Bool is not atomic

The Go race detector doesn't like concurrent access to bool. This change
moves the bool to use atomicBool, and extends atomicBool with the swap()
method, to handle conditional use.

file.go

@@ -23,6 +23,13 @@ type atomicBool int32
 func (b *atomicBool) isSet() bool { return atomic.LoadInt32((*int32)(b)) != 0 }
 func (b *atomicBool) setFalse()   { atomic.StoreInt32((*int32)(b), 0) }
 func (b *atomicBool) setTrue()    { atomic.StoreInt32((*int32)(b), 1) }
+func (b *atomicBool) swap(new bool) bool {
+	var newInt int32
+	if new {
+		newInt = 1
+	}
+	return atomic.SwapInt32((*int32)(b), newInt) == 1
+}
 
 const (
 	cFILE_SKIP_COMPLETION_PORT_ON_SUCCESS = 1
@@ -71,7 +78,7 @@ func initIo() {
 type win32File struct {
 	handle        syscall.Handle
 	wg            sync.WaitGroup
-	closing       bool
+	closing       atomicBool
 	readDeadline  deadlineHandler
 	writeDeadline deadlineHandler
 }
@@ -107,9 +114,9 @@ func MakeOpenFile(h syscall.Handle) (io.ReadWriteCloser, error) {
 
 // closeHandle closes the resources associated with a Win32 handle
 func (f *win32File) closeHandle() {
-	if !f.closing {
+	// Atomically set that we are closing, releasing the resources only once.
+	if !f.closing.swap(true) {
 		// cancel all IO and wait for it to complete
-		f.closing = true
 		cancelIoEx(f.handle, nil)
 		f.wg.Wait()
 		// at this point, no new IO can start
@@ -127,7 +134,7 @@ func (f *win32File) Close() error {
 // prepareIo prepares for a new IO operation.
 // The caller must call f.wg.Done() when the IO is finished, prior to Close() returning.
 func (f *win32File) prepareIo() (*ioOperation, error) {
-	if f.closing {
+	if f.closing.isSet() {
 		return nil, ErrFileClosed
 	}
 	f.wg.Add(1)
@@ -159,7 +166,7 @@ func (f *win32File) asyncIo(c *ioOperation, d *deadlineHandler, bytes uint32, er
 		return int(bytes), err
 	}
 
-	if f.closing {
+	if f.closing.isSet() {
 		cancelIoEx(f.handle, &c.o)
 	}
 
@@ -175,7 +182,7 @@ func (f *win32File) asyncIo(c *ioOperation, d *deadlineHandler, bytes uint32, er
 	case r = <-c.ch:
 		err = r.err
 		if err == syscall.ERROR_OPERATION_ABORTED {
-			if f.closing {
+			if f.closing.isSet() {
 				err = ErrFileClosed
 			}
 		}