Map selects to a typed custom enum in zebra-network

[teor2345]

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

In zebra_network, we use nested selects, then match on Either (or a nested Either<Either<_, _>, _>). This makes the code hard to read and maintain.

Describe the solution you'd like

Map Either to a custom typed enum, with variants named after the action taken for each of the selected futures.

In peer::connection::Connection::run:

• map Either<PeerMessage, Either<ResponseTimeout, CancelChannel>> to a custom enum with PeerMessage, ResponseTimeout, and CancelChannel variants
  • https://github.com/ZcashFoundation/zebra/blob/main/zebra-network/src/peer/connection.rs#L408
  • PR Refactor connection.rs to make fail_with errors impossible #1721 moves this code down a few lines

In peer_set::initialize::initialize:

• map Either<Either<DemandSignal, CrawlTimer>, Handshake> to a custom enum with DemandSignal, CrawlTimer, and Handshake variants

  • https://github.com/ZcashFoundation/zebra/blob/main/zebra-network/src/peer_set/initialize.rs#L310

• Search for any other uses of the select function or macro

• Discard unused fields while mapping to the enum, to simplify the match code

• Match on the new enums

Describe alternatives you've considered

Do nothing: the code will continue to be hard to read and modify.

We could create a generic Either3, but that would only be slightly easier to read than Either<Either<_, _>, _>.

[teor2345]

This is a useful refactor, but it's not needed.