net: provide public access to file descriptors

Fixes #918.

R=rsc, rog, brainman
CC=golang-dev
https://golang.org/cl/2904041

src/pkg/net/fd.go

@@ -517,3 +517,17 @@ func (fd *netFD) accept(toAddr func(syscall.Sockaddr) Addr) (nfd *netFD, err os.
 	}
 	return nfd, nil
 }
+
+func (fd *netFD) dup() (f *os.File, err os.Error) {
+	ns, e := syscall.Dup(fd.sysfd)
+	if e != 0 {
+		return nil, &OpError{"dup", fd.net, fd.laddr, os.Errno(e)}
+	}
+
+	// We want blocking mode for the new fd, hence the double negative.
+	if e = syscall.SetNonblock(ns, false); e != 0 {
+		return nil, &OpError{"setnonblock", fd.net, fd.laddr, os.Errno(e)}
+	}
+
+	return os.NewFile(ns, fd.sysfile.Name()), nil
+}

src/pkg/net/fd_windows.go

@@ -373,3 +373,8 @@ func init() {
 		initErr = os.NewSyscallError("WSAStartup", e)
 	}
 }
+
+func (fd *netFD) dup() (f *os.File, err os.Error) {
+	// TODO: Implement this
+	return nil, os.NewSyscallError("dup", syscall.EWINDOWS)
+}

src/pkg/net/tcpsock.go

@@ -208,6 +208,11 @@ func (c *TCPConn) SetNoDelay(noDelay bool) os.Error {
 	return setNoDelay(c.fd, noDelay)
 }
 
+// File returns a copy of the underlying os.File, set to blocking mode.
+// It is the caller's responsibility to close f when finished.
+// Closing c does not affect f, and closing f does not affect c.
+func (c *TCPConn) File() (f *os.File, err os.Error) { return c.fd.dup() }
+
 // DialTCP is like Dial but can only connect to TCP networks
 // and returns a TCPConn structure.
 func DialTCP(net string, laddr, raddr *TCPAddr) (c *TCPConn, err os.Error) {
@@ -281,3 +286,8 @@ func (l *TCPListener) Close() os.Error {
 
 // Addr returns the listener's network address, a *TCPAddr.
 func (l *TCPListener) Addr() Addr { return l.fd.laddr }
+
+// File returns a copy of the underlying os.File, set to blocking mode.
+// It is the caller's responsibility to close f when finished.
+// Closing c does not affect f, and closing f does not affect c.
+func (l *TCPListener) File() (f *os.File, err os.Error) { return l.fd.dup() }

src/pkg/net/udpsock.go

@@ -274,3 +274,8 @@ func (c *UDPConn) BindToDevice(device string) os.Error {
 	defer c.fd.decref()
 	return os.NewSyscallError("setsockopt", syscall.BindToDevice(c.fd.sysfd, device))
 }
+
+// File returns a copy of the underlying os.File, set to blocking mode.
+// It is the caller's responsibility to close f when finished.
+// Closing c does not affect f, and closing f does not affect c.
+func (c *UDPConn) File() (f *os.File, err os.Error) { return c.fd.dup() }

src/pkg/net/unixsock.go

@@ -277,6 +277,11 @@ func (c *UnixConn) WriteTo(b []byte, addr Addr) (n int, err os.Error) {
 	return c.WriteToUnix(b, a)
 }
 
+// File returns a copy of the underlying os.File, set to blocking mode.
+// It is the caller's responsibility to close f when finished.
+// Closing c does not affect f, and closing f does not affect c.
+func (c *UnixConn) File() (f *os.File, err os.Error) { return c.fd.dup() }
+
 // DialUnix connects to the remote address raddr on the network net,
 // which must be "unix" or "unixgram".  If laddr is not nil, it is used
 // as the local address for the connection.
@@ -369,6 +374,11 @@ func (l *UnixListener) Close() os.Error {
 // Addr returns the listener's network address.
 func (l *UnixListener) Addr() Addr { return l.fd.laddr }
 
+// File returns a copy of the underlying os.File, set to blocking mode.
+// It is the caller's responsibility to close f when finished.
+// Closing c does not affect f, and closing f does not affect c.
+func (l *UnixListener) File() (f *os.File, err os.Error) { return l.fd.dup() }
+
 // ListenUnixgram listens for incoming Unix datagram packets addressed to the
 // local address laddr.  The returned connection c's ReadFrom
 // and WriteTo methods can be used to receive and send UDP