urus/ROADMAP.md

# urus — Roadmap

> Companion to `urus-spec.md` (artefact store, URUS project). v1 shipped
> 2026-05-26, built against pre-v0.4 smarm — before cancellation, supervisors,
> links, gen_server, registry, select, and the slot-slab runtime existed.
> smarm is now at v0.8. This roadmap covers catching up, then extending.

## State assessment (2026-06-11)

Verified against smarm master (`393cdd0`: v0.8 slot slab + park-epoch +
select, plus RFC 008 phase 1 fd arms / timed fd waits):

- **Builds clean, 24/24 tests pass.** The scheduler surface urus consumes
  (`spawn`, `wait_readable`, `wait_writable`, `sleep`, `init`, `Config`,
  `run`, `JoinHandle`) survived the v0.5–v0.8 runtime rework unchanged.
- **Timeouts are decorative.** `Config::{request_timeout, keep_alive_timeout}`
  are threaded into `ConnLimits` and never read. `read_some` parks in
  `wait_readable` with no deadline → an idle keep-alive connection pins its
  actor (and its 64 KiB+ of buffers) forever. Slowloris-shaped.
- **No supervision.** Spec §2.1 calls for a supervised listener pool;
  v1 predates `Strategy`/`ChildSpec` and uses bare `spawn` + `join`.
  A panicking listener silently shrinks the accept pool.
- **No graceful shutdown.** Spec deferred it; smarm now has the pieces:
  `request_stop` + sentinel unwind, and the 06-10 io fix (*deregister a
  stopped actor's fd wait on the unwind path*) makes stopping an actor
  parked in `wait_readable` safe. The `OwnedFd`-drops-on-unwind discipline
  in `conn_actor.rs` already composes with this.
- **No streaming response path.** `RespBody` is `Empty | Bytes`. Blocks SSE
  (a spec v1 goal that slipped) and chunked responses.
- **Full-duplex constraint (matters for WebSocket).** smarm's epoll model is
  one waiter per RawFd (`AlreadyExists`, now surfaced as `Err` via the
  `try_select` twins) and one direction per parked actor. RFC 008 phase 1
  (landed `393cdd0`) gives first-of(fd, channel) via fd arms in select —
  the duplex options in v0.4-3 are both open now. Decide in v0.4 design,
  before the frame codec.

---

## v0.2 — OTP refresh ✅ (landed 2026-06, chunks 1-4 / commits 5fe6969..HEAD)

Adopt the primitives urus predates. No protocol surface changes; `Plug`/`Conn`
untouched. Chunks are independently landable, in order:

### 1. Supervised listener pool ✅
Landed: `serve_with` runs a `Strategy::OneForOne` supervisor on the root
actor, one `ChildSpec` per dup'd listener fd, `Restart::Permanent`. A
listener panic (or a transient `wait_readable` failure, which now exits
and restarts instead of being fatal) restarts that listener; the pool
never silently shrinks. Connection actors stay unsupervised bare spawns
(per spec: a connection is cheap and its failure is local — a 500 path,
not a restart path); `spawn` parents them under the listener, which has no
supervisor channel, so their deaths are invisible to the pool supervisor
by construction.

Open question resolved as leaned: the ChildSpec factory owns the fd via
`Arc<OwnedFd>` (`OwnedFd` gained `Sync` for this) and every (re)start
reuses it. No re-dup, no window where a pool slot has no fd. Covered by
`tests/integration.rs::panicking_listener_restarts` (pool of 1, panic
injected before `accept` so the pending connection must survive into the
restarted listener).

### 2. Graceful shutdown ✅
Landed (c658ac0): `serve_with_shutdown(config, pipeline, signal)` +
`shutdown_handle()`. Drain-then-stop as decided below; design details in
the commit body. `serve_with` keeps v1 run-forever semantics by dropping
its own Handle.
`serve_with` gains a shutdown signal (an `urus::Handle` with `.shutdown()`,
backed by a channel + `request_stop` fan-out):
1. Stop listeners (no new accepts; listener fds close).
2. **Decided: drain-then-stop with a deadline** — in-flight requests get
   to finish; `request_stop` after the deadline (unwind closes fds cleanly
   via `OwnedFd::drop`). Drain also keeps the door open for an
   apache-reload-style restart later.
3. Supervisor winds down; `rt.run` returns; `serve_with` returns.

Requires tracking live connection pids. Cheapest honest version: a
`gen_server` connection registry — listener casts `{Started, pid}`, conn
actor monitors-free self-deregisters via a drop guard cast. Pid set only
(no activity stamps — chunk 3 went per-conn timed waits, no reaper); still
the future ws/channels introspection point, so build it once here.

### 3. Enforce the timeouts we advertise ✅
Landed (a60687b): timed waits as decided below; the leaning held — a
per-request `Instant` deadline keeps `request_timeout` an honest
whole-request wall clock (mid-head expiry: best-effort non-parking 408;
mid-body: close). Details in the commit body.
**Decided: smarm-native timed waits** (the fork's option (a), a reaper in
urus, died when RFC 008 landed at `393cdd0` — `wait_readable_timeout` /
`wait_writable_timeout` exist upstream now, so there is nothing for the
reaper to be the stopgap for):

`read_some` parks in `wait_readable_timeout` instead of `wait_readable`;
`Ok(false)` means the deadline passed and the conn actor closes. Per-conn
semantics, no activity stamping, no sweep ticker. Semantics note: a timed
read wait enforces *idle-between-reads* — `keep_alive_timeout` between
requests, and slowloris dies because each stalled read has a deadline.
`request_timeout` as a whole-request wall clock is NOT what this gives;
decide in implementation whether to track a per-request deadline across
reads (cheap: one `Instant` in the conn loop, pass the min of both
budgets to each wait) or redefine `request_timeout` as per-read. Leaning
the `Instant`: it keeps the advertised config honest.

urus is the first real consumer of the timed wrappers — note the RFC 008
reviewer flag (`wait_fd` wraps in `NoPreempt`, select arms don't); the
crud example under load doubles as that confirmation.

### 4. Hygiene ✅
Landed: registry names + whereis test (in-runtime probe via a handler —
tests are foreign threads), README rewrite, smarm-trace compile verified.
- Registry names: `urus.server`, `urus.listener.{n}` via `smarm::register`
  (debuggability; `whereis` in tests).
- README: rewrite Roadmap section to point here; document `Handle`.
- `smarm-trace` feature passthrough already exists — verify it still
  compiles under `--features smarm-trace`.

**Exit criteria:** kill -TERM equivalent drains cleanly in the crud example;
an idle keep-alive conn is reaped at `keep_alive_timeout`; a deliberately
panicking listener restarts under load without dropped accepts (test each).

---

## v0.3 — Streaming bodies + SSE ✅ (landed 2026-06, chunks 1-3)

The slipped v1 goal, and the WebSocket prerequisite (an upgraded connection
is "a response that never ends").

1. `RespBody::Stream(StreamBody)` ✅ — **decided: pull** (user call at the
   v0.3 design fork): the handler hands back a `smarm::Receiver<Vec<u8>>`
   and the producer is an actor the handler spawned; the conn actor pumps
   (`pump_stream`) and keeps sole ownership of the socket and write
   deadlines. The recv() park between chunks is stoppable by the draining
   registry, so infinite streams die at the drain deadline with no
   polling. HTTP/1.1: chunked TE, keep-alive after the terminator;
   HTTP/1.0: raw bytes, forced close, EOF-delimited. End of stream =
   every Sender dropped = 0-chunk. Timeout split (handoff Q3, as
   proposed): `request_timeout` covers the READ phase only; new
   `Config.write_timeout` (default 30s) bounds every response write —
   fresh budget per streamed chunk, and the fixed-body + error-response
   writes went deadline-bounded too (write-side slowloris closed).
2. Chunked **request** bodies ✅ — `read_chunked_body` decodes
   incrementally on the request deadline; extensions ignored, trailers
   consumed + discarded, decoded size capped at max_body_bytes (413),
   size-line/trailer spam capped (400). CL + TE: chunked, or chunked on
   HTTP/1.0, rejected 400 at parse (smuggling ambiguity). Keep-alive
   drain accounts RAW framing length, so pipelined requests behind a
   chunked body land exactly.
3. SSE sugar ✅ — `Conn::sse()` / `sse_with_heartbeat` → `EventSender`
   (`send(event, data)` / `data` / `comment`, Clone, `Err(SseClosed)`
   once the stream is gone). Heartbeat lives in the pump as leaned: with
   `StreamBody.heartbeat` set, the chunk wait is `recv_timeout(interval)`
   and expiry writes a `: keep-alive` comment chunk. Liveness from the
   ping (dead client = heartbeat write stalls past write_timeout), not
   from any request clock.

**Exit criteria (verified):** chunked round-trips both directions with
keep-alive and pipelining intact; an infinite SSE stream heartbeats while
silent, is killed by write_timeout when the client stops reading, and is
force-stopped at the drain deadline on shutdown; full suite green.

---

## v0.4 — WebSocket upgrade path

1. **Handshake**: detect `Connection: Upgrade` + `Upgrade: websocket` in
   the pipeline; `Conn::upgrade(handler)` short-circuits — conn actor
   validates `Sec-WebSocket-Key`, emits 101, exits the HTTP loop.
   (SHA-1 + base64 needed; vendor a tiny SHA-1 rather than pulling a
   crate — urus has two deps and it should stay that way.)
2. **Frame codec**: RFC 6455 framing — fragmentation, masking (client
   frames are always masked), control frames (ping/pong/close), 125-byte
   control-payload rule, close-code handshake.
3. **Duplex topology** (the design decision from the assessment):
   - **(a) Two actors, dup'd fd**: reader actor on the dup, writer actor
     on the original, writer owns a `Receiver<Frame>`; the user handler
     talks to both via channels. Proven trick (listener pool), works today.
   - **(b) One actor, smarm first-of(fd-readable, channel)**: fd arms in
     select landed with RFC 008 (`393cdd0`) — no longer blocked. Simpler
     topology, no dup; use the fallible `try_select` twins (registration
     errors, incl. `AlreadyExists`, surface as `Err` and the wait fully
     retires).
   Was "default (a), revisit if (b) lands" — (b) landed, so this is a
   live choice for the v0.4 design pass. Note RFC 008 is phase 1:
   single-waiter-per-fd stands, `dup` remains the documented answer for
   true simultaneous read+write waits.
4. Handler API sketch (settle in design pass): trait with
   `on_message/on_close` + a `WsSender` handle — gen_server-shaped, so a
   ws connection *is* an actor you can monitor, link, and register.

## v0.5 — PubSub

`urus::pubsub`, phoenix_pubsub-shaped, local node only (no distribution —
smarm has none):

- `subscribe(topic)`, `unsubscribe(topic)`, `broadcast(topic, msg)`,
  `broadcast_from(self, topic, msg)`.
- Topic table: gen_server owning `HashMap<Topic, Vec<(Pid, Sender<Arc<M>>)>>`;
  shard N ways by topic hash if the single server measures hot (don't
  pre-shard; bench first — smarm channel sends are cheap).
- Dead subscriber cleanup via `monitor` + `handle_down` (this is exactly
  what 06-10's gen_server `handle_down` was built for).
- `Arc<M>` payloads: one allocation per broadcast, not per subscriber —
  the shared-heap advantage over BEAM, take it.

Deliberately independent of HTTP — usable from any smarm app. Separate
module now, candidate for crate extraction later.

## v0.6 — Channels

Phoenix channels: the join/leave/event protocol over WebSocket transport,
PubSub underneath.

- `Channel` trait: `join(topic, payload, socket)`, `handle_in(event,
  payload, socket)`, `handle_out`, `terminate`.
- Topic router (`"room:*"` patterns) → channel actor per (conn, topic),
  spawned under the ws connection, linked so conn death reaps channels.
- Wire format: Phoenix V2 JSON serializer
  (`[join_ref, ref, topic, event, payload]`) — free interop with
  phoenix.js clients is the killer feature; needs a JSON dep or a
  hand-rolled mini-codec (decision point — this would be dep #3).
- Presence: explicitly out of scope until distribution exists somewhere.

---

## Later / icebox

- **HTTP/2** — spec §2.2 already designs the stream-actor demux (zero-copy
  ownership transfer, conn actor owns HPACK + flow control, stream actors
  run the pipeline). Big: HPACK, flow control, priorities. The Stressgrid
  constraint (no stream-actor cost on HTTP/1.1) stays law.
- **SSE ergonomics round 2** — auto-reconnect support (`Last-Event-ID`),
  event-id bookkeeping, backpressure policy on slow consumers.
- **TLS** — still deferred; acceptor design still must not preclude it.
- **Middleware ecosystem** — compression, static files, auth plugs.
- **Bench suite** — `urus-bench-spec.md` exists in the artefact store;
  wire it up once v0.2 lands (supervision changes the hot path not at all,
  but prove it).