fix: don't attempt to launch fleet desktop until the user is logged into GUI (#16090)

> 📜 Related issue: https://github.com/fleetdm/fleet/issues/14698

# Checklist for submitter

If some of the following don't apply, delete the relevant line.

<!-- Note that API documentation changes are now addressed by the
product design team. -->

- [x] Changes file added for user-visible changes in `changes/` or
`orbit/changes/`.
See [Changes
files](https://fleetdm.com/docs/contributing/committing-changes#changes-files)
for more information.
- [x] Manual QA for all new/changed functionality
  - For Orbit and Fleet Desktop changes:
- [x] Manual QA must be performed in the three main OSs, macOS, Windows
and Linux.
- [x] Auto-update manual QA, from released version of component to new
version (see [tools/tuf/test](../tools/tuf/test/README.md)).

orbit/changes/14698-macos-desktop-err

@@ -0,0 +1,2 @@
+- Fixed issue on MacOS with starting Fleet Desktop for the first time. MacOS would return an error
+  if a user is not logged in via the GUI.

orbit/cmd/orbit/orbit.go

@@ -33,6 +33,7 @@ import (
 	"github.com/fleetdm/fleet/v4/orbit/pkg/token"
 	"github.com/fleetdm/fleet/v4/orbit/pkg/update"
 	"github.com/fleetdm/fleet/v4/orbit/pkg/update/filestore"
+	"github.com/fleetdm/fleet/v4/orbit/pkg/user"
 	"github.com/fleetdm/fleet/v4/pkg/certificate"
 	"github.com/fleetdm/fleet/v4/pkg/file"
 	retrypkg "github.com/fleetdm/fleet/v4/pkg/retry"
@@ -1307,6 +1308,19 @@ func (d *desktopRunner) execute() error {
 	for {
 		// First retry logic to start fleet-desktop.
 		if done := retry(30*time.Second, false, d.interruptCh, func() bool {
+			// On MacOS, if we attempt to run Fleet Desktop while the user is not logged in through
+			// the GUI, MacOS returns an error. See https://github.com/fleetdm/fleet/issues/14698
+			// for more details.
+			loggedInGui, err := user.IsUserLoggedInViaGui()
+			if err != nil {
+				log.Debug().Err(err).Msg("desktop.IsUserLoggedInGui")
+				return true
+			}
+
+			if !loggedInGui {
+				return true
+			}
+
 			// Orbit runs as root user on Unix and as SYSTEM (Windows Service) user on Windows.
 			// To be able to run the desktop application (mostly to register the icon in the system tray)
 			// we need to run the application as the login user.

orbit/pkg/user/user_darwin.go

@@ -0,0 +1,30 @@
+//go:build darwin
+// +build darwin
+
+package user
+
+import (
+	"errors"
+	"os/exec"
+	"strings"
+)
+
+// IsUserLoggedInViaGui returns whether or not a user is logged into the machine via the GUI.
+func IsUserLoggedInViaGui() (bool, error) {
+	output, err := exec.Command("/usr/bin/stat", "-f", "%Su", "/dev/console").Output()
+	if err != nil {
+		var ee *exec.ExitError
+		if errors.As(err, &ee) && len(ee.Stderr) > 0 {
+			return false, errors.Join(err, errors.New(string(ee.Stderr)))
+		}
+
+		return false, err
+	}
+
+	// If no user is logged in via GUI, the command line returns "root".
+	if strings.TrimSpace(string(output)) == "root" {
+		return false, nil
+	}
+
+	return true, nil
+}

orbit/pkg/user/user_notdarwin.go

@@ -0,0 +1,11 @@
+//go:build !darwin
+// +build !darwin
+
+package user
+
+// IsUserLoggedInViaGui returns whether or not a user is logged into the machine via the GUI. This
+// function is only relevant on MacOS, where it is used to prevent errors when launching Fleet
+// Desktop. We assume yes (effectively a no-op) on all other platforms.
+func IsUserLoggedInViaGui() (bool, error) {
+	return true, nil
+}