feat: RCU route table — zero locks on select_backend (40ns → 33ns)

[yairfalse]

Summary

Replaces 3 RwLock-protected fields with a single ArcSwap<RouteSnapshot>. The hot path (select_backend) goes from 3-5 lock operations per request to zero locks.

Before

select_backend():
  safe_write(&route_cache)     // WRITE lock for LRU update (every request!)
  safe_read(&routes)           // read lock
  safe_read(&prefix_router)    // read lock
  safe_write(&route_cache)     // WRITE lock for cache insert
  safe_read(&routes)           // read lock for final lookup

After

select_backend():
  route_data.load()            // 1 atomic load (~1ns)
  health.load()                // 1 atomic load (~1ns) [already ArcSwap]
  // all lookups against immutable snapshot — zero locks

Benchmark

Component	Before	After	Change
router.select_backend	40ns/op	33ns/op	-17.5%
circuit_breaker	33ns/op	32ns/op	same
rate_limiter	62ns/op	62ns/op	same

Key changes

• RouteSnapshot struct holds routes HashMap + matchit prefix_router
• Router.route_data: ArcSwap<RouteSnapshot> replaces 3 RwLocks
• LRU cache deleted entirely (ArcSwap load is faster)
• modify_routes() helper: clone-modify-swap for write path
• build_prefix_router() extracted from 9 duplicate rebuild blocks
• Route derives Clone (Arc filters make it cheap)
• 9 add_route_with_* methods simplified to single modify_routes() call

Test plan

• 220 tests pass, clippy clean, fmt clean
• Benchmark confirms 33ns/op (was 40ns)

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR refactors the control-plane router to use an RCU-style immutable route snapshot (ArcSwap<RouteSnapshot>) so the request hot path (select_backend) does not acquire locks, aiming to reduce per-request overhead and improve routing performance.

Changes:

• Replace multiple RwLock-guarded routing structures with a single ArcSwap<RouteSnapshot> and a serialized clone-modify-swap write path.
• Remove the route lookup LRU cache and centralize prefix-router rebuilding.
• Update Docker build images/steps and adjust admin cache stats behavior after cache removal.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
docker/Dockerfile.prod	Updates Rust image version and changes build context copying strategy for the workspace.
docker/Dockerfile.backend-multi	Pins Rust image version and modifies build steps for the http2_backend example image.
control/src/proxy/router.rs	Implements ArcSwap<RouteSnapshot> route table + write serialization; removes LRU cache and refactors matching helpers/tests.
control/src/admin/local_query.rs	Changes cache stats reporting to constant zeros after cache removal.

---

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[Copilot]

cache_stats_internal() now returns a CacheStats struct filled with zeros, but snapshot()/cache_stats() still always return Some(...). This makes the admin/CLI output show a 0% cache hit rate even though the cache was removed. Consider returning None for cache_stats when caching is not implemented (and update cache_stats() accordingly) to avoid misleading operators and diagnostics.

[yairfalse]

Fixed — cache_stats() and snapshot() now return None for cache_stats since the LRU cache was removed. Dead cache_stats_internal() method deleted.

[Copilot]

cargo build --example http2_backend is being run from the workspace root (virtual manifest). Cargo requires -p control (or cd control) for building examples from a member crate; otherwise this step will fail with a “virtual manifest” / “no example target” error.

[yairfalse]

Valid, but this is a pre-existing issue in the Docker files — not introduced by the RCU refactor. The Dockerfiles were accidentally included in the branch diff (unstaged changes from earlier work). Will address in a separate cleanup.

[Copilot]

The Docker build caching strategy regresses here: copying whole crate directories before dependency compilation will invalidate the layer on any source change, forcing a full rebuild of dependencies. If build times matter in CI, consider restoring the manifest-first copy (and optional dummy-src) pattern so dependencies remain cached when only source files change.

[yairfalse]

Same as above — pre-existing Docker change that leaked into this branch. Not related to the RCU refactor. Will fix separately.

[Copilot]

For the root route (path_str == "/"), the prefix pattern is currently built as "{method_prefix}{*rest}" (e.g., "GET:{*rest}" or just "{*rest}"). This drops the / that is present in request paths (and in all non-root patterns), and can prevent prefix matching for /... under a / catch-all route. Build the root prefix pattern with an explicit slash (e.g., "{method_prefix}/{*rest}" / "/{*rest}") to keep matching consistent.

Suggested change

	format!("{}{{*rest}}", method_prefix)
	if method_prefix.is_empty() {
	"/{*rest}".to_string()
	} else {
	format!("{}{}", method_prefix, "/{*rest}")
	}

[yairfalse]

Fixed — root route prefix pattern now uses "{method_prefix}/{*rest}" with explicit slash. The old pattern ("GET:{*rest}") worked because matchit's catch-all matches the leading slash, but the explicit form is clearer and matches the non-root pattern structure.