src/pkg/exp/gui/x11/conn.go - go - Git at Google

 // 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.

 // Package x11 implements an X11 backend for the exp/gui package.
 //
 // The X protocol specification is at ftp://ftp.x.org/pub/X11R7.0/doc/PDF/proto.pdf.
 // A summary of the wire format can be found in XCB's xproto.xml.
 package x11

 import (
 	"bufio"
 	"exp/gui"
 	"image"
 	"image/draw"
 	"io"
 	"log"
 	"net"
 	"os"
 	"strconv"
 	"strings"
 	"time"
 )

 type resID uint32 // X resource IDs.

 // TODO(nigeltao): Handle window resizes.
 const (
 	windowHeight = 600
 	windowWidth  = 800
 )

 const (
 	keymapLo = 8
 	keymapHi = 255
 )

 type conn struct {
 	c io.Closer
 	r *bufio.Reader
 	w *bufio.Writer

 	gc, window, root, visual resID

 	img        *image.RGBA
 	eventc     chan interface{}
 	mouseState gui.MouseEvent

 	buf [256]byte // General purpose scratch buffer.

 	flush     chan bool
 	flushBuf0 [24]byte
 	flushBuf1 [4 * 1024]byte
 }

 // writeSocket runs in its own goroutine, serving both FlushImage calls
 // directly from the exp/gui client and indirectly from X expose events.
 // It paints c.img to the X server via PutImage requests.
 func (c *conn) writeSocket() {
 	defer c.c.Close()
 	for _ = range c.flush {
 		b := c.img.Bounds()
 		if b.Empty() {
 			continue
 		}
 		// Each X request has a 16-bit length (in terms of 4-byte units). To avoid going over
 		// this limit, we send PutImage for each row of the image, rather than trying to paint
 		// the entire image in one X request. This approach could easily be optimized (or the
 		// X protocol may have an escape sequence to delimit very large requests).
 		// TODO(nigeltao): See what XCB's xcb_put_image does in this situation.
 		units := 6 + b.Dx()
 		if units > 0xffff || b.Dy() > 0xffff {
 			log.Print("x11: window is too large for PutImage")
 			return
 		}

 		c.flushBuf0[0] = 0x48 // PutImage opcode.
 		c.flushBuf0[1] = 0x02 // XCB_IMAGE_FORMAT_Z_PIXMAP.
 		c.flushBuf0[2] = uint8(units)
 		c.flushBuf0[3] = uint8(units >> 8)
 		setU32LE(c.flushBuf0[4:8], uint32(c.window))
 		setU32LE(c.flushBuf0[8:12], uint32(c.gc))
 		setU32LE(c.flushBuf0[12:16], 1<<16|uint32(b.Dx()))
 		c.flushBuf0[21] = 0x18 // depth = 24 bits.

 		for y := b.Min.Y; y < b.Max.Y; y++ {
 			setU32LE(c.flushBuf0[16:20], uint32(y<<16))
 			if _, err := c.w.Write(c.flushBuf0[:24]); err != nil {
 				if err != os.EOF {
 					log.Println("x11:", err.String())
 				}
 				return
 			}
 			p := c.img.Pix[(y-b.Min.Y)*c.img.Stride:]
 			for x, dx := 0, 4*b.Dx(); x < dx; {
 				nx := dx - x
 				if nx > len(c.flushBuf1) {
 					nx = len(c.flushBuf1) &^ 3
 				}
 				for i := 0; i < nx; i += 4 {
 					// X11's order is BGRX, not RGBA.
 					c.flushBuf1[i+0] = p[x+i+2]
 					c.flushBuf1[i+1] = p[x+i+1]
 					c.flushBuf1[i+2] = p[x+i+0]
 				}
 				x += nx
 				if _, err := c.w.Write(c.flushBuf1[:nx]); err != nil {
 					if err != os.EOF {
 						log.Println("x11:", err.String())
 					}
 					return
 				}
 			}
 		}
 		if err := c.w.Flush(); err != nil {
 			if err != os.EOF {
 				log.Println("x11:", err.String())
 			}
 			return
 		}
 	}
 }

 func (c *conn) Screen() draw.Image { return c.img }

 func (c *conn) FlushImage() {
 	select {
 	case c.flush <- false:
 		// Flush notification sent.
 	default:
 		// Could not send.
 		// Flush notification must be pending already.
 	}
 }

 func (c *conn) Close() os.Error {
 	// Shut down the writeSocket goroutine. This will close the socket to the
 	// X11 server, which will cause c.eventc to close.
 	close(c.flush)
 	for _ = range c.eventc {
 		// Drain the channel to allow the readSocket goroutine to shut down.
 	}
 	return nil
 }

 func (c *conn) EventChan() <-chan interface{} { return c.eventc }

 // readSocket runs in its own goroutine, reading X events and sending gui
 // events on c's EventChan.
 func (c *conn) readSocket() {
 	var (
 		keymap            [256][]int
 		keysymsPerKeycode int
 	)
 	defer close(c.eventc)
 	for {
 		// X events are always 32 bytes long.
 		if _, err := io.ReadFull(c.r, c.buf[:32]); err != nil {
 			if err != os.EOF {
 				c.eventc <- gui.ErrEvent{err}
 			}
 			return
 		}
 		switch c.buf[0] {
 		case 0x01: // Reply from a request (e.g. GetKeyboardMapping).
 			cookie := int(c.buf[3])<<8 | int(c.buf[2])
 			if cookie != 1 {
 				// We issued only one request (GetKeyboardMapping) with a cookie of 1,
 				// so we shouldn't get any other reply from the X server.
 				c.eventc <- gui.ErrEvent{os.NewError("x11: unexpected cookie")}
 				return
 			}
 			keysymsPerKeycode = int(c.buf[1])
 			b := make([]int, 256*keysymsPerKeycode)
 			for i := range keymap {
 				keymap[i] = b[i*keysymsPerKeycode : (i+1)*keysymsPerKeycode]
 			}
 			for i := keymapLo; i <= keymapHi; i++ {
 				m := keymap[i]
 				for j := range m {
 					u, err := readU32LE(c.r, c.buf[:4])
 					if err != nil {
 						if err != os.EOF {
 							c.eventc <- gui.ErrEvent{err}
 						}
 						return
 					}
 					m[j] = int(u)
 				}
 			}
 		case 0x02, 0x03: // Key press, key release.
 			// X Keyboard Encoding is documented at http://tronche.com/gui/x/xlib/input/keyboard-encoding.html
 			// TODO(nigeltao): Do we need to implement the "MODE SWITCH / group modifier" feature
 			// or is that some no-longer-used X construct?
 			if keysymsPerKeycode < 2 {
 				// Either we haven't yet received the GetKeyboardMapping reply or
 				// the X server has sent one that's too short.
 				continue
 			}
 			keycode := int(c.buf[1])
 			shift := int(c.buf[28]) & 0x01
 			keysym := keymap[keycode][shift]
 			if keysym == 0 {
 				keysym = keymap[keycode][0]
 			}
 			// TODO(nigeltao): Should we send KeyEvents for Shift/Ctrl/Alt? Should Shift-A send
 			// the same int down the channel as the sent on just the A key?
 			// TODO(nigeltao): How should IME events (e.g. key presses that should generate CJK text) work? Or
 			// is that outside the scope of the gui.Window interface?
 			if c.buf[0] == 0x03 {
 				keysym = -keysym
 			}
 			c.eventc <- gui.KeyEvent{keysym}
 		case 0x04, 0x05: // Button press, button release.
 			mask := 1 << (c.buf[1] - 1)
 			if c.buf[0] == 0x04 {
 				c.mouseState.Buttons |= mask
 			} else {
 				c.mouseState.Buttons &^= mask
 			}
 			c.mouseState.Nsec = time.Nanoseconds()
 			c.eventc <- c.mouseState
 		case 0x06: // Motion notify.
 			c.mouseState.Loc.X = int(int16(c.buf[25])<<8 | int16(c.buf[24]))
 			c.mouseState.Loc.Y = int(int16(c.buf[27])<<8 | int16(c.buf[26]))
 			c.mouseState.Nsec = time.Nanoseconds()
 			c.eventc <- c.mouseState
 		case 0x0c: // Expose.
 			// A single user action could trigger multiple expose events (e.g. if moving another
 			// window with XShape'd rounded corners over our window). In that case, the X server will
 			// send a uint16 count (in bytes 16-17) of the number of additional expose events coming.
 			// We could parse each event for the (x, y, width, height) and maintain a minimal dirty
 			// rectangle, but for now, the simplest approach is to paint the entire window, when
 			// receiving the final event in the series.
 			if c.buf[17] == 0 && c.buf[16] == 0 {
 				// TODO(nigeltao): Should we ignore the very first expose event? A freshly mapped window
 				// will trigger expose, but until the first c.FlushImage call, there's probably nothing to
 				// paint but black. For an 800x600 window, at 4 bytes per pixel, each repaint writes about
 				// 2MB over the socket.
 				c.FlushImage()
 			}
 			// TODO(nigeltao): Should we listen to DestroyNotify (0x11) and ResizeRequest (0x19) events?
 			// What about EnterNotify (0x07) and LeaveNotify (0x08)?
 		}
 	}
 }

 // connect connects to the X server given by the full X11 display name (e.g.
 // ":12.0") and returns the connection as well as the portion of the full name
 // that is the display number (e.g. "12").
 // Examples:
 //	connect(":1")                 // calls net.Dial("unix", "", "/tmp/.X11-unix/X1"), displayStr="1"
 //	connect("/tmp/launch-123/:0") // calls net.Dial("unix", "", "/tmp/launch-123/:0"), displayStr="0"
 //	connect("hostname:2.1")       // calls net.Dial("tcp", "", "hostname:6002"), displayStr="2"
 //	connect("tcp/hostname:1.0")   // calls net.Dial("tcp", "", "hostname:6001"), displayStr="1"
 func connect(display string) (conn net.Conn, displayStr string, err os.Error) {
 	colonIdx := strings.LastIndex(display, ":")
 	if colonIdx < 0 {
 		return nil, "", os.NewError("bad display: " + display)
 	}
 	// Parse the section before the colon.
 	var protocol, host, socket string
 	if display[0] == '/' {
 		socket = display[:colonIdx]
 	} else {
 		if i := strings.LastIndex(display, "/"); i < 0 {
 			// The default protocol is TCP.
 			protocol = "tcp"
 			host = display[:colonIdx]
 		} else {
 			protocol = display[:i]
 			host = display[i+1 : colonIdx]
 		}
 	}
 	// Parse the section after the colon.
 	after := display[colonIdx+1:]
 	if after == "" {
 		return nil, "", os.NewError("bad display: " + display)
 	}
 	if i := strings.LastIndex(after, "."); i < 0 {
 		displayStr = after
 	} else {
 		displayStr = after[:i]
 	}
 	displayInt, err := strconv.Atoi(displayStr)
 	if err != nil || displayInt < 0 {
 		return nil, "", os.NewError("bad display: " + display)
 	}
 	// Make the connection.
 	if socket != "" {
 		conn, err = net.Dial("unix", socket+":"+displayStr)
 	} else if host != "" {
 		conn, err = net.Dial(protocol, host+":"+strconv.Itoa(6000+displayInt))
 	} else {
 		conn, err = net.Dial("unix", "/tmp/.X11-unix/X"+displayStr)
 	}
 	if err != nil {
 		return nil, "", os.NewError("cannot connect to " + display + ": " + err.String())
 	}
 	return
 }

 // authenticate authenticates ourselves with the X server.
 // displayStr is the "12" out of ":12.0".
 func authenticate(w *bufio.Writer, displayStr string) os.Error {
 	key, value, err := readAuth(displayStr)
 	if err != nil {
 		return err
 	}
 	// Assume that the authentication protocol is "MIT-MAGIC-COOKIE-1".
 	if len(key) != 18 || len(value) != 16 {
 		return os.NewError("unsupported Xauth")
 	}
 	// 0x006c means little-endian. 0x000b, 0x0000 means X major version 11, minor version 0.
 	// 0x0012 and 0x0010 means the auth key and value have lengths 18 and 16.
 	// The final 0x0000 is padding, so that the string length is a multiple of 4.
 	_, err = io.WriteString(w, "\x6c\x00\x0b\x00\x00\x00\x12\x00\x10\x00\x00\x00")
 	if err != nil {
 		return err
 	}
 	_, err = io.WriteString(w, key)
 	if err != nil {
 		return err
 	}
 	// Again, the 0x0000 is padding.
 	_, err = io.WriteString(w, "\x00\x00")
 	if err != nil {
 		return err
 	}
 	_, err = io.WriteString(w, value)
 	if err != nil {
 		return err
 	}
 	err = w.Flush()
 	if err != nil {
 		return err
 	}
 	return nil
 }

 // readU8 reads a uint8 from r, using b as a scratch buffer.
 func readU8(r io.Reader, b []byte) (uint8, os.Error) {
 	_, err := io.ReadFull(r, b[:1])
 	if err != nil {
 		return 0, err
 	}
 	return uint8(b[0]), nil
 }

 // readU16LE reads a little-endian uint16 from r, using b as a scratch buffer.
 func readU16LE(r io.Reader, b []byte) (uint16, os.Error) {
 	_, err := io.ReadFull(r, b[:2])
 	if err != nil {
 		return 0, err
 	}
 	return uint16(b[0]) | uint16(b[1])<<8, nil
 }

 // readU32LE reads a little-endian uint32 from r, using b as a scratch buffer.
 func readU32LE(r io.Reader, b []byte) (uint32, os.Error) {
 	_, err := io.ReadFull(r, b[:4])
 	if err != nil {
 		return 0, err
 	}
 	return uint32(b[0]) | uint32(b[1])<<8 | uint32(b[2])<<16 | uint32(b[3])<<24, nil
 }

 // setU32LE sets b[:4] to be the little-endian representation of u.
 func setU32LE(b []byte, u uint32) {
 	b[0] = byte((u >> 0) & 0xff)
 	b[1] = byte((u >> 8) & 0xff)
 	b[2] = byte((u >> 16) & 0xff)
 	b[3] = byte((u >> 24) & 0xff)
 }

 // checkPixmapFormats checks that we have an agreeable X pixmap Format.
 func checkPixmapFormats(r io.Reader, b []byte, n int) (agree bool, err os.Error) {
 	for i := 0; i < n; i++ {
 		_, err = io.ReadFull(r, b[:8])
 		if err != nil {
 			return
 		}
 		// Byte 0 is depth, byte 1 is bits-per-pixel, byte 2 is scanline-pad, the rest (5) is padding.
 		if b[0] == 24 && b[1] == 32 {
 			agree = true
 		}
 	}
 	return
 }

 // checkDepths checks that we have an agreeable X Depth (i.e. one that has an agreeable X VisualType).
 func checkDepths(r io.Reader, b []byte, n int, visual uint32) (agree bool, err os.Error) {
 	for i := 0; i < n; i++ {
 		depth, err := readU16LE(r, b)
 		if err != nil {
 			return
 		}
 		depth &= 0xff
 		visualsLen, err := readU16LE(r, b)
 		if err != nil {
 			return
 		}
 		// Ignore 4 bytes of padding.
 		_, err = io.ReadFull(r, b[:4])
 		if err != nil {
 			return
 		}
 		for j := 0; j < int(visualsLen); j++ {
 			// Read 24 bytes: visual(4), class(1), bits per rgb value(1), colormap entries(2),
 			// red mask(4), green mask(4), blue mask(4), padding(4).
 			v, err := readU32LE(r, b)
 			_, err = readU32LE(r, b)
 			rm, err := readU32LE(r, b)
 			gm, err := readU32LE(r, b)
 			bm, err := readU32LE(r, b)
 			_, err = readU32LE(r, b)
 			if err != nil {
 				return
 			}
 			if v == visual && rm == 0xff0000 && gm == 0xff00 && bm == 0xff && depth == 24 {
 				agree = true
 			}
 		}
 	}
 	return
 }

 // checkScreens checks that we have an agreeable X Screen.
 func checkScreens(r io.Reader, b []byte, n int) (root, visual uint32, err os.Error) {
 	for i := 0; i < n; i++ {
 		root0, err := readU32LE(r, b)
 		if err != nil {
 			return
 		}
 		// Ignore the next 7x4 bytes, which is: colormap, whitepixel, blackpixel, current input masks,
 		// width and height (pixels), width and height (mm), min and max installed maps.
 		_, err = io.ReadFull(r, b[:28])
 		if err != nil {
 			return
 		}
 		visual0, err := readU32LE(r, b)
 		if err != nil {
 			return
 		}
 		// Next 4 bytes: backing stores, save unders, root depth, allowed depths length.
 		x, err := readU32LE(r, b)
 		if err != nil {
 			return
 		}
 		nDepths := int(x >> 24)
 		agree, err := checkDepths(r, b, nDepths, visual0)
 		if err != nil {
 			return
 		}
 		if agree && root == 0 {
 			root = root0
 			visual = visual0
 		}
 	}
 	return
 }

 // handshake performs the protocol handshake with the X server, and ensures
 // that the server provides a compatible Screen, Depth, etc.
 func (c *conn) handshake() os.Error {
 	_, err := io.ReadFull(c.r, c.buf[:8])
 	if err != nil {
 		return err
 	}
 	// Byte 0 should be 1 (success), bytes 2:6 should be 0xb0000000 (major/minor version 11.0).
 	if c.buf[0] != 1 || c.buf[2] != 11 || c.buf[3] != 0 || c.buf[4] != 0 || c.buf[5] != 0 {
 		return os.NewError("unsupported X version")
 	}
 	// Ignore the release number.
 	_, err = io.ReadFull(c.r, c.buf[:4])
 	if err != nil {
 		return err
 	}
 	// Read the resource ID base.
 	resourceIdBase, err := readU32LE(c.r, c.buf[:4])
 	if err != nil {
 		return err
 	}
 	// Read the resource ID mask.
 	resourceIdMask, err := readU32LE(c.r, c.buf[:4])
 	if err != nil {
 		return err
 	}
 	if resourceIdMask < 256 {
 		return os.NewError("X resource ID mask is too small")
 	}
 	// Ignore the motion buffer size.
 	_, err = io.ReadFull(c.r, c.buf[:4])
 	if err != nil {
 		return err
 	}
 	// Read the vendor length and round it up to a multiple of 4,
 	// for X11 protocol alignment reasons.
 	vendorLen, err := readU16LE(c.r, c.buf[:2])
 	if err != nil {
 		return err
 	}
 	vendorLen = (vendorLen + 3) &^ 3
 	// Read the maximum request length.
 	maxReqLen, err := readU16LE(c.r, c.buf[:2])
 	if err != nil {
 		return err
 	}
 	if maxReqLen != 0xffff {
 		return os.NewError("unsupported X maximum request length")
 	}
 	// Read the roots length.
 	rootsLen, err := readU8(c.r, c.buf[:1])
 	if err != nil {
 		return err
 	}
 	// Read the pixmap formats length.
 	pixmapFormatsLen, err := readU8(c.r, c.buf[:1])
 	if err != nil {
 		return err
 	}
 	// Ignore some things that we don't care about (totaling 10 + vendorLen bytes):
 	// imageByteOrder(1), bitmapFormatBitOrder(1), bitmapFormatScanlineUnit(1) bitmapFormatScanlinePad(1),
 	// minKeycode(1), maxKeycode(1), padding(4), vendor (vendorLen).
 	if 10+int(vendorLen) > cap(c.buf) {
 		return os.NewError("unsupported X vendor")
 	}
 	_, err = io.ReadFull(c.r, c.buf[:10+int(vendorLen)])
 	if err != nil {
 		return err
 	}
 	// Check that we have an agreeable pixmap format.
 	agree, err := checkPixmapFormats(c.r, c.buf[:8], int(pixmapFormatsLen))
 	if err != nil {
 		return err
 	}
 	if !agree {
 		return os.NewError("unsupported X pixmap formats")
 	}
 	// Check that we have an agreeable screen.
 	root, visual, err := checkScreens(c.r, c.buf[:24], int(rootsLen))
 	if err != nil {
 		return err
 	}
 	if root == 0 || visual == 0 {
 		return os.NewError("unsupported X screen")
 	}
 	c.gc = resID(resourceIdBase)
 	c.window = resID(resourceIdBase + 1)
 	c.root = resID(root)
 	c.visual = resID(visual)
 	return nil
 }

 // NewWindow calls NewWindowDisplay with $DISPLAY.
 func NewWindow() (gui.Window, os.Error) {
 	display := os.Getenv("DISPLAY")
 	if len(display) == 0 {
 		return nil, os.NewError("$DISPLAY not set")
 	}
 	return NewWindowDisplay(display)
 }

 // NewWindowDisplay returns a new gui.Window, backed by a newly created and
 // mapped X11 window. The X server to connect to is specified by the display
 // string, such as ":1".
 func NewWindowDisplay(display string) (gui.Window, os.Error) {
 	socket, displayStr, err := connect(display)
 	if err != nil {
 		return nil, err
 	}
 	c := new(conn)
 	c.c = socket
 	c.r = bufio.NewReader(socket)
 	c.w = bufio.NewWriter(socket)
 	err = authenticate(c.w, displayStr)
 	if err != nil {
 		return nil, err
 	}
 	err = c.handshake()
 	if err != nil {
 		return nil, err
 	}

 	// Now that we're connected, show a window, via three X protocol messages.
 	// First, issue a GetKeyboardMapping request. This is the first request, and
 	// will be associated with a cookie of 1.
 	setU32LE(c.buf[0:4], 0x00020065) // 0x65 is the GetKeyboardMapping opcode, and the message is 2 x 4 bytes long.
 	setU32LE(c.buf[4:8], uint32((keymapHi-keymapLo+1)<<8|keymapLo))
 	// Second, create a graphics context (GC).
 	setU32LE(c.buf[8:12], 0x00060037) // 0x37 is the CreateGC opcode, and the message is 6 x 4 bytes long.
 	setU32LE(c.buf[12:16], uint32(c.gc))
 	setU32LE(c.buf[16:20], uint32(c.root))
 	setU32LE(c.buf[20:24], 0x00010004) // Bit 2 is XCB_GC_FOREGROUND, bit 16 is XCB_GC_GRAPHICS_EXPOSURES.
 	setU32LE(c.buf[24:28], 0x00000000) // The Foreground is black.
 	setU32LE(c.buf[28:32], 0x00000000) // GraphicsExposures' value is unused.
 	// Third, create the window.
 	setU32LE(c.buf[32:36], 0x000a0001) // 0x01 is the CreateWindow opcode, and the message is 10 x 4 bytes long.
 	setU32LE(c.buf[36:40], uint32(c.window))
 	setU32LE(c.buf[40:44], uint32(c.root))
 	setU32LE(c.buf[44:48], 0x00000000) // Initial (x, y) is (0, 0).
 	setU32LE(c.buf[48:52], windowHeight<<16|windowWidth)
 	setU32LE(c.buf[52:56], 0x00010000) // Border width is 0, XCB_WINDOW_CLASS_INPUT_OUTPUT is 1.
 	setU32LE(c.buf[56:60], uint32(c.visual))
 	setU32LE(c.buf[60:64], 0x00000802) // Bit 1 is XCB_CW_BACK_PIXEL, bit 11 is XCB_CW_EVENT_MASK.
 	setU32LE(c.buf[64:68], 0x00000000) // The Back-Pixel is black.
 	setU32LE(c.buf[68:72], 0x0000804f) // Key/button press and release, pointer motion, and expose event masks.
 	// Fourth, map the window.
 	setU32LE(c.buf[72:76], 0x00020008) // 0x08 is the MapWindow opcode, and the message is 2 x 4 bytes long.
 	setU32LE(c.buf[76:80], uint32(c.window))
 	// Write the bytes.
 	_, err = c.w.Write(c.buf[:80])
 	if err != nil {
 		return nil, err
 	}
 	err = c.w.Flush()
 	if err != nil {
 		return nil, err
 	}

 	c.img = image.NewRGBA(image.Rect(0, 0, windowWidth, windowHeight))
 	c.eventc = make(chan interface{}, 16)
 	c.flush = make(chan bool, 1)
 	go c.readSocket()
 	go c.writeSocket()
 	return c, nil
 }
	// 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.

	// Package x11 implements an X11 backend for the exp/gui package.
	//
	// The X protocol specification is at ftp://ftp.x.org/pub/X11R7.0/doc/PDF/proto.pdf.
	// A summary of the wire format can be found in XCB's xproto.xml.
	package x11

	import (
	"bufio"
	"exp/gui"
	"image"
	"image/draw"
	"io"
	"log"
	"net"
	"os"
	"strconv"
	"strings"
	"time"
	)

	type resID uint32 // X resource IDs.

	// TODO(nigeltao): Handle window resizes.
	const (
	windowHeight = 600
	windowWidth = 800
	)

	const (
	keymapLo = 8
	keymapHi = 255
	)

	type conn struct {
	c io.Closer
	r *bufio.Reader
	w *bufio.Writer

	gc, window, root, visual resID

	img *image.RGBA
	eventc chan interface{}
	mouseState gui.MouseEvent

	buf [256]byte // General purpose scratch buffer.

	flush chan bool
	flushBuf0 [24]byte
	flushBuf1 [4 * 1024]byte
	}

	// writeSocket runs in its own goroutine, serving both FlushImage calls
	// directly from the exp/gui client and indirectly from X expose events.
	// It paints c.img to the X server via PutImage requests.
	func (c *conn) writeSocket() {
	defer c.c.Close()
	for _ = range c.flush {
	b := c.img.Bounds()
	if b.Empty() {
	continue
	}
	// Each X request has a 16-bit length (in terms of 4-byte units). To avoid going over
	// this limit, we send PutImage for each row of the image, rather than trying to paint
	// the entire image in one X request. This approach could easily be optimized (or the
	// X protocol may have an escape sequence to delimit very large requests).
	// TODO(nigeltao): See what XCB's xcb_put_image does in this situation.
	units := 6 + b.Dx()
	if units > 0xffff \|\| b.Dy() > 0xffff {
	log.Print("x11: window is too large for PutImage")
	return
	}

	c.flushBuf0[0] = 0x48 // PutImage opcode.
	c.flushBuf0[1] = 0x02 // XCB_IMAGE_FORMAT_Z_PIXMAP.
	c.flushBuf0[2] = uint8(units)
	c.flushBuf0[3] = uint8(units >> 8)
	setU32LE(c.flushBuf0[4:8], uint32(c.window))
	setU32LE(c.flushBuf0[8:12], uint32(c.gc))
	setU32LE(c.flushBuf0[12:16], 1<<16\|uint32(b.Dx()))
	c.flushBuf0[21] = 0x18 // depth = 24 bits.

	for y := b.Min.Y; y < b.Max.Y; y++ {
	setU32LE(c.flushBuf0[16:20], uint32(y<<16))
	if _, err := c.w.Write(c.flushBuf0[:24]); err != nil {
	if err != os.EOF {
	log.Println("x11:", err.String())
	}
	return
	}
	p := c.img.Pix[(y-b.Min.Y)*c.img.Stride:]
	for x, dx := 0, 4*b.Dx(); x < dx; {
	nx := dx - x
	if nx > len(c.flushBuf1) {
	nx = len(c.flushBuf1) &^ 3
	}
	for i := 0; i < nx; i += 4 {
	// X11's order is BGRX, not RGBA.
	c.flushBuf1[i+0] = p[x+i+2]
	c.flushBuf1[i+1] = p[x+i+1]
	c.flushBuf1[i+2] = p[x+i+0]
	}
	x += nx
	if _, err := c.w.Write(c.flushBuf1[:nx]); err != nil {
	if err != os.EOF {
	log.Println("x11:", err.String())
	}
	return
	}
	}
	}
	if err := c.w.Flush(); err != nil {
	if err != os.EOF {
	log.Println("x11:", err.String())
	}
	return
	}
	}
	}

	func (c *conn) Screen() draw.Image { return c.img }

	func (c *conn) FlushImage() {
	select {
	case c.flush <- false:
	// Flush notification sent.
	default:
	// Could not send.
	// Flush notification must be pending already.
	}
	}

	func (c *conn) Close() os.Error {
	// Shut down the writeSocket goroutine. This will close the socket to the
	// X11 server, which will cause c.eventc to close.
	close(c.flush)
	for _ = range c.eventc {
	// Drain the channel to allow the readSocket goroutine to shut down.
	}
	return nil
	}

	func (c *conn) EventChan() <-chan interface{} { return c.eventc }

	// readSocket runs in its own goroutine, reading X events and sending gui
	// events on c's EventChan.
	func (c *conn) readSocket() {
	var (
	keymap [256][]int
	keysymsPerKeycode int
	)
	defer close(c.eventc)
	for {
	// X events are always 32 bytes long.
	if _, err := io.ReadFull(c.r, c.buf[:32]); err != nil {
	if err != os.EOF {
	c.eventc <- gui.ErrEvent{err}
	}
	return
	}
	switch c.buf[0] {
	case 0x01: // Reply from a request (e.g. GetKeyboardMapping).
	cookie := int(c.buf[3])<<8 \| int(c.buf[2])
	if cookie != 1 {
	// We issued only one request (GetKeyboardMapping) with a cookie of 1,
	// so we shouldn't get any other reply from the X server.
	c.eventc <- gui.ErrEvent{os.NewError("x11: unexpected cookie")}
	return
	}
	keysymsPerKeycode = int(c.buf[1])
	b := make([]int, 256*keysymsPerKeycode)
	for i := range keymap {
	keymap[i] = b[ikeysymsPerKeycode : (i+1)keysymsPerKeycode]
	}
	for i := keymapLo; i <= keymapHi; i++ {
	m := keymap[i]
	for j := range m {
	u, err := readU32LE(c.r, c.buf[:4])
	if err != nil {
	if err != os.EOF {
	c.eventc <- gui.ErrEvent{err}
	}
	return
	}
	m[j] = int(u)
	}
	}
	case 0x02, 0x03: // Key press, key release.
	// X Keyboard Encoding is documented at http://tronche.com/gui/x/xlib/input/keyboard-encoding.html
	// TODO(nigeltao): Do we need to implement the "MODE SWITCH / group modifier" feature
	// or is that some no-longer-used X construct?
	if keysymsPerKeycode < 2 {
	// Either we haven't yet received the GetKeyboardMapping reply or
	// the X server has sent one that's too short.
	continue
	}
	keycode := int(c.buf[1])
	shift := int(c.buf[28]) & 0x01
	keysym := keymap[keycode][shift]
	if keysym == 0 {
	keysym = keymap[keycode][0]
	}
	// TODO(nigeltao): Should we send KeyEvents for Shift/Ctrl/Alt? Should Shift-A send
	// the same int down the channel as the sent on just the A key?
	// TODO(nigeltao): How should IME events (e.g. key presses that should generate CJK text) work? Or
	// is that outside the scope of the gui.Window interface?
	if c.buf[0] == 0x03 {
	keysym = -keysym
	}
	c.eventc <- gui.KeyEvent{keysym}
	case 0x04, 0x05: // Button press, button release.
	mask := 1 << (c.buf[1] - 1)
	if c.buf[0] == 0x04 {
	c.mouseState.Buttons \|= mask
	} else {
	c.mouseState.Buttons &^= mask
	}
	c.mouseState.Nsec = time.Nanoseconds()
	c.eventc <- c.mouseState
	case 0x06: // Motion notify.
	c.mouseState.Loc.X = int(int16(c.buf[25])<<8 \| int16(c.buf[24]))
	c.mouseState.Loc.Y = int(int16(c.buf[27])<<8 \| int16(c.buf[26]))
	c.mouseState.Nsec = time.Nanoseconds()
	c.eventc <- c.mouseState
	case 0x0c: // Expose.
	// A single user action could trigger multiple expose events (e.g. if moving another
	// window with XShape'd rounded corners over our window). In that case, the X server will
	// send a uint16 count (in bytes 16-17) of the number of additional expose events coming.
	// We could parse each event for the (x, y, width, height) and maintain a minimal dirty
	// rectangle, but for now, the simplest approach is to paint the entire window, when
	// receiving the final event in the series.
	if c.buf[17] == 0 && c.buf[16] == 0 {
	// TODO(nigeltao): Should we ignore the very first expose event? A freshly mapped window
	// will trigger expose, but until the first c.FlushImage call, there's probably nothing to
	// paint but black. For an 800x600 window, at 4 bytes per pixel, each repaint writes about
	// 2MB over the socket.
	c.FlushImage()
	}
	// TODO(nigeltao): Should we listen to DestroyNotify (0x11) and ResizeRequest (0x19) events?
	// What about EnterNotify (0x07) and LeaveNotify (0x08)?
	}
	}
	}

	// connect connects to the X server given by the full X11 display name (e.g.
	// ":12.0") and returns the connection as well as the portion of the full name
	// that is the display number (e.g. "12").
	// Examples:
	// connect(":1") // calls net.Dial("unix", "", "/tmp/.X11-unix/X1"), displayStr="1"
	// connect("/tmp/launch-123/:0") // calls net.Dial("unix", "", "/tmp/launch-123/:0"), displayStr="0"
	// connect("hostname:2.1") // calls net.Dial("tcp", "", "hostname:6002"), displayStr="2"
	// connect("tcp/hostname:1.0") // calls net.Dial("tcp", "", "hostname:6001"), displayStr="1"
	func connect(display string) (conn net.Conn, displayStr string, err os.Error) {
	colonIdx := strings.LastIndex(display, ":")
	if colonIdx < 0 {
	return nil, "", os.NewError("bad display: " + display)
	}
	// Parse the section before the colon.
	var protocol, host, socket string
	if display[0] == '/' {
	socket = display[:colonIdx]
	} else {
	if i := strings.LastIndex(display, "/"); i < 0 {
	// The default protocol is TCP.
	protocol = "tcp"
	host = display[:colonIdx]
	} else {
	protocol = display[:i]
	host = display[i+1 : colonIdx]
	}
	}
	// Parse the section after the colon.
	after := display[colonIdx+1:]
	if after == "" {
	return nil, "", os.NewError("bad display: " + display)
	}
	if i := strings.LastIndex(after, "."); i < 0 {
	displayStr = after
	} else {
	displayStr = after[:i]
	}
	displayInt, err := strconv.Atoi(displayStr)
	if err != nil \|\| displayInt < 0 {
	return nil, "", os.NewError("bad display: " + display)
	}
	// Make the connection.
	if socket != "" {
	conn, err = net.Dial("unix", socket+":"+displayStr)
	} else if host != "" {
	conn, err = net.Dial(protocol, host+":"+strconv.Itoa(6000+displayInt))
	} else {
	conn, err = net.Dial("unix", "/tmp/.X11-unix/X"+displayStr)
	}
	if err != nil {
	return nil, "", os.NewError("cannot connect to " + display + ": " + err.String())
	}
	return
	}

	// authenticate authenticates ourselves with the X server.
	// displayStr is the "12" out of ":12.0".
	func authenticate(w *bufio.Writer, displayStr string) os.Error {
	key, value, err := readAuth(displayStr)
	if err != nil {
	return err
	}
	// Assume that the authentication protocol is "MIT-MAGIC-COOKIE-1".
	if len(key) != 18 \|\| len(value) != 16 {
	return os.NewError("unsupported Xauth")
	}
	// 0x006c means little-endian. 0x000b, 0x0000 means X major version 11, minor version 0.
	// 0x0012 and 0x0010 means the auth key and value have lengths 18 and 16.
	// The final 0x0000 is padding, so that the string length is a multiple of 4.
	_, err = io.WriteString(w, "\x6c\x00\x0b\x00\x00\x00\x12\x00\x10\x00\x00\x00")
	if err != nil {
	return err
	}
	_, err = io.WriteString(w, key)
	if err != nil {
	return err
	}
	// Again, the 0x0000 is padding.
	_, err = io.WriteString(w, "\x00\x00")
	if err != nil {
	return err
	}
	_, err = io.WriteString(w, value)
	if err != nil {
	return err
	}
	err = w.Flush()
	if err != nil {
	return err
	}
	return nil
	}

	// readU8 reads a uint8 from r, using b as a scratch buffer.
	func readU8(r io.Reader, b []byte) (uint8, os.Error) {
	_, err := io.ReadFull(r, b[:1])
	if err != nil {
	return 0, err
	}
	return uint8(b[0]), nil
	}

	// readU16LE reads a little-endian uint16 from r, using b as a scratch buffer.
	func readU16LE(r io.Reader, b []byte) (uint16, os.Error) {
	_, err := io.ReadFull(r, b[:2])
	if err != nil {
	return 0, err
	}
	return uint16(b[0]) \| uint16(b[1])<<8, nil
	}

	// readU32LE reads a little-endian uint32 from r, using b as a scratch buffer.
	func readU32LE(r io.Reader, b []byte) (uint32, os.Error) {
	_, err := io.ReadFull(r, b[:4])
	if err != nil {
	return 0, err
	}
	return uint32(b[0]) \| uint32(b[1])<<8 \| uint32(b[2])<<16 \| uint32(b[3])<<24, nil
	}

	// setU32LE sets b[:4] to be the little-endian representation of u.
	func setU32LE(b []byte, u uint32) {
	b[0] = byte((u >> 0) & 0xff)
	b[1] = byte((u >> 8) & 0xff)
	b[2] = byte((u >> 16) & 0xff)
	b[3] = byte((u >> 24) & 0xff)
	}

	// checkPixmapFormats checks that we have an agreeable X pixmap Format.
	func checkPixmapFormats(r io.Reader, b []byte, n int) (agree bool, err os.Error) {
	for i := 0; i < n; i++ {
	_, err = io.ReadFull(r, b[:8])
	if err != nil {
	return
	}
	// Byte 0 is depth, byte 1 is bits-per-pixel, byte 2 is scanline-pad, the rest (5) is padding.
	if b[0] == 24 && b[1] == 32 {
	agree = true
	}
	}
	return
	}

	// checkDepths checks that we have an agreeable X Depth (i.e. one that has an agreeable X VisualType).
	func checkDepths(r io.Reader, b []byte, n int, visual uint32) (agree bool, err os.Error) {
	for i := 0; i < n; i++ {
	depth, err := readU16LE(r, b)
	if err != nil {
	return
	}
	depth &= 0xff
	visualsLen, err := readU16LE(r, b)
	if err != nil {
	return
	}
	// Ignore 4 bytes of padding.
	_, err = io.ReadFull(r, b[:4])
	if err != nil {
	return
	}
	for j := 0; j < int(visualsLen); j++ {
	// Read 24 bytes: visual(4), class(1), bits per rgb value(1), colormap entries(2),
	// red mask(4), green mask(4), blue mask(4), padding(4).
	v, err := readU32LE(r, b)
	_, err = readU32LE(r, b)
	rm, err := readU32LE(r, b)
	gm, err := readU32LE(r, b)
	bm, err := readU32LE(r, b)
	_, err = readU32LE(r, b)
	if err != nil {
	return
	}
	if v == visual && rm == 0xff0000 && gm == 0xff00 && bm == 0xff && depth == 24 {
	agree = true
	}
	}
	}
	return
	}

	// checkScreens checks that we have an agreeable X Screen.
	func checkScreens(r io.Reader, b []byte, n int) (root, visual uint32, err os.Error) {
	for i := 0; i < n; i++ {
	root0, err := readU32LE(r, b)
	if err != nil {
	return
	}
	// Ignore the next 7x4 bytes, which is: colormap, whitepixel, blackpixel, current input masks,
	// width and height (pixels), width and height (mm), min and max installed maps.
	_, err = io.ReadFull(r, b[:28])
	if err != nil {
	return
	}
	visual0, err := readU32LE(r, b)
	if err != nil {
	return
	}
	// Next 4 bytes: backing stores, save unders, root depth, allowed depths length.
	x, err := readU32LE(r, b)
	if err != nil {
	return
	}
	nDepths := int(x >> 24)
	agree, err := checkDepths(r, b, nDepths, visual0)
	if err != nil {
	return
	}
	if agree && root == 0 {
	root = root0
	visual = visual0
	}
	}
	return
	}

	// handshake performs the protocol handshake with the X server, and ensures
	// that the server provides a compatible Screen, Depth, etc.
	func (c *conn) handshake() os.Error {
	_, err := io.ReadFull(c.r, c.buf[:8])
	if err != nil {
	return err
	}
	// Byte 0 should be 1 (success), bytes 2:6 should be 0xb0000000 (major/minor version 11.0).
	if c.buf[0] != 1 \|\| c.buf[2] != 11 \|\| c.buf[3] != 0 \|\| c.buf[4] != 0 \|\| c.buf[5] != 0 {
	return os.NewError("unsupported X version")
	}
	// Ignore the release number.
	_, err = io.ReadFull(c.r, c.buf[:4])
	if err != nil {
	return err
	}
	// Read the resource ID base.
	resourceIdBase, err := readU32LE(c.r, c.buf[:4])
	if err != nil {
	return err
	}
	// Read the resource ID mask.
	resourceIdMask, err := readU32LE(c.r, c.buf[:4])
	if err != nil {
	return err
	}
	if resourceIdMask < 256 {
	return os.NewError("X resource ID mask is too small")
	}
	// Ignore the motion buffer size.
	_, err = io.ReadFull(c.r, c.buf[:4])
	if err != nil {
	return err
	}
	// Read the vendor length and round it up to a multiple of 4,
	// for X11 protocol alignment reasons.
	vendorLen, err := readU16LE(c.r, c.buf[:2])
	if err != nil {
	return err
	}
	vendorLen = (vendorLen + 3) &^ 3
	// Read the maximum request length.
	maxReqLen, err := readU16LE(c.r, c.buf[:2])
	if err != nil {
	return err
	}
	if maxReqLen != 0xffff {
	return os.NewError("unsupported X maximum request length")
	}
	// Read the roots length.
	rootsLen, err := readU8(c.r, c.buf[:1])
	if err != nil {
	return err
	}
	// Read the pixmap formats length.
	pixmapFormatsLen, err := readU8(c.r, c.buf[:1])
	if err != nil {
	return err
	}
	// Ignore some things that we don't care about (totaling 10 + vendorLen bytes):
	// imageByteOrder(1), bitmapFormatBitOrder(1), bitmapFormatScanlineUnit(1) bitmapFormatScanlinePad(1),
	// minKeycode(1), maxKeycode(1), padding(4), vendor (vendorLen).
	if 10+int(vendorLen) > cap(c.buf) {
	return os.NewError("unsupported X vendor")
	}
	_, err = io.ReadFull(c.r, c.buf[:10+int(vendorLen)])
	if err != nil {
	return err
	}
	// Check that we have an agreeable pixmap format.
	agree, err := checkPixmapFormats(c.r, c.buf[:8], int(pixmapFormatsLen))
	if err != nil {
	return err
	}
	if !agree {
	return os.NewError("unsupported X pixmap formats")
	}
	// Check that we have an agreeable screen.
	root, visual, err := checkScreens(c.r, c.buf[:24], int(rootsLen))
	if err != nil {
	return err
	}
	if root == 0 \|\| visual == 0 {
	return os.NewError("unsupported X screen")
	}
	c.gc = resID(resourceIdBase)
	c.window = resID(resourceIdBase + 1)
	c.root = resID(root)
	c.visual = resID(visual)
	return nil
	}

	// NewWindow calls NewWindowDisplay with $DISPLAY.
	func NewWindow() (gui.Window, os.Error) {
	display := os.Getenv("DISPLAY")
	if len(display) == 0 {
	return nil, os.NewError("$DISPLAY not set")
	}
	return NewWindowDisplay(display)
	}

	// NewWindowDisplay returns a new gui.Window, backed by a newly created and
	// mapped X11 window. The X server to connect to is specified by the display
	// string, such as ":1".
	func NewWindowDisplay(display string) (gui.Window, os.Error) {
	socket, displayStr, err := connect(display)
	if err != nil {
	return nil, err
	}
	c := new(conn)
	c.c = socket
	c.r = bufio.NewReader(socket)
	c.w = bufio.NewWriter(socket)
	err = authenticate(c.w, displayStr)
	if err != nil {
	return nil, err
	}
	err = c.handshake()
	if err != nil {
	return nil, err
	}

	// Now that we're connected, show a window, via three X protocol messages.
	// First, issue a GetKeyboardMapping request. This is the first request, and
	// will be associated with a cookie of 1.
	setU32LE(c.buf[0:4], 0x00020065) // 0x65 is the GetKeyboardMapping opcode, and the message is 2 x 4 bytes long.
	setU32LE(c.buf[4:8], uint32((keymapHi-keymapLo+1)<<8\|keymapLo))
	// Second, create a graphics context (GC).
	setU32LE(c.buf[8:12], 0x00060037) // 0x37 is the CreateGC opcode, and the message is 6 x 4 bytes long.
	setU32LE(c.buf[12:16], uint32(c.gc))
	setU32LE(c.buf[16:20], uint32(c.root))
	setU32LE(c.buf[20:24], 0x00010004) // Bit 2 is XCB_GC_FOREGROUND, bit 16 is XCB_GC_GRAPHICS_EXPOSURES.
	setU32LE(c.buf[24:28], 0x00000000) // The Foreground is black.
	setU32LE(c.buf[28:32], 0x00000000) // GraphicsExposures' value is unused.
	// Third, create the window.
	setU32LE(c.buf[32:36], 0x000a0001) // 0x01 is the CreateWindow opcode, and the message is 10 x 4 bytes long.
	setU32LE(c.buf[36:40], uint32(c.window))
	setU32LE(c.buf[40:44], uint32(c.root))
	setU32LE(c.buf[44:48], 0x00000000) // Initial (x, y) is (0, 0).
	setU32LE(c.buf[48:52], windowHeight<<16\|windowWidth)
	setU32LE(c.buf[52:56], 0x00010000) // Border width is 0, XCB_WINDOW_CLASS_INPUT_OUTPUT is 1.
	setU32LE(c.buf[56:60], uint32(c.visual))
	setU32LE(c.buf[60:64], 0x00000802) // Bit 1 is XCB_CW_BACK_PIXEL, bit 11 is XCB_CW_EVENT_MASK.
	setU32LE(c.buf[64:68], 0x00000000) // The Back-Pixel is black.
	setU32LE(c.buf[68:72], 0x0000804f) // Key/button press and release, pointer motion, and expose event masks.
	// Fourth, map the window.
	setU32LE(c.buf[72:76], 0x00020008) // 0x08 is the MapWindow opcode, and the message is 2 x 4 bytes long.
	setU32LE(c.buf[76:80], uint32(c.window))
	// Write the bytes.
	_, err = c.w.Write(c.buf[:80])
	if err != nil {
	return nil, err
	}
	err = c.w.Flush()
	if err != nil {
	return nil, err
	}

	c.img = image.NewRGBA(image.Rect(0, 0, windowWidth, windowHeight))
	c.eventc = make(chan interface{}, 16)
	c.flush = make(chan bool, 1)
	go c.readSocket()
	go c.writeSocket()
	return c, nil
	}