

But could also be useful for giving users portable commands
during debugging without worrying about which OS they're on.

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

cmd/tailscale/cli/debug.go

@@ -69,6 +69,11 @@ var debugCmd = &ffcli.Command{
 			Exec:      runEnv,
 			ShortHelp: "print cmd/tailscale environment",
 		},
+		{
+			Name:      "stat",
+			Exec:      runStat,
+			ShortHelp: "stat a file",
+		},
 		{
 			Name:      "hostinfo",
 			Exec:      runHostinfo,
@@ -284,6 +289,28 @@ func runEnv(ctx context.Context, args []string) error {
 	return nil
 }
 
+func runStat(ctx context.Context, args []string) error {
+	for _, a := range args {
+		fi, err := os.Lstat(a)
+		if err != nil {
+			fmt.Printf("%s: %v\n", a, err)
+			continue
+		}
+		fmt.Printf("%s: %v, %v\n", a, fi.Mode(), fi.Size())
+		if fi.IsDir() {
+			ents, _ := os.ReadDir(a)
+			for i, ent := range ents {
+				if i == 25 {
+					fmt.Printf("  ...\n")
+					break
+				}
+				fmt.Printf("  - %s\n", ent.Name())
+			}
+		}
+	}
+	return nil
+}
+
 func runHostinfo(ctx context.Context, args []string) error {
 	hi := hostinfo.New()
 	j, _ := json.MarshalIndent(hi, "", "  ")