cmd/tailscale/cli: add "help" alias for --help

Fixes #14053

Change-Id: I0a13e11af089f02b0656fea0d316543c67591fb5
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -93,8 +93,13 @@ func Run(args []string) (err error) {
 
 	args = CleanUpArgs(args)
 
-	if len(args) == 1 && (args[0] == "-V" || args[0] == "--version") {
+	if len(args) == 1 {
-		args = []string{"version"}
+		switch args[0] {
+		case "-V", "--version":
+			args = []string{"version"}
+		case "help":
+			args = []string{"--help"}
+		}
 	}
 
 	var warnOnce sync.Once

cmd/tailscale/cli/cli_test.go

@@ -9,6 +9,7 @@ import (
 	"encoding/json"
 	"flag"
 	"fmt"
+	"io"
 	"net/netip"
 	"reflect"
 	"strings"
@@ -1480,3 +1481,33 @@ func TestParseNLArgs(t *testing.T) {
 		})
 	}
 }
+
+func TestHelpAlias(t *testing.T) {
+	var stdout, stderr bytes.Buffer
+	tstest.Replace[io.Writer](t, &Stdout, &stdout)
+	tstest.Replace[io.Writer](t, &Stderr, &stderr)
+
+	gotExit0 := false
+	defer func() {
+		if !gotExit0 {
+			t.Error("expected os.Exit(0) to be called")
+			return
+		}
+		if !strings.Contains(stderr.String(), "SUBCOMMANDS") {
+			t.Errorf("expected help output to contain SUBCOMMANDS; got stderr=%q; stdout=%q", stderr.String(), stdout.String())
+		}
+	}()
+	defer func() {
+		if e := recover(); e != nil {
+			if strings.Contains(fmt.Sprint(e), "unexpected call to os.Exit(0)") {
+				gotExit0 = true
+			} else {
+				t.Errorf("unexpected panic: %v", e)
+			}
+		}
+	}()
+	err := Run([]string{"help"})
+	if err != nil {
+		t.Fatalf("Run: %v", err)
+	}
+}