cmd/tailscale: add systray subcommand on Linux builds

This will start including the sytray app in unstable builds for Linux,
unless the `ts_omit_systray` build flag is specified.

If we decide not to include it in the v1.88 release, we can pull it
back out or restrict it to unstable builds.

Updates #1708

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

cmd/tailscale/cli/cli.go

@@ -261,6 +261,7 @@ change in the future.
 			driveCmd,
 			idTokenCmd,
 			configureHostCmd(),
+			systrayCmd,
 		),
 		FlagSet: rootfs,
 		Exec: func(ctx context.Context, args []string) error {

cmd/tailscale/cli/systray.go

@@ -0,0 +1,24 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build linux && !ts_omit_systray
+
+package cli
+
+import (
+	"context"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+	"tailscale.com/client/systray"
+)
+
+var systrayCmd = &ffcli.Command{
+	Name:       "systray",
+	ShortUsage: "tailscale systray",
+	ShortHelp:  "Run a systray application to manage Tailscale",
+	Exec: func(_ context.Context, _ []string) error {
+		// TODO(will): pass localClient to menu to use the global --socket flag
+		new(systray.Menu).Run()
+		return nil
+	},
+}

cmd/tailscale/cli/systray_omit.go

@@ -0,0 +1,31 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build !linux || ts_omit_systray
+
+package cli
+
+import (
+	"context"
+	"fmt"
+	"strings"
+
+	"github.com/peterbourgon/ff/v3/ffcli"
+)
+
+// TODO(will): update URL to KB article when available
+var systrayHelp = strings.TrimSpace(`
+The Tailscale systray app is not included in this client build.
+To run it manually, see https://github.com/tailscale/tailscale/tree/main/cmd/systray
+`)
+
+var systrayCmd = &ffcli.Command{
+	Name:       "systray",
+	ShortUsage: "tailscale systray",
+	ShortHelp:  "Not available in this client build",
+	LongHelp:   hidden + systrayHelp,
+	Exec: func(_ context.Context, _ []string) error {
+		fmt.Println(systrayHelp)
+		return nil
+	},
+}