wgengine/monitor: add Windows linkchange monitor

Updates tailscale/corp#553

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/interfaces/interfaces.go

@@ -43,7 +43,8 @@ func Tailscale() (net.IP, *net.Interface, error) {
 // maybeTailscaleInterfaceName reports whether s is an interface
 // name that might be used by Tailscale.
 func maybeTailscaleInterfaceName(s string) bool {
-	return strings.HasPrefix(s, "wg") ||
+	return s == "Tailscale" ||
+		strings.HasPrefix(s, "wg") ||
 		strings.HasPrefix(s, "ts") ||
 		strings.HasPrefix(s, "tailscale") ||
 		strings.HasPrefix(s, "utun")
@@ -179,7 +180,8 @@ func (s *State) Equal(s2 *State) bool {
 // /^tailscale/)
 func (s *State) RemoveTailscaleInterfaces() {
 	for name := range s.InterfaceIPs {
-		if strings.HasPrefix(name, "tailscale") { // TODO: use --tun flag value, etc; see TODO in method doc
+		if name == "Tailscale" || // as it is on Windows
+			strings.HasPrefix(name, "tailscale") { // TODO: use --tun flag value, etc; see TODO in method doc
 			delete(s.InterfaceIPs, name)
 			delete(s.InterfaceUp, name)
 		}

wgengine/monitor/monitor_unsupported.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build !linux,!freebsd android
+// +build !linux,!freebsd,!windows android
 
 package monitor
 

wgengine/monitor/monitor_windows.go

@@ -0,0 +1,131 @@
+// 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.
+
+package monitor
+
+import (
+	"context"
+	"errors"
+	"sync"
+	"syscall"
+	"time"
+	"unsafe"
+
+	"golang.org/x/sys/windows"
+	"tailscale.com/types/logger"
+)
+
+var (
+	iphlpapi              = syscall.NewLazyDLL("iphlpapi.dll")
+	notifyAddrChangeProc  = iphlpapi.NewProc("NotifyAddrChange")
+	notifyRouteChangeProc = iphlpapi.NewProc("NotifyRouteChange")
+)
+
+type unspecifiedMessage struct{}
+
+func (unspecifiedMessage) ignore() bool { return false }
+
+type winMon struct {
+	ctx    context.Context
+	cancel context.CancelFunc
+
+	logf logger.Logf
+
+	mu    sync.Mutex
+	event windows.Handle
+}
+
+func newOSMon(logf logger.Logf) (osMon, error) {
+	ctx, cancel := context.WithCancel(context.Background())
+	return &winMon{
+		logf:   logf,
+		ctx:    ctx,
+		cancel: cancel,
+	}, nil
+}
+
+func (m *winMon) Close() error {
+	m.cancel()
+
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if h := m.event; h != 0 {
+		// Wake up any reader blocked in Receive.
+		windows.SetEvent(h)
+	}
+
+	return nil
+}
+
+var errClosed = errors.New("closed")
+
+func (m *winMon) Receive() (message, error) {
+	if m.ctx.Err() != nil {
+		return nil, errClosed
+	}
+
+	var o windows.Overlapped
+	h, err := windows.CreateEvent(nil, 1 /* true*/, 0 /* unsignaled */, nil /* no name */)
+	if err != nil {
+		m.logf("CreateEvent: %v", err)
+		return nil, err
+	}
+	defer windows.CloseHandle(h)
+
+	m.mu.Lock()
+	m.event = h
+	m.mu.Unlock()
+
+	o.HEvent = h
+
+	err = notifyAddrChange(&h, &o)
+	if err != nil {
+		m.logf("notifyAddrChange: %v", err)
+		return nil, err
+	}
+	err = notifyRouteChange(&h, &o)
+	if err != nil {
+		m.logf("notifyRouteChange: %v", err)
+		return nil, err
+	}
+
+	t0 := time.Now()
+	evt, err := windows.WaitForSingleObject(o.HEvent, windows.INFINITE)
+	if m.ctx.Err() != nil {
+		return nil, errClosed
+	}
+	if err != nil {
+		m.logf("notifyRouteChange: %v", err)
+		return nil, err
+	}
+	d := time.Since(t0)
+	m.logf("got windows change event after %v: %+v", d, evt)
+
+	m.mu.Lock()
+	m.event = 0
+	m.mu.Unlock()
+
+	return unspecifiedMessage{}, nil
+}
+
+func notifyAddrChange(h *windows.Handle, o *windows.Overlapped) error {
+	return callNotifyProc(notifyAddrChangeProc, h, o)
+}
+
+func notifyRouteChange(h *windows.Handle, o *windows.Overlapped) error {
+	return callNotifyProc(notifyAddrChangeProc, h, o)
+}
+
+func callNotifyProc(p *syscall.LazyProc, h *windows.Handle, o *windows.Overlapped) error {
+	r1, _, e1 := p.Call(uintptr(unsafe.Pointer(h)), uintptr(unsafe.Pointer(o)))
+	expect := uintptr(0)
+	if h != nil || o != nil {
+		const ERROR_IO_PENDING = 997
+		expect = ERROR_IO_PENDING
+	}
+	if r1 == expect {
+		return nil
+	}
+	return e1
+}