Initial exp/draw/x11 implementation.

This provides an experimental X11 backend for the exp/draw interface.
It does not aim to provide a complete implementation of the X11 client protocol.

This works for me (Ubuntu Hardy 8.04, GOARCH=386). Your mileage my vary.

R=r, rsc, r1
CC=golang-dev
https://golang.org/cl/156109

src/pkg/exp/draw/x11/Makefile

+# 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.
+
+include $(GOROOT)/src/Make.$(GOARCH)
+
+TARG=exp/draw/x11
+GOFILES=\
+	auth.go\
+	conn.go\
+
+include $(GOROOT)/src/Make.pkg

src/pkg/exp/draw/x11/auth.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.
+
+package x11
+
+import (
+	"bufio";
+	"io";
+	"os";
+)
+
+// Reads the DISPLAY environment variable, and returns the "12" in ":12.0".
+func getDisplay() string {
+	d := os.Getenv("DISPLAY");
+	if len(d) < 1 || d[0] != ':' {
+		return ""
+	}
+	i := 1;
+	for ; i < len(d); i++ {
+		if d[i] < '0' || d[i] > '9' {
+			break
+		}
+	}
+	return d[1:i];
+}
+
+// Reads a big-endian uint16 from r, using b as a scratch buffer.
+func readU16BE(r io.Reader, b []byte) (uint16, os.Error) {
+	_, err := io.ReadFull(r, b[0:2]);
+	if err != nil {
+		return 0, err
+	}
+	return uint16(b[0])<<8 + uint16(b[1]), nil;
+}
+
+// Reads a length-prefixed string from r, using b as a scratch buffer.
+func readStr(r io.Reader, b []byte) (s string, err os.Error) {
+	n, err := readU16BE(r, b);
+	if err != nil {
+		return
+	}
+	if int(n) > len(b) {
+		return s, os.NewError("Xauthority entry too long for buffer")
+	}
+	_, err = io.ReadFull(r, b[0:n]);
+	if err != nil {
+		return
+	}
+	return string(b[0:n]), nil;
+}
+
+// Reads the ~/.Xauthority file and returns the name/data pair for the DISPLAY.
+// b is a scratch buffer to use, and should be at least 256 bytes long (i.e. it should be able to hold a hostname).
+func readAuth(b []byte) (name, data string, err os.Error) {
+	// As per /usr/include/X11/Xauth.h.
+	const familyLocal = 256;
+
+	home := os.Getenv("HOME");
+	if len(home) == 0 {
+		err = os.NewError("unknown HOME");
+		return;
+	}
+	r, err := os.Open(home+"/.Xauthority", os.O_RDONLY, 0444);
+	if err != nil {
+		return
+	}
+	defer r.Close();
+	br := bufio.NewReader(r);
+
+	hostname, err := os.Hostname();
+	if err != nil {
+		return
+	}
+	display := getDisplay();
+	for {
+		family, err := readU16BE(br, b[0:2]);
+		if err != nil {
+			return
+		}
+		addr, err := readStr(br, b[0:]);
+		if err != nil {
+			return
+		}
+		disp, err := readStr(br, b[0:]);
+		if err != nil {
+			return
+		}
+		name0, err := readStr(br, b[0:]);
+		if err != nil {
+			return
+		}
+		data0, err := readStr(br, b[0:]);
+		if err != nil {
+			return
+		}
+		if family == familyLocal && addr == hostname && disp == display {
+			return name0, data0, nil
+		}
+	}
+	panic("unreachable");
+}