another attempt at avoiding IPv6 when it's not supported.

dsymonds confirms that this one works.

R=r
DELTA=50  (23 added, 17 deleted, 10 changed)
OCL=28433
CL=28444

src/lib/net/ip.go

@@ -42,7 +42,7 @@ type IPMask []byte;
 // IPv4 returns the IP address (in 16-byte form) of the
 // IPv4 address a.b.c.d.
 func IPv4(a, b, c, d byte) IP {
-	p := make([]byte, IPv6len);
+	p := make(IP, IPv6len);
 	for i := 0; i < 10; i++ {
 		p[i] = 0
 	}
@@ -60,6 +60,7 @@ var (
 	IPv4bcast = IPv4(255, 255, 255, 255);	// broadcast
 	IPv4allsys = IPv4(224, 0, 0, 1);	// all systems
 	IPv4allrouter = IPv4(224, 0, 0, 2);	// all routers
+	IPv4zero = IPv4(0, 0, 0, 0);	// all zeros
 )
 
 // Well-known IPv6 addresses
@@ -68,7 +69,7 @@ var (
 )
 
 // Is p all zeros?
-func isZeros(p []byte) bool {
+func isZeros(p IP) bool {
 	for i := 0; i < len(p); i++ {
 		if p[i] != 0 {
 			return false
@@ -106,9 +107,9 @@ func (ip IP) To16() IP {
 
 // Default route masks for IPv4.
 var (
-	classAMask IPMask = IPv4(0xff, 0, 0, 0);
-	classBMask IPMask = IPv4(0xff, 0xff, 0, 0);
-	classCMask IPMask = IPv4(0xff, 0xff, 0xff, 0);
+	classAMask = IPMask(IPv4(0xff, 0, 0, 0));
+	classBMask = IPMask(IPv4(0xff, 0xff, 0, 0));
+	classCMask = IPMask(IPv4(0xff, 0xff, 0xff, 0));
 )
 
 // DefaultMask returns the default IP mask for the IP address ip.
@@ -227,7 +228,7 @@ func (ip IP) String() string {
 
 // If mask is a sequence of 1 bits followed by 0 bits,
 // return the number of 1 bits.
-func simpleMaskLength(mask IP) int {
+func simpleMaskLength(mask IPMask) int {
 	var i int;
 	for i = 0; i < len(mask); i++ {
 		if mask[i] != 0xFF {

src/lib/net/net.go

@@ -19,6 +19,24 @@ var (
 	UnknownSocketFamily = os.NewError("unknown socket family");
 )
 
+
+// Should we try to use the IPv4 socket interface if we're
+// only dealing with IPv4 sockets?  As long as the host system
+// understands IPv6, it's okay to pass IPv4 addresses to the IPv6
+// interface.  That simplifies our code and is most general.
+// Unfortunately, we need to run on kernels built without IPv6 support too.
+// So probe the kernel to figure it out.
+func kernelSupportsIPv6() bool {
+	fd, e := syscall.Socket(syscall.AF_INET6, syscall.SOCK_STREAM, syscall.IPPROTO_TCP);
+	if fd >= 0 {
+		syscall.Close(fd)
+	}
+	return e == 0
+}
+
+var preferIPv4 = !kernelSupportsIPv6()
+
+
 func LookupHost(name string) (cname string, addrs []string, err os.Error)
 
 // Split "host:port" into "host" and "port".
@@ -63,17 +81,21 @@ func joinHostPort(host, port string) string {
 // Convert "host:port" into IP address and port.
 // For now, host and port must be numeric literals.
 // Eventually, we'll have name resolution.
-func hostPortToIP(net, hostport, mode string) (ip []byte, iport int, err os.Error) {
+func hostPortToIP(net, hostport, mode string) (ip IP, iport int, err os.Error) {
 	var host, port string;
 	host, port, err = splitHostPort(hostport);
 	if err != nil {
 		return nil, 0, err
 	}
 
-	var addr []byte;
+	var addr IP;
 	if host == "" {
 		if mode == "listen" {
-			addr = IPzero;	// wildcard - listen to all
+			if preferIPv4 {
+				addr = IPv4zero;
+			} else {
+				addr = IPzero;	// wildcard - listen to all
+			}
 		} else {
 			return nil, 0, MissingAddress;
 		}
@@ -296,23 +318,6 @@ func (c *connBase) SetLinger(sec int) os.Error {
 
 // Internet sockets (TCP, UDP)
 
-// Should we try to use the IPv4 socket interface if we're
-// only dealing with IPv4 sockets?  As long as the host system
-// understands IPv6, it's okay to pass IPv4 addresses to the IPv6
-// interface.  That simplifies our code and is most general.
-// Unfortunately, we need to run on kernels built without IPv6 support too.
-// So probe the kernel to figure it out.
-func kernelSupportsIPv6() bool {
-	fd, e := syscall.Socket(syscall.AF_INET6, syscall.SOCK_STREAM, syscall.IPPROTO_TCP);
-	if fd >= 0 {
-		syscall.Close(fd)
-	}
-	return e == 0
-}
-
-var preferIPv4 = !kernelSupportsIPv6()
-
-
 func internetSocket(net, laddr, raddr string, proto int64, mode string)
 	(fd *netFD, err os.Error)
 {
@@ -352,7 +357,7 @@ func internetSocket(net, laddr, raddr string, proto int64, mode string)
 		}
 	}
 
-	var cvt func(addr []byte, port int) (sa *syscall.Sockaddr, err os.Error);
+	var cvt func(addr IP, port int) (sa *syscall.Sockaddr, err os.Error);
 	var family int64;
 	if vers == 4 {
 		cvt = v4ToSockaddr;