Support Ubuntu 24.04 with Xorg (by detecting user's DISPLAY environment variable) (#18996)

#18925 (Should also fix #17660.)

Tests:
- Ubuntu 22.04.2
	- Wayland
		- Works with chrome ✅ 
		- Doesn't work with Firefox. ❌
	- Xorg
		- Works with Chrome. ✅ 
		- Works with Firefox. ✅
- Ubuntu 24.04
	- Wayland
		- Doesn't work with Chrome. ❌
		- Doesn't work with Firefox. ❌
- Xorg (when using Xorg it defaults to `DISPLAY=:1`, and with the
changes in this PR it works):
		- Works with Chrome. ✅ 
		- Works with Firefox. ✅ 
---

How to change between Wayland and Xorg:

- Set `WaylandEnable=false` in `/etc/gdm3/custom.conf` and reboot.

---

How to determine what's running:
```sh
$ loginctl
SESSION  UID USER SEAT  TTY 
      2 1000 luk  seat0 tty2
     c2 1000 luk        

$ loginctl show-session 2 -p Type
# will output
Type=wayland
or
Type=x11
```
---

- [X] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://fleetdm.com/docs/contributing/committing-changes#changes-files)
for more information.
- [X] Added/updated tests
- [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/pkg/execuser/execuser_linux.go

@@ -1,11 +1,15 @@
 package execuser
 
 import (
+	"bufio"
+	"bytes"
 	"errors"
 	"fmt"
+	"io"
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"strconv"
 	"strings"
 
@@ -19,23 +23,44 @@ func run(path string, opts eopts) error {
 		return fmt.Errorf("get user: %w", err)
 	}
 
+	// TODO(lucas): Default to display :0 if user DISPLAY environment variable
+	// could not be found, revisit when working on multi-user/multi-session support.
+	// This assumes there's only one desktop session and belongs to the
+	// user returned in `getLoginUID'.
+	defaultDisplay := ":0"
+
 	log.Info().
 		Str("user", user.name).
 		Int64("id", user.id).
-		Msg("running sudo")
+		Msg("attempting to get user's DISPLAY")
 
-	// Flag `-i` is needed to run the command with the user's context, from `man sudo`:
-	// "The command is run with an environment similar to the one a user would receive at log in"
-	arg := []string{"-i", "-u", user.name, "-H"}
-	for _, nv := range opts.env {
-		arg = append(arg, fmt.Sprintf("%s=%s", nv[0], nv[1]))
+	display, err := getUserDisplay(user.name, opts)
+	if err != nil {
+		log.Error().
+			Str("user", user.name).
+			Int64("id", user.id).
+			Err(err).
+			Msgf("failed to get user's DISPLAY, using default %s", defaultDisplay)
+		display = defaultDisplay
+	} else if display == "" {
+		log.Warn().
+			Str("user", user.name).
+			Int64("id", user.id).
+			Msgf("user's DISPLAY not found, using default %s", defaultDisplay)
+		display = defaultDisplay
 	}
 
-	arg = append(arg,
-		// TODO(lucas): Default to display 0, revisit when working on
-		// multi-user/multi-session support. This assumes there's only
-		// one desktop session and belongs to the user returned in `getLoginUID'.
-		"DISPLAY=:0",
+	log.Info().
+		Str("path", path).
+		Str("user", user.name).
+		Int64("id", user.id).
+		Str("display", display).
+		Msg("running sudo")
+
+	args := argsForSudo(user, opts)
+
+	args = append(args,
+		"DISPLAY="+display,
 		// DBUS_SESSION_BUS_ADDRESS sets the location of the user login session bus.
 		// Required by the libayatana-appindicator3 library to display a tray icon
 		// on the desktop session.
@@ -44,11 +69,14 @@ func run(path string, opts eopts) error {
 		// (because it's already part of the user).
 		fmt.Sprintf("DBUS_SESSION_BUS_ADDRESS=unix:path=/run/user/%d/bus", user.id),
 		// Append the packaged libayatana-appindicator3 libraries path to LD_LIBRARY_PATH.
+		//
+		// Fleet Desktop doesn't use libayatana-appindicator3 since 1.18.3, but we need to
+		// keep this to support older versions of Fleet Desktop.
 		fmt.Sprintf("LD_LIBRARY_PATH=%s:%s", filepath.Dir(path), os.ExpandEnv("$LD_LIBRARY_PATH")),
 		path,
 	)
 
-	cmd := exec.Command("sudo", arg...)
+	cmd := exec.Command("sudo", args...)
 	cmd.Stderr = os.Stderr
 	cmd.Stdout = os.Stdout
 	log.Printf("cmd=%s", cmd.String())
@@ -64,6 +92,18 @@ type user struct {
 	id   int64
 }
 
+func argsForSudo(u *user, opts eopts) []string {
+	// -H: "[...] to set HOME environment to what's specified in the target's user password database entry."
+	// -i: needed to run the command with the user's context, from `man sudo`:
+	// "The command is run with an environment similar to the one a user would receive at log in"
+	// -u: "[..]Run the command as a user other than the default target user (usually root)."
+	args := []string{"-i", "-u", u.name, "-H"}
+	for _, nv := range opts.env {
+		args = append(args, fmt.Sprintf("%s=%s", nv[0], nv[1]))
+	}
+	return args
+}
+
 // getLoginUID returns the name and uid of the first login user
 // as reported by the `users' command.
 //
@@ -120,3 +160,30 @@ func parseIDOutput(s string) (int64, error) {
 	}
 	return uid, nil
 }
+
+var whoLineRegexp = regexp.MustCompile(`(\w+)\s+(:\d+)\s+`)
+
+func getUserDisplay(user string, opts eopts) (string, error) {
+	cmd := exec.Command("who")
+	var stdout bytes.Buffer
+	cmd.Stdout = &stdout
+	if err := cmd.Run(); err != nil {
+		return "", fmt.Errorf("run 'who' to get user display: %w", err)
+	}
+	return parseWhoOutputForDisplay(&stdout, user)
+}
+
+func parseWhoOutputForDisplay(output io.Reader, user string) (string, error) {
+	scanner := bufio.NewScanner(output)
+	for scanner.Scan() {
+		line := scanner.Text()
+		matches := whoLineRegexp.FindStringSubmatch(line)
+		if len(matches) > 1 && matches[1] == user {
+			return matches[2], nil
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", fmt.Errorf("scanner error: %w", err)
+	}
+	return "", nil
+}