Server-side Observe (RFC 7641, roadmap 2.1)

[cvik]

Summary

Server-side RFC 7641 Observe — the last Tier 2 feature. Handlers register observers, application pushes notifications from any thread.

API

// At startup:
const temp_rid = server.allocateResource().?;

// In handler (on Observe=0):
_ = req.observeResource(temp_rid);

// From any thread:
server.notify(temp_rid, coap.Response.ok("22.5"));

Implementation

• observe.zig: Pre-allocated ObserverRegistry with resource slots, per-resource observer lists, lock-free MPSC notification queue
• handler.zig: Request.observeResource(rid) and Request.removeObserver(rid)
• Server.zig: Config (max_observers, max_observe_resources), init/deinit, public notify()/allocateResource(), tick-loop drainNotifications(), RST observer eviction

Design

• Thread-safe notify() encodes NON notification with Observe sequence into MPSC queue
• Tick loop drains queue and sends to all registered observers
• Observers evicted on RST from client
• Zero overhead when no observers registered (early return on empty queue)

Test plan

• zig build test — all 148+ tests pass (8 new observe registry tests)
• Tier 2 now fully complete (2.1-2.4 all done)