net: do not call syscall.Bind twice on windows

Fixes #5355.

R=golang-dev, mikioh.mikioh, bradfitz, r
CC=golang-dev
https://golang.org/cl/8966046

src/pkg/net/dial_test.go

@@ -372,3 +372,38 @@ func TestDialFailPDLeak(t *testing.T) {
 		}
 	}
 }
+
+func TestDialer(t *testing.T) {
+	ln, err := Listen("tcp4", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen failed: %v", err)
+	}
+	defer ln.Close()
+	ch := make(chan error, 1)
+	go func() {
+		var err error
+		c, err := ln.Accept()
+		if err != nil {
+			ch <- fmt.Errorf("Accept failed: %v", err)
+			return
+		}
+		defer c.Close()
+		ch <- nil
+	}()
+
+	laddr, err := ResolveTCPAddr("tcp4", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("ResolveTCPAddr failed: %v", err)
+	}
+	d := &Dialer{LocalAddr: laddr}
+	c, err := d.Dial("tcp4", ln.Addr().String())
+	if err != nil {
+		t.Fatalf("Dial failed: %v", err)
+	}
+	defer c.Close()
+	c.Read(make([]byte, 1))
+	err = <-ch
+	if err != nil {
+		t.Error(err)
+	}
+}

src/pkg/net/fd_unix.go

@@ -79,7 +79,7 @@ func (fd *netFD) name() string {
 	return fd.net + ":" + ls + "->" + rs
 }
 
-func (fd *netFD) connect(ra syscall.Sockaddr) error {
+func (fd *netFD) connect(la, ra syscall.Sockaddr) error {
 	fd.wio.Lock()
 	defer fd.wio.Unlock()
 	if err := fd.pd.PrepareWrite(); err != nil {

src/pkg/net/fd_windows.go

@@ -364,22 +364,23 @@ func (o *connectOp) Name() string {
 	return "ConnectEx"
 }
 
-func (fd *netFD) connect(ra syscall.Sockaddr) error {
+func (fd *netFD) connect(la, ra syscall.Sockaddr) error {
 	if !canUseConnectEx(fd.net) {
 		return syscall.Connect(fd.sysfd, ra)
 	}
 	// ConnectEx windows API requires an unconnected, previously bound socket.
-	var la syscall.Sockaddr
-	switch ra.(type) {
-	case *syscall.SockaddrInet4:
-		la = &syscall.SockaddrInet4{}
-	case *syscall.SockaddrInet6:
-		la = &syscall.SockaddrInet6{}
-	default:
-		panic("unexpected type in connect")
-	}
-	if err := syscall.Bind(fd.sysfd, la); err != nil {
-		return err
+	if la == nil {
+		switch ra.(type) {
+		case *syscall.SockaddrInet4:
+			la = &syscall.SockaddrInet4{}
+		case *syscall.SockaddrInet6:
+			la = &syscall.SockaddrInet6{}
+		default:
+			panic("unexpected type in connect")
+		}
+		if err := syscall.Bind(fd.sysfd, la); err != nil {
+			return err
+		}
 	}
 	// Call ConnectEx API.
 	var o connectOp

src/pkg/net/sock_posix.go

@@ -57,7 +57,7 @@ func socket(net string, f, t, p int, ipv6only bool, ulsa, ursa syscall.Sockaddr,
 		if !deadline.IsZero() {
 			setWriteDeadline(fd, deadline)
 		}
-		if err = fd.connect(ursa); err != nil {
+		if err = fd.connect(ulsa, ursa); err != nil {
 			fd.Close()
 			return nil, err
 		}