hostinfo, tailcfg: add desktop detection on Linux to hostinfo

From the machines tab its hard to differenciate desktop Linux installs from
server Linux installs. Transmitting this information should make this
determination a lot easier.

Due to the reality that tailscaled is likely a system process, the standard
checks based on XDG_SESSION_TYPE or DISPLAY environment variables are not
possible (those variables won't be set). Instead, we look for listening
unix sockets that are typical of desktop installs.

Signed-off-by: Tom DNetto <tom@tailscale.com>

hostinfo/hostinfo.go

@@ -31,6 +31,7 @@ func New() *tailcfg.Hostinfo {
 		Hostname:    hostname,
 		OS:          version.OS(),
 		OSVersion:   GetOSVersion(),
+		Desktop:     desktop(),
 		Package:     packageTypeCached(),
 		GoArch:      runtime.GOARCH,
 		DeviceModel: deviceModel(),
@@ -97,6 +98,7 @@ func GetEnvType() EnvType {
 var (
 	deviceModelAtomic atomic.Value // of string
 	osVersionAtomic   atomic.Value // of string
+	desktopAtomic     atomic.Value // of bool
 	packagingType     atomic.Value // of string
 )
 
@@ -106,6 +108,9 @@ func SetDeviceModel(model string) { deviceModelAtomic.Store(model) }
 // SetOSVersion sets the OS version.
 func SetOSVersion(v string) { osVersionAtomic.Store(v) }
 
+// SetDesktop sets the desktop for use in HostInfo updates.
+func SetDesktop(d bool) { desktopAtomic.Store(d) }
+
 // SetPackage sets the packaging type for the app.
 //
 // As of 2022-03-25, this is used by Android ("nogoogle" for the
@@ -117,6 +122,11 @@ func deviceModel() string {
 	return s
 }
 
+func desktop() bool {
+	s, _ := desktopAtomic.Load().(bool)
+	return s
+}
+
 func getEnvType() EnvType {
 	if inKnative() {
 		return KNative

hostinfo/hostinfo_linux.go

@@ -26,6 +26,27 @@ func init() {
 	if v := linuxDeviceModel(); v != "" {
 		SetDeviceModel(v)
 	}
+	SetDesktop(linuxDesktop())
+}
+
+// linuxDesktop attempts to detect the class of desktop being used by the
+// system, if any. Given that tailscaled is likely to be run as a system
+// service, the usual environment-variable checks are not possible. Instead,
+// we look for unix sockets typical of a desktop Linux setup.
+func linuxDesktop() bool {
+	seenDesktop := false
+
+	err := lineread.File("/proc/net/unix", func(line []byte) error {
+		seenDesktop = seenDesktop || strings.Contains(string(line), " @/tmp/dbus-")
+		seenDesktop = seenDesktop || strings.Contains(string(line), ".X11-unix")
+		seenDesktop = seenDesktop || strings.Contains(string(line), "/wayland-1")
+		return nil
+	})
+	if err != nil {
+		return false
+	}
+
+	return seenDesktop
 }
 
 func linuxDeviceModel() string {

tailcfg/tailcfg.go

@@ -460,6 +460,7 @@ type Hostinfo struct {
 	BackendLogID  string             `json:",omitempty"` // logtail ID of backend instance
 	OS            string             `json:",omitempty"` // operating system the client runs on (a version.OS value)
 	OSVersion     string             `json:",omitempty"` // operating system version, with optional distro prefix ("Debian 10.4", "Windows 10 Pro 10.0.19041")
+	Desktop       bool               `json:",omitempty"` // if a desktop was detected on Linux
 	Package       string             `json:",omitempty"` // Tailscale package to disambiguate ("choco", "appstore", etc; "" for unknown)
 	DeviceModel   string             `json:",omitempty"` // mobile phone model ("Pixel 3a", "iPhone12,3")
 	Hostname      string             `json:",omitempty"` // name of the host the client runs on

tailcfg/tailcfg_test.go

@@ -28,7 +28,7 @@ func fieldsOf(t reflect.Type) (fields []string) {
 func TestHostinfoEqual(t *testing.T) {
 	hiHandles := []string{
 		"IPNVersion", "FrontendLogID", "BackendLogID",
-		"OS", "OSVersion", "Package", "DeviceModel", "Hostname",
+		"OS", "OSVersion", "Desktop", "Package", "DeviceModel", "Hostname",
 		"ShieldsUp", "ShareeNode",
 		"GoArch",
 		"RoutableIPs", "RequestTags",