Limit the initial crawler demand for outbound peers, based on seed peer connections

[teor2345]

Motivation

When Zebra starts up, the crawler task tries to dial peerset_initial_target_size peers. This is in addition to the peerset_initial_target_size peers from the initial seed peer connection limit.

But we only want peerset_initial_target_size initial peers, not peerset_initial_target_size * 2 initial peers. In PR #2944, this extra demand gets dropped, but it's a small waste of CPU and RAM.

Suggested Design

Initialize the peer connection count demand_tx in peer_set::initialize based on the number of connections made in add_initial_peers.

Edge Cases

The initial peer limit counts connection attempts. But the connection limit is for open connections. So we have to count the right thing here.

We should make sure the calculation doesn't underflow and panic.

Related Work

These tickets might modify the same functions as this ticket:

• Security: Rate-limit initial seed peer connections #2326

[mpguerra]

Hey team! Please add your planning poker estimate with ZenHub @conradoplg @dconnolly @jvff @oxarbitrage @teor2345 @upbqdn

[teor2345]

The implementation in PR #2944 covers the security issue here.

So this is now a much smaller cleanup ticket, rather than a security ticket.