Make discovery server optional via Option<u16> port

[ivonnyssen]

Summary

Changes Server's discovery port from u16 to Option<u16>, allowing the discovery server to be disabled entirely by passing None. Also makes DEFAULT_DISCOVERY_PORT public so consumers can reference it.

Motivation

When running multiple Alpaca servers in integration tests, each server tries to bind the discovery UDP socket, causing port conflicts. Making the discovery server optional allows test servers to skip discovery binding entirely, avoiding these conflicts.

Changes

• src/server/mod.rs:
  • Server::discovery_port changed from u16 to Option<u16>
  • BoundServer::discovery changed from BoundDiscoveryServer to Option<BoundDiscoveryServer>
  • BoundServer::discovery_listen_addr() returns Option<SocketAddr>
  • BoundServer::start() restructured to handle both cases
• src/discovery.rs: DEFAULT_DISCOVERY_PORT changed from pub(crate) to pub with documentation

Test plan

• cargo clippy passes (full CI feature matrix verified)
• Server::new(...) with Some(port) behaves identically to before
• Server::new(...) with None skips discovery server entirely
• DEFAULT_DISCOVERY_PORT accessible from downstream crates

[RReverser]

@ivonnyssen Can you please mark PRs that are ready for review as such? Looks like there's a bunch now in draft mode, and Github requires them to be merged "ready for review" before I can approve :)

[RReverser]

When running multiple Alpaca servers in integration tests, each server tries to bind the discovery UDP socket, causing port conflicts.

Hm I thought I ran tests in parallel with no issues so far... What error are you getting? Would set_reuse_port help instead?

[ivonnyssen]

When running multiple Alpaca servers in integration tests, each server tries to bind the discovery UDP socket, causing port conflicts.

Hm I thought I ran tests in parallel with no issues so far... What error are you getting? Would set_reuse_port help instead?

The issues are on MacOS. There the set_reuse_port does not help, as the os is holding on to the port for a bit, making tests either very slow - wait for port to be re-usable - or forcing them to be completely sequential.

[Copilot]

Pull request overview

Updates the server configuration to allow disabling the UDP discovery responder by making the discovery port optional, primarily to avoid port-binding conflicts when running multiple servers (e.g., in integration tests).

Changes:

• Changed Server::discovery_port from u16 to Option<u16> with None disabling discovery.
• Updated BoundServer to hold Option<BoundDiscoveryServer> and return Option<SocketAddr> from discovery_listen_addr().
• Made DEFAULT_DISCOVERY_PORT public and documented it for downstream use.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/server/mod.rs	Makes discovery binding optional and threads Option through the bound server APIs and startup logic.
src/discovery.rs	Exposes DEFAULT_DISCOVERY_PORT publicly with rustdoc.

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