code review 163052: Fix netFD.Close races demonstrated in Issues 321 and 271.

src/pkg/net/udpsock.go

 // Copyright 2009 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.
 
 // UDP sockets
 
 package net
 
 import (
         "os";
@@ skipping 55 matching lines @@
 }
 
 // UDPConn is the implementation of the Conn and PacketConn
 // interfaces for UDP network connections.
 type UDPConn struct {
         fd *netFD;
 }
 
 func newUDPConn(fd *netFD) *UDPConn {
         c := &UDPConn{fd};
-        fd.getref();
-        defer fd.delref();
         setsockoptInt(fd.sysfd, syscall.SOL_SOCKET, syscall.SO_BROADCAST, 1);
         return c;
 }
 
 func (c *UDPConn) ok() bool     { return c != nil && c.fd != nil }
 
 // Implementation of the Conn interface - see Conn for documentation.
 
 // Read reads data from a single UDP packet on the connection.
 // If the slice b is smaller than the arriving packet,
@@ skipping 193 matching lines @@
         }
         if laddr == nil {
                 return nil, &OpError{"listen", "udp", nil, errMissingAddress}
         }
         fd, e := internetSocket(net, laddr.toAddr(), nil, syscall.SOCK_DGRAM, "dial", sockaddrToUDP);
         if e != nil {
                 return nil, e
         }
         return newUDPConn(fd), nil;
 }