correctly format ip4-in-6 addresses

fixes part one of #77

multiaddr_test.go

@@ -451,6 +451,7 @@ func TestBinaryRepresentation(t *testing.T) {
 func TestRoundTrip(t *testing.T) {
 	for _, s := range []string{
 		"/unix/a/b/c/d",
+		"/ip6/::ffff:127.0.0.1/tcp/111",
 		"/ip4/127.0.0.1/tcp/123",
 		"/ip4/127.0.0.1/udp/123",
 		"/ip4/127.0.0.1/udp/123/ip6/::",

transcoders.go

@@ -46,8 +46,8 @@ func (t twrp) ValidateBytes(b []byte) error {
 	return t.validbyte(b)
 }
 
-var TranscoderIP4 = NewTranscoderFromFunctions(ip4StB, ipBtS, nil)
-var TranscoderIP6 = NewTranscoderFromFunctions(ip6StB, ipBtS, nil)
+var TranscoderIP4 = NewTranscoderFromFunctions(ip4StB, ip4BtS, nil)
+var TranscoderIP6 = NewTranscoderFromFunctions(ip6StB, ip6BtS, nil)
 var TranscoderIP6Zone = NewTranscoderFromFunctions(ip6zoneStB, ip6zoneBtS, ip6zoneVal)
 
 func ip4StB(s string) ([]byte, error) {
@@ -88,7 +88,16 @@ func ip6StB(s string) ([]byte, error) {
 	return i, nil
 }
 
-func ipBtS(b []byte) (string, error) {
+func ip6BtS(b []byte) (string, error) {
+	ip := net.IP(b)
+	if ip4 := ip.To4(); ip4 != nil {
+		// Go fails to prepend the `::ffff:` part.
+		return "::ffff:" + ip4.String(), nil
+	}
+	return ip.String(), nil
+}
+
+func ip4BtS(b []byte) (string, error) {
 	return net.IP(b).String(), nil
 }