control/controlclient: use API to get Windows version number

Fixes #1478

Signed-off-by: Aleksandar Pesic <peske.nis@gmail.com>

control/controlclient/hostinfo_windows.go

@@ -5,9 +5,9 @@
 package controlclient
 
 import (
-	"os/exec"
-	"strings"
-	"syscall"
+	"fmt"
+
+	"golang.org/x/sys/windows"
 )
 
 func init() {
@@ -15,16 +15,6 @@ func init() {
 }
 
 func osVersionWindows() string {
-	cmd := exec.Command("cmd", "/c", "ver")
-	cmd.SysProcAttr = &syscall.SysProcAttr{HideWindow: true}
-	out, _ := cmd.Output() // "\nMicrosoft Windows [Version 10.0.19041.388]\n\n"
-	s := strings.TrimSpace(string(out))
-	s = strings.TrimPrefix(s, "Microsoft Windows [")
-	s = strings.TrimSuffix(s, "]")
-
-	// "Version 10.x.y.z", with "Version" localized. Keep only stuff after the space.
-	if sp := strings.Index(s, " "); sp != -1 {
-		s = s[sp+1:]
-	}
-	return s // "10.0.19041.388", ideally
+	major, minor, build := windows.RtlGetNtVersionNumbers()
+	return fmt.Sprintf("%d.%d.%d", major, minor, build)
 }

control/controlclient/hostinfo_windows_test.go

@@ -0,0 +1,28 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package controlclient
+
+import (
+	"os/exec"
+	"regexp"
+	"testing"
+)
+
+func TestOsVersionWindows(t *testing.T) {
+	out, err := exec.Command("cmd", "/c", "ver").Output()
+	if err != nil {
+		t.Fatalf("`ver` error: %v", err)
+	}
+	// Extract the version number from the output, and isolate the first three parts (major.minor.build)
+	rx := regexp.MustCompile(`(\d+\.\d+\.\d+)(\.\d+)?`)
+	m := rx.FindStringSubmatch(string(out))
+	if m == nil {
+		t.Fatalf("no version number in `ver` output: %q", out)
+	}
+	got := osVersionWindows()
+	if m[1] != got {
+		t.Errorf("osVersionWindows got %q want %q", got, m[1])
+	}
+}