Security: Track the number of open connections in Zebra

[teor2345]

Motivation

To limit the number of connections in Zebra, we need to be able to track how many open connections there are.

Suggested Design

1. In zebra_network::init, create:

   • a variable that tracks the number of open connections
   • a mpsc channel that gets a signal when a Connection is dropped

2. Get the channel from zebra_network::init to each Connection

3. In Connection, send a signal on the channel when each connection is dropped.

Edge Cases

Zebra should check and increase the connection limit as early as possible, before it uses any resources for the connection.

Zebra should send the drop signal as late as possible, after it has dropped everything else that's used by the connection.

Rejected Alternatives

We could limit the number of AddressBook peers in the Responded state. But this doesn't work for inbound connections, because some PeerSet addresses aren't stored in the address book. (Including the null address, and addresses from peers that don't advertise the NODE service bit.)

We could limit the number of PeerSet inbound and outbound peers peers in the Responded state. But there could be a significant delay between PeerSet peers getting dropped, and the corresponding Connection getting dropped. (We can't be sure that the generic is an owned Connection. And the tasks and channels might be slow under load.)