wgengine/monitor: close closeHandle

eccc167 introduced closeHandle which opened the handle,
but never closed it.

Windows handles should be closed.

Updates #921

Signed-off-by: Alex Brainman <alex.brainman@gmail.com>

wgengine/monitor/monitor_windows.go

@@ -58,6 +58,7 @@ type winMon struct {
 	pollTicker      *time.Ticker
 	lastState       *interfaces.State
 	closeHandle     windows.Handle // signaled upon close
+	goroutineDoneCh chan struct{}  // closed when awaitIPAndRouteChanges goroutine has stopped
 
 	mu            sync.Mutex
 	lastNetChange time.Time
@@ -78,6 +79,7 @@ func newOSMon(logf logger.Logf) (osMon, error) {
 		messagec:        make(chan messageOrError, 1),
 		pollTicker:      time.NewTicker(pollIntervalSlow),
 		closeHandle:     closeHandle,
+		goroutineDoneCh: make(chan struct{}),
 	}
 	go m.awaitIPAndRouteChanges()
 	return m, nil
@@ -87,6 +89,12 @@ func (m *winMon) Close() error {
 	m.cancel()
 	m.pollTicker.Stop()
 	windows.SetEvent(m.closeHandle) // wakes up any reader blocked in Receive
+
+	// Wait for awaitIPAndRouteChannges to be done, which could
+	// still be using the m.closeHandle.
+	<-m.goroutineDoneCh
+
+	windows.CloseHandle(m.closeHandle)
 	return nil
 }
 
@@ -125,6 +133,7 @@ func (m *winMon) stateChanged() bool {
 }
 
 func (m *winMon) awaitIPAndRouteChanges() {
+	defer close(m.goroutineDoneCh)
 	for {
 		msg, err := m.getIPOrRouteChangeMessage()
 		if err == errClosed {