net: make Dial("tcp", ln.Addr().String()) work even with bad IPv6 config

Some machines can be configured (or came/come configured) in such a
state that IPv6 only half works: you can bind on [::]:n but not
connect back to it.

This implements a fallback such that it's guaranteed that this pattern
works:

   ln, err := Listen("tcp", ":0")
   ...
   addr := ln.Addr().String() // "[::]:n"
   c, err := Dial("tcp", addr)

... which is also now tested. It will first try to dial "[::]:n", as
before, but if that dial fails, it will also try "0.0.0.0:n".

Fixes #18806 (contains more details)
Fixes #20611 (I was going to fix nacl later, but it was easy enough)

Change-Id: I1107eb197e902ae8185c781ad1bc4e2bc61d1f4c
Reviewed-on: https://go-review.googlesource.com/45088Reviewed-by: Paul Marks <pmarks@google.com>

src/net/dial_test.go

@@ -887,3 +887,21 @@ func TestCancelAfterDial(t *testing.T) {
 		try()
 	}
 }
+
+// Issue 18806: it should always be possible to net.Dial a
+// net.Listener().Addr().String when the listen address was ":n", even
+// if the machine has halfway configured IPv6 such that it can bind on
+// "::" not connect back to that same address.
+func TestDialListenerAddr(t *testing.T) {
+	ln, err := Listen("tcp", ":0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer ln.Close()
+	addr := ln.Addr().String()
+	c, err := Dial("tcp", addr)
+	if err != nil {
+		t.Fatalf("for addr %q, dial error: %v", addr, err)
+	}
+	c.Close()
+}

src/net/ipsock.go

@@ -254,6 +254,13 @@ func (r *Resolver) internetAddrList(ctx context.Context, net, addr string) (addr
 		ips = []IPAddr{{IP: ip}}
 	} else if ip, zone := parseIPv6(host, true); ip != nil {
 		ips = []IPAddr{{IP: ip, Zone: zone}}
+		// Issue 18806: if the machine has halfway configured
+		// IPv6 such that it can bind on "::" (IPv6unspecified)
+		// but not connect back to that same address, fall
+		// back to dialing 0.0.0.0.
+		if ip.Equal(IPv6unspecified) {
+			ips = append(ips, IPAddr{IP: IPv4zero})
+		}
 	} else {
 		// Try as a DNS name.
 		ips, err = r.LookupIPAddr(ctx, host)

src/syscall/net_nacl.go

@@ -177,6 +177,11 @@ func (sa *SockaddrInet4) copy() Sockaddr {
 
 func (sa *SockaddrInet4) key() interface{} { return *sa }
 
+func isIPv4Localhost(sa Sockaddr) bool {
+	sa4, ok := sa.(*SockaddrInet4)
+	return ok && sa4.Addr == [4]byte{127, 0, 0, 1}
+}
+
 type SockaddrInet6 struct {
 	Port   int
 	ZoneId uint32
@@ -601,6 +606,14 @@ func (f *netFile) connect(sa Sockaddr) error {
 		return EISCONN
 	}
 	l, ok := net.listener[netAddr{f.proto, f.sotype, sa.key()}]
+	if !ok {
+		// If we're dialing 127.0.0.1 but found nothing, try
+		// 0.0.0.0 also. (Issue 20611)
+		if isIPv4Localhost(sa) {
+			sa = &SockaddrInet4{Port: sa.(*SockaddrInet4).Port}
+			l, ok = net.listener[netAddr{f.proto, f.sotype, sa.key()}]
+		}
+	}
 	if !ok || l.listenerClosed() {
 		net.Unlock()
 		return ECONNREFUSED