net: Make Listen(":port") use IPv6 when IPv4 is not supported.

When running an experimental kernel with IPv4 disabled, Listen(":port")
currently tries to create an AF_INET socket, and fails.  Instead, it
should see !supportsIPv4, and use an AF_INET6 socket.

This sort of environment is quite esoteric at the moment, but I can
force the tests to fail on regular Linux using the following tweaks:

- net/net.go: supportsIPv4, supportsIPv6, supportsIPv4map = false, true, false
- net/sockopt_linux.go: ipv6only=true
- net/ipsock_posix.go: Revert this fix
- ./make.bash && ../bin/go test net

Also, make the arrows in server_test.go point to the left, because
server<-client is easier to read.

Fixes #12510

Change-Id: I0cc3b6b08d5e6908d2fbf8594f652ba19815aa4b
Reviewed-on: https://go-review.googlesource.com/14334
Run-TryBot: Paul Marks <pmarks@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/ipsock_posix.go

@@ -101,10 +101,11 @@ func probeIPv6Stack() (supportsIPv6, supportsIPv4map bool) {
 //
 //	1. A wild-wild listen, "tcp" + ""
 //	If the platform supports both IPv6 and IPv6 IPv4-mapping
-//	capabilities, we assume that the user want to listen on
-//	both IPv4 and IPv6 wildcard address over an AF_INET6
-//	socket with IPV6_V6ONLY=0.  Otherwise we prefer an IPv4
-//	wildcard address listen over an AF_INET socket.
+//	capabilities, or does not support IPv4, we assume that
+//	the user wants to listen on both IPv4 and IPv6 wildcard
+//	addresses over an AF_INET6 socket with IPV6_V6ONLY=0.
+//	Otherwise we prefer an IPv4 wildcard address listen over
+//	an AF_INET socket.
 //
 //	2. A wild-ipv4wild listen, "tcp" + "0.0.0.0"
 //	Same as 1.
@@ -137,7 +138,7 @@ func favoriteAddrFamily(net string, laddr, raddr sockaddr, mode string) (family
 	}
 
 	if mode == "listen" && (laddr == nil || laddr.isWildcard()) {
-		if supportsIPv4map {
+		if supportsIPv4map || !supportsIPv4 {
 			return syscall.AF_INET6, false
 		}
 		if laddr == nil {

src/net/platform_test.go

@@ -134,7 +134,7 @@ func testableListenArgs(network, address, client string) bool {
 
 	// Test functionality of IPv4 communication using AF_INET6
 	// sockets.
-	if !supportsIPv4map && (network == "tcp" || network == "udp" || network == "ip") && wildcard {
+	if !supportsIPv4map && supportsIPv4 && (network == "tcp" || network == "udp" || network == "ip") && wildcard {
 		// At this point, we prefer IPv4 when ip is nil.
 		// See favoriteAddrFamily for further information.
 		if ip.To16() != nil && ip.To4() == nil && cip.To4() != nil { // a pair of IPv6 server and IPv4 client

src/net/server_test.go

@@ -55,7 +55,7 @@ func TestTCPServer(t *testing.T) {
 
 	for i, tt := range tcpServerTests {
 		if !testableListenArgs(tt.snet, tt.saddr, tt.taddr) {
-			t.Logf("skipping %s test", tt.snet+" "+tt.saddr+"->"+tt.taddr)
+			t.Logf("skipping %s test", tt.snet+" "+tt.saddr+"<-"+tt.taddr)
 			continue
 		}
 
@@ -251,7 +251,7 @@ var udpServerTests = []struct {
 func TestUDPServer(t *testing.T) {
 	for i, tt := range udpServerTests {
 		if !testableListenArgs(tt.snet, tt.saddr, tt.taddr) {
-			t.Logf("skipping %s test", tt.snet+" "+tt.saddr+"->"+tt.taddr)
+			t.Logf("skipping %s test", tt.snet+" "+tt.saddr+"<-"+tt.taddr)
 			continue
 		}
 
@@ -329,7 +329,7 @@ var unixgramServerTests = []struct {
 func TestUnixgramServer(t *testing.T) {
 	for i, tt := range unixgramServerTests {
 		if !testableListenArgs("unixgram", tt.saddr, "") {
-			t.Logf("skipping %s test", "unixgram "+tt.saddr+"->"+tt.caddr)
+			t.Logf("skipping %s test", "unixgram "+tt.saddr+"<-"+tt.caddr)
 			continue
 		}