net: make WriteTo fail when UDPConn is already connected

Fixes #2773.

R=rsc
CC=golang-dev
https://golang.org/cl/5571056

src/pkg/net/fd.go

@@ -22,15 +22,16 @@ type netFD struct {
 	closing bool
 
 	// immutable until Close
-	sysfd   int
-	family  int
-	sotype  int
-	sysfile *os.File
-	cr      chan bool
-	cw      chan bool
-	net     string
-	laddr   Addr
-	raddr   Addr
+	sysfd       int
+	family      int
+	sotype      int
+	isConnected bool
+	sysfile     *os.File
+	cr          chan bool
+	cw          chan bool
+	net         string
+	laddr       Addr
+	raddr       Addr
 
 	// owned by client
 	rdeadline int64

src/pkg/net/fd_windows.go

@@ -228,14 +228,15 @@ type netFD struct {
 	closing bool
 
 	// immutable until Close
-	sysfd   syscall.Handle
-	family  int
-	sotype  int
-	net     string
-	laddr   Addr
-	raddr   Addr
-	resultc [2]chan ioResult // read/write completion results
-	errnoc  [2]chan error    // read/write submit or cancel operation errors
+	sysfd       syscall.Handle
+	family      int
+	sotype      int
+	isConnected bool
+	net         string
+	laddr       Addr
+	raddr       Addr
+	resultc     [2]chan ioResult // read/write completion results
+	errnoc      [2]chan error    // read/write submit or cancel operation errors
 
 	// owned by client
 	rdeadline int64

src/pkg/net/sock.go

@@ -49,6 +49,7 @@ func socket(net string, f, t, p int, la, ra syscall.Sockaddr, toAddr func(syscal
 			fd.Close()
 			return nil, err
 		}
+		fd.isConnected = true
 	}
 
 	sa, _ := syscall.Getsockname(s)

src/pkg/net/udp_test.go

@@ -0,0 +1,87 @@
+// Copyright 2012 The Go 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 net
+
+import (
+	"runtime"
+	"testing"
+)
+
+func TestWriteToUDP(t *testing.T) {
+	if runtime.GOOS == "plan9" {
+		return
+	}
+
+	l, err := ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("Listen failed: %v", err)
+	}
+	defer l.Close()
+
+	testWriteToConn(t, l.LocalAddr().String())
+	testWriteToPacketConn(t, l.LocalAddr().String())
+}
+
+func testWriteToConn(t *testing.T, raddr string) {
+	c, err := Dial("udp", raddr)
+	if err != nil {
+		t.Fatalf("Dial failed: %v", err)
+	}
+	defer c.Close()
+
+	ra, err := ResolveUDPAddr("udp", raddr)
+	if err != nil {
+		t.Fatalf("ResolveUDPAddr failed: %v", err)
+	}
+
+	_, err = c.(*UDPConn).WriteToUDP([]byte("Connection-oriented mode socket"), ra)
+	if err == nil {
+		t.Fatal("WriteToUDP should be failed")
+	}
+	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("WriteToUDP should be failed as ErrWriteToConnected: %v", err)
+	}
+
+	_, err = c.(*UDPConn).WriteTo([]byte("Connection-oriented mode socket"), ra)
+	if err == nil {
+		t.Fatal("WriteTo should be failed")
+	}
+	if err != nil && err.(*OpError).Err != ErrWriteToConnected {
+		t.Fatalf("WriteTo should be failed as ErrWriteToConnected: %v", err)
+	}
+
+	_, err = c.Write([]byte("Connection-oriented mode socket"))
+	if err != nil {
+		t.Fatalf("Write failed: %v", err)
+	}
+}
+
+func testWriteToPacketConn(t *testing.T, raddr string) {
+	c, err := ListenPacket("udp", "127.0.0.1:0")
+	if err != nil {
+		t.Fatalf("ListenPacket failed: %v", err)
+	}
+	defer c.Close()
+
+	ra, err := ResolveUDPAddr("udp", raddr)
+	if err != nil {
+		t.Fatalf("ResolveUDPAddr failed: %v", err)
+	}
+
+	_, err = c.(*UDPConn).WriteToUDP([]byte("Connection-less mode socket"), ra)
+	if err != nil {
+		t.Fatalf("WriteToUDP failed: %v", err)
+	}
+
+	_, err = c.WriteTo([]byte("Connection-less mode socket"), ra)
+	if err != nil {
+		t.Fatalf("WriteTo failed: %v", err)
+	}
+
+	_, err = c.(*UDPConn).Write([]byte("Connection-less mode socket"))
+	if err == nil {
+		t.Fatal("Write should be failed")
+	}
+}

src/pkg/net/udpsock_posix.go

@@ -9,11 +9,14 @@
 package net
 
 import (
+	"errors"
 	"os"
 	"syscall"
 	"time"
 )
 
+var ErrWriteToConnected = errors.New("use of WriteTo with pre-connected UDP")
+
 func sockaddrToUDP(sa syscall.Sockaddr) Addr {
 	switch sa := sa.(type) {
 	case *syscall.SockaddrInet4:
@@ -182,6 +185,9 @@ func (c *UDPConn) WriteToUDP(b []byte, addr *UDPAddr) (int, error) {
 	if !c.ok() {
 		return 0, os.EINVAL
 	}
+	if c.fd.isConnected {
+		return 0, &OpError{"write", c.fd.net, addr, ErrWriteToConnected}
+	}
 	sa, err := addr.sockaddr(c.fd.family)
 	if err != nil {
 		return 0, &OpError{"write", c.fd.net, addr, err}