perf(session): optimise ProducerBuffer — swap field roles

[Tehsmash]

Description

ProducerBuffer is the fixed-capacity ring buffer used in the reliable-mode hot path to store messages for retransmission. The previous layout kept full Message objects in a Vec<Option<Message>> ring and message IDs in a HashMap<usize, usize> index.

The bottleneck in push() was the eviction step: to evict the oldest slot we had to read buffer[self.next] — a large protobuf struct unlikely to be cache-warm — just to extract its .get_id().

This PR swaps the field roles:

// Before
buffer: Vec<Option<Message>>   — ring slots (large structs)
map:    HashMap<usize, usize>  — message_id → slot index

// After
messages: HashMap<usize, Message>  — message_id → Message
ring:     Vec<usize>               — compact ring of IDs (usize::MAX sentinel = empty)

The ring Vec is 512 × 8 B = 4 KB — fits entirely in L1. Eviction now reads a single integer instead of dereferencing a large struct.

Per-operation impact:

Operation	Before	After
push eviction	Access Vec<Option<Message>>[next] → .get_id()	Read Vec<usize>[next] — integer, L1-hot
get	HashMap → Vec index → clone	HashMap → clone (one fewer indirection)
clear	Reallocates vec![None; capacity]	HashMap::clear() only, no allocation
iter	Vec scan with Option unwrap	Ring-ordered walk (cold flush path only)

iter() is only called on the cold flush-on-connect path so its slightly more complex traversal has no hot-path impact.

All call sites in session_sender.rs use the same public API (push, get, iter, clear) — no changes needed outside producer_buffer.rs.

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Other (please describe)

Performance optimisation — no behaviour change.

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

[github-actions]

The latest Buf updates on your PR. Results from workflow ci-buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jun 9, 2026, 9:59 AM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[micpapal]

LGTM, with minor changes

[micpapal]

Here we can drop the return value as we always return true