wgengine/router/dns: move to net/dns.

Preparation for merging the APIs and whatnot.

Signed-off-by: David Anderson <danderson@tailscale.com>

net/dns/registry_windows.go

@@ -0,0 +1,76 @@
+// Copyright (c) 2020 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.
+//
+// The code in this file originates from https://git.zx2c4.com/wireguard-go:
+// Copyright (C) 2017-2020 WireGuard LLC. All Rights Reserved.
+// Copying license: https://git.zx2c4.com/wireguard-go/tree/COPYING
+
+package dns
+
+import (
+	"fmt"
+	"runtime"
+	"strings"
+	"time"
+
+	"golang.org/x/sys/windows"
+	"golang.org/x/sys/windows/registry"
+)
+
+func openKeyWait(k registry.Key, path string, access uint32, timeout time.Duration) (registry.Key, error) {
+	runtime.LockOSThread()
+	defer runtime.UnlockOSThread()
+
+	deadline := time.Now().Add(timeout)
+	pathSpl := strings.Split(path, "\\")
+	for i := 0; ; i++ {
+		keyName := pathSpl[i]
+		isLast := i+1 == len(pathSpl)
+
+		event, err := windows.CreateEvent(nil, 0, 0, nil)
+		if err != nil {
+			return 0, fmt.Errorf("windows.CreateEvent: %v", err)
+		}
+		defer windows.CloseHandle(event)
+
+		var key registry.Key
+		for {
+			err = windows.RegNotifyChangeKeyValue(windows.Handle(k), false, windows.REG_NOTIFY_CHANGE_NAME, event, true)
+			if err != nil {
+				return 0, fmt.Errorf("windows.RegNotifyChangeKeyValue: %v", err)
+			}
+
+			var accessFlags uint32
+			if isLast {
+				accessFlags = access
+			} else {
+				accessFlags = registry.NOTIFY
+			}
+			key, err = registry.OpenKey(k, keyName, accessFlags)
+			if err == windows.ERROR_FILE_NOT_FOUND || err == windows.ERROR_PATH_NOT_FOUND {
+				timeout := time.Until(deadline) / time.Millisecond
+				if timeout < 0 {
+					timeout = 0
+				}
+				s, err := windows.WaitForSingleObject(event, uint32(timeout))
+				if err != nil {
+					return 0, fmt.Errorf("windows.WaitForSingleObject: %v", err)
+				}
+				if s == uint32(windows.WAIT_TIMEOUT) { // windows.WAIT_TIMEOUT status const is misclassified as error in golang.org/x/sys/windows
+					return 0, fmt.Errorf("timeout waiting for registry key")
+				}
+			} else if err != nil {
+				return 0, fmt.Errorf("registry.OpenKey(%v): %v", path, err)
+			} else {
+				if isLast {
+					return key, nil
+				}
+				defer key.Close()
+				break
+			}
+		}
+
+		k = key
+	}
+}