client: enhance CNI address handling for backward compatibility

Add fallback logic in cniToAllocNet to populate the Address field with the IPv6 address when no IPv4 address is available. This change ensures compatibility with existing code that relies on the Address field for service registration, particularly in scenarios where only IPv6 addresses are present.

Author: kemko

client/allocrunner/networking_cni.go

@@ -507,6 +507,13 @@ func (c *cniNetworkConfigurator) cniToAllocNet(res *cni.Result) (*structs.AllocN
 
 	}
 
+	// Fallback: if no IPv4 address but we have IPv6, copy it to Address field
+	// for backward compatibility with code that only checks Address field
+	// (e.g. service registration with address_mode="alloc")
+	if netStatus.Address == "" && netStatus.AddressIPv6 != "" {
+		netStatus.Address = netStatus.AddressIPv6
+	}
+
 	// Use the first DNS results, if non-empty
 	if len(res.DNS) > 0 {
 		cniDNS := res.DNS[0]