Race during adding and updating same port in L3 agent's info can generate wrong radvd config file

[Slawek Kaplonski]

There is possibility that because of some race in processing adding/updating internal ports info in RouterInfo class, same port, with 2 different revisions and different subnets configured will be added to RouterInfo.internal_ports twice in RouterInfo._process_internal_ports method (https://github.com/openstack/neutron/blob/master/neutron/agent/l3/router_info.py#L544).
If ports have got IPv6 gateway configured and radvd daemon should be started for such router, this may lead to generate radvd config file with duplicate interfaces, like:

interface qr-29c030a8-26
{
   AdvSendAdvert on;
   MinRtrAdvInterval 30;
   MaxRtrAdvInterval 100;
   AdvLinkMTU 1500;
   AdvOtherConfigFlag on;

   prefix 2003:0:0:1::/64
   {
        AdvOnLink on;
        AdvAutonomous on;
   };

   prefix 2003::/64
   {
        AdvOnLink on;
        AdvAutonomous on;
   };
};interface qr-29c030a8-26
{
   AdvSendAdvert on;
   MinRtrAdvInterval 30;
   MaxRtrAdvInterval 100;
   AdvLinkMTU 1500;

   AdvOtherConfigFlag on;

   prefix 2003::/64
   {
        AdvOnLink on;
        AdvAutonomous on;
   };
};

In some cases this may lead to crash radvd daemon. See also https://bugzilla.redhat.com/show_bug.cgi?id=1630167 for more details.