Prevent Clients from being Buffered in zebra-network

[teor2345]

Is your feature request related to a problem? Please describe.

PeerSet calls poll_ready multiple times on its (possibly wrapped) Client services. This polling is part of its design.

However, if these services are ever Buffered, these polls would fill up their Buffer reservations, causing hangs.

Describe the solution you'd like

In peer::peer_set::set::PeerSet::<impl Service>

• document the constraints on the PeerSet's D::Service type
• prevent Buffering of Clients at compile-time or runtime, if possible:
  • assert!(!format!("{:?}", service).contains("Buffer")) in PeerSet::poll_ready?

Describe alternatives you've considered

Do nothing: future changes could cause hangs.

Additional context

See #1593 for background.

[teor2345]

The tower crate was fixed a while ago (tower-rs/tower#476 and tower-rs/tower#480). Both these fixes are before Zebra's pinned git hash of tower.

But we didn't know these bugs were fixed until yesterday, because the tower::Buffer comments were outdated, and the tower::Buffer tests were incomplete.

Since tower is already fixed, this ticket is obsolete.