cmd/tailscale: print a helpful error for Taildrive CLI on macOS GUI

Rather than printing `unknown subcommand: drive` for any Taildrive
commands run in the macOS GUI, print an error message directing the user
to the GUI client and the docs page.

Updates #17210
Fixes #18823

Change-Id: I6435007b5911baee79274b56e3ee101e6bb6d809
Signed-off-by: Alex Chan <alexc@tailscale.com>

cmd/tailscale/cli/drive_macgui.go

@@ -0,0 +1,33 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_drive && ts_mac_gui
+
+package cli
+
+import (
+	"context"
+	"errors"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+func init() {
+	maybeDriveCmd = driveCmdStub
+}
+
+func driveCmdStub() *ffcli.Command {
+	return &ffcli.Command{
+		Name:       "drive",
+		ShortHelp:  "Share a directory with your tailnet",
+		ShortUsage: "tailscale drive [...any]",
+		LongHelp:   hidden + "Taildrive allows you to share directories with other machines on your tailnet.",
+		Exec: func(_ context.Context, args []string) error {
+			return errors.New(
+				"Taildrive CLI commands are not supported when using the macOS GUI app. " +
+					"Please use the Tailscale menu bar icon to configure Taildrive in Settings.\n\n" +
+					"See https://tailscale.com/docs/features/taildrive",
+			)
+		},
+	}
+}

cmd/tailscale/cli/drive_macgui_test.go

@@ -0,0 +1,66 @@
+// Copyright (c) Tailscale Inc & contributors
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !ts_omit_drive && ts_mac_gui
+
+package cli
+
+import (
+	"bytes"
+	"context"
+	"flag"
+	"strings"
+	"testing"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+// In macOS GUI builds, the `drive` command should not appear in
+// the help text generated by ffcli.
+func TestDriveCommandHiddenInHelpText(t *testing.T) {
+	root := newRootCmd()
+
+	var buf bytes.Buffer
+	root.FlagSet = flag.NewFlagSet("tailscale", flag.ContinueOnError)
+	root.FlagSet.SetOutput(&buf)
+
+	ffcli.DefaultUsageFunc(root)
+
+	output := buf.String()
+
+	if strings.Contains(output, "drive") {
+		t.Errorf("found hidden command 'drive' in help output:\n%q", output)
+	}
+}
+
+// Running the drive command always prints an error pointing you to
+// the GUI app, regardless of input.
+func TestDriveCommandPrintsError(t *testing.T) {
+	commands := [][]string{
+		{"drive"},
+		{"drive", "share", "myfile.txt", "/path/to/myfile.txt"},
+		{"drive", "rename", "oldname.txt", "newname.txt"},
+		{"drive", "unshare", "myfile.txt"},
+		{"drive", "list"},
+	}
+
+	for _, args := range commands {
+		root := newRootCmd()
+
+		if err := root.Parse(args); err != nil {
+			t.Errorf("unable to parse args %q, got err %v", args, err)
+			continue
+		}
+
+		t.Logf("running `tailscale drive %q`", strings.Join(args, " "))
+		err := root.Run(context.Background())
+		if err == nil {
+			t.Error("expected error, but got nil", args)
+		}
+
+		expectedText := "Taildrive CLI commands are not supported when using the macOS GUI app."
+		if !strings.Contains(err.Error(), expectedText) {
+			t.Errorf("error was not expected: want %q, got %q", expectedText, err.Error())
+		}
+	}
+}