Early rescan when all network connections closed

[willscott]

Partially address #34

[Stebalien]

What's the motivation for this change?

[willscott]

The real event of 'network change' that should reset autonat is likely to be an approximation - mostly because the underlying host won't always or reliably know that the network environment has changed.

Some options:

• We could poll net.InterfaceAddrs() to see if local addresses have changed from DHCP.
• We could try platform specific methods like netlink sockets to try to get notified of network changes as the OS understands them.

The heuristic here is that if your network gets disconnected or switched, your existing connections will all time out / close. If you find yourself with no active connections (and you expect to generally be maintaining at least some connections in a passive state) that's an indication that you probably should have less confidence in your previous externally visible address remaining the same.

[Stebalien]

Got it. My concern here is that, when we disconnect from all peers, we likely have no internet so this will likely fail. Then, when we start connecting to new peers, we won't take any additional actions.

The correct way to do this would be to listen for the EvtLocalAddressesUpdated event (defined in go-libp2p-core) from the libp2p event bus (host.EventBus()).

Unfortunately..., it doesn't look like we've hooked that up. However, I'd just go ahead and do that anyways (i.e., use events) then file an issue in go-libp2p about adding address change detection to the swarm host.

[Stebalien]

We could poll net.InterfaceAddrs() to see if local addresses have changed from DHCP.

We actually do this (effectively) in "BasicHost" (defined in go-libp2p).

[raulk]

The hooking up of the “local addrs updated” event in the host is here: libp2p/go-libp2p#747

[aarshkshah1992]

@willscott This PR can be closed as I think you plan to address this change by consuming the EvtLocalAddressesUpdated event here.