control/controlclient: use "getprop net.hostname" for Android hostname

Updates #409

control/controlclient/direct.go

@@ -18,7 +18,9 @@
 	"net/http"
 	"net/url"
 	"os"
+	"os/exec"
 	"reflect"
+	"runtime"
 	"strconv"
 	"strings"
 	"sync"
@@ -159,7 +161,13 @@ func NewDirect(opts Options) (*Direct, error) {
 }
 
 func NewHostinfo() *tailcfg.Hostinfo {
-	hostname, _ := os.Hostname()
+	var hostname string
+	if runtime.GOOS == "android" {
+		hostname = androidHostname()
+	}
+	if hostname == "" {
+		hostname, _ = os.Hostname()
+	}
 	return &tailcfg.Hostinfo{
 		IPNVersion: version.LONG,
 		Hostname:   hostname,
@@ -167,6 +175,20 @@ func NewHostinfo() *tailcfg.Hostinfo {
 	}
 }
 
+var androidHostnameOnce struct {
+	_ structs.Incomparable
+	sync.Once
+	v string
+}
+
+func androidHostname() string {
+	androidHostnameOnce.Do(func() {
+		out, _ := exec.Command("getprop", "net.hostname").Output()
+		androidHostnameOnce.v = strings.TrimSpace(string(out))
+	})
+	return androidHostnameOnce.v
+}
+
 // SetHostinfo clones the provided Hostinfo and remembers it for the
 // next update. It reports whether the Hostinfo has changed.
 func (c *Direct) SetHostinfo(hi *tailcfg.Hostinfo) bool {