# smarm — task.md (next steps) Handoff for a future session reusing this sandbox. Read top to bottom once before starting; the gotchas section is hard-won and will save you a faceplant. ## Resume the environment - Repo: `/home/claude/work/smarm`, branch `arm-port`, currently 6 commits past `master`: - `a8fec02` aarch64 context-switch port - `183fd05` monitors (`monitor()` → `Receiver`) - `f617266` one-for-one supervisor (restart policies + intensity cap) - `a8ddb4a` cooperative cancellation (roadmap #1 — done) - `e80334b` fix: orphaned timers no longer block runtime shutdown - `351dc9c` one_for_all / rest_for_one + ordered shutdown (roadmap #2 — done) - Toolchain is installed but NOT on PATH in a fresh shell. First line of every session: `. "$HOME/.cargo/env"` (rustc/cargo 1.96). - Build `cargo build` · all tests `cargo test` · one suite `cargo test --test monitor`. - Bench probe `cargo bench --bench general` (custom print-only harness; compiles tokio in release the first time — slow — but `target/` persists across git checkouts so it's paid once). - Perf regression check: `git checkout ` → `cargo bench --bench general | tee before.txt` → `git checkout arm-port` → run again → diff the `smarm 1-thread` medians for `chained_spawn` and `yield_many` (those exercise spawn/finalize/scheduler). Numbers are noisy on this shared CPU; treat as "regression beyond noise?" not a precise delta. ## Roadmap (dependency order) ### 1. Cooperative cancellation — the keystone ✅ DONE (`a8ddb4a`) Everything below (one_for_all/rest_for_one, links) needs a *safe* way to stop a running peer. Forcible teardown of another green thread's stack is unsound here (shared heap + Drop). So: cooperative stop the actor observes and unwinds itself. Shipped as designed (sentinel unwind, not Result-threading). Notes for what came next / future readers: - Stop flag lives on `Actor` behind `Arc` (fresh per spawn), NOT a `Slot` field — sidesteps the three-place reset, at the cost of one small alloc per spawn. The scheduler hands the resume path a raw `*const AtomicBool` (no per-resume refcount traffic); `yield_many` bench stayed at baseline, `chained_spawn` ~+6% from that alloc (left as-is; move to a `Slot` field if it ever matters). - Observation points: amortised `maybe_preempt`/`check!()` path + the wakeup side of `park_current`/`yield_now`. Sentinel = `StopSentinel` (zero-size), recognised in the trampoline → `Outcome::Stopped`. `join()` on a stopped actor returns `Ok(())` (no payload to propagate; reason is on the monitor channel). - Documented gaps confirmed by tests: no-observation-point loop can't be stopped (same as preemption); a user `catch_unwind` can swallow the sentinel but the flag stays set so the next yield re-raises. Original plan, for reference: - Add a per-actor stop flag (Slot field + atomic, or check via shared state). - `request_stop(pid)`: set the flag, unpark if parked. - Realize the stop as a **controlled unwind**: when the scheduler resumes a stop-requested actor, inject a sentinel panic (dedicated payload type) so the existing `trampoline` `catch_unwind` tears the stack down and runs Drop. The trampoline recognizes the sentinel and reports a new `Outcome::Stopped` (distinct from a user `Panic`). This avoids changing every blocking-op signature. - Alternative considered: thread `Result<_, Cancelled>` through recv/sleep/ lock/io. Rejected — large API churn. Go with the sentinel unwind. - Caveat to document: user code with its own `catch_unwind` can swallow the sentinel (cf. Erlang `catch`); re-check the flag at the next yield and/or re-raise. And a tight no-alloc loop without `check!()` can't be stopped — same inherent limitation as preemption. - Observation points: `maybe_preempt()`/`check!()` (cheap flag check) and the blocking parks (recv/sleep/mutex/io) on the stop-driven unpark. - Tests: looping actor on `check!()` gets stopped → `Outcome::Stopped`, Drop guards ran; parked-on-recv actor gets stopped; no-check loop documents the gap. ### 2. one_for_all / rest_for_one + ordered shutdown ✅ DONE (`351dc9c`) Shipped. What landed vs the plan: - `Strategy::{OneForOne,OneForAll,RestForOne}` selected via `.strategy()`, default `OneForOne`. The struct keeps the `OneForOne` name (compat; existing tests untouched) — a rename to `Supervisor` is a deferred refactor. - The *triggering* child's `Restart` policy decides whether anything restarts; the strategy decides which live siblings are cycled (all / index-> after the failed one). Survivors are `request_stop`'d in reverse start order, awaited on the existing `supervisor_channel` funnel (no new channel, no `select`), restarted in start order. One failure = one intensity tick regardless of group size. Out-of-band signals during an await are stashed and replayed. - `Signal::Stopped(pid)` + `DownReason::Stopped` added (kept distinct from Exit, as planned). A `Stopped` signal counts as abnormal for the restart decision. - Ordered shutdown: on cap-trip / mailbox-close-with-survivors, stop remaining children in reverse start order and await them (no-op on the normal exit). - ⚠️ Surfaced + fixed a latent keystone bug (`e80334b`): a cancelled sleeping/timeout actor orphans its timer entry, and the scheduler's shutdown check counted pending timers → `run()` hung until the dead actor's deadline fired (a `sleep(30s)` sleeper hung shutdown 30s). Fix: timers no longer gate shutdown (`live == 0` already implies nothing a timer could wake); heap is cleared on exit. Independent of the supervisor work. - Tests: all-restart (sibling cycled despite clean exit), suffix-restart (prefix child left alone), reverse-order teardown. Original plan, for reference: - one_for_all: on any child failure, `request_stop` all siblings, await their termination signals, restart all per spec. - rest_for_one: stop+restart the failed child and those started after it. - Supervisor shutdown: stop children in reverse start order. - Decide signal surface: add `Signal::Stopped(pid)` + `DownReason::Stopped` rather than folding into Exit (clearer for the supervisor's await logic). - Tests: all-restart, suffix-restart, reverse-order shutdown. ### 3. Links + trap_exit ← NEXT - `Slot.links: Vec` (bidirectional); `link`/`unlink`; per-actor `trap_exit` flag. - On finalize, for each linked peer: abnormal death → `request_stop(peer)` unless peer traps, in which case deliver an exit *message* instead. Normal exit does NOT propagate. Linking an already-dead pid delivers an immediate exit signal. - Open decision: which channel a trapping actor reads exit messages from (reuse a Down-style channel vs a dedicated trap inbox) — ties into the no-unified-mailbox constraint below. (`Signal::Stopped` + the reverse-order stop helper from #2 are now available to build on.) - Tests: linked pair one panics → other stopped; with trap_exit → other gets a message and survives; normal exit doesn't propagate. ### 4. Selective receive (independent track) Channels are strict FIFO MPSC; there's no pattern-matched `receive`. - Add `Receiver::recv_match(pred) -> T`: scan the queued `VecDeque`, remove+return first match, leave the rest in order; park and re-scan on new arrivals. - This changes channel wakeup: a parked selective receiver must wake on ANY send (not just empty→nonempty) and re-scan. Touches `channel.rs` carefully — the stress tests guard lost-wakeup invariants; keep them green. - Tests: messages arrive out of interest-order; match pulled first; non-matches remain for a later `recv`. ### 5. Grab-bag (each its own small commit) - `demonitor`: needs a per-monitor id to remove a specific sender. Decide the monitor API NOW before more code depends on it — likely return a `Monitor { id, rx }` instead of a bare `Receiver`. - Named registry: `register(name,pid)`/`whereis`/`send_by_name`; a `HashMap` in `SharedState`. - gen_server-style call/cast: request-reply correlation as a thin layer over channels (`call` sends `{req, reply_tx}`, awaits `reply_rx`); no runtime change. - Docs lag: README still says "x86-64 Linux only — ARM64 … deferred" and the module table calls `context` x86-64-only; update for the aarch64 backend. ## Gotchas / invariants (respect these) - **Shared mutex is non-reentrant.** `Sender::send` can call `unpark` → `with_shared`. NEVER send on a channel while holding the shared lock. Pattern: `mem::take` the senders/data under the lock, send after releasing. See `finalize_actor` (supervisor signal + monitor Downs both sent post-lock). - **`finalize_actor` order:** take stack/waiters/monitors under lock + set Done/outcome → recycle stack (post-lock) → deliver supervisor Signal + monitor Downs (post-lock) → unpark joiners → reclaim slot iff `outstanding_handles==0`. Death notifications always precede reclamation, so a pid carried in a Signal/Down is still matchable even as its slot is about to be reused. - **Slot lifecycle is reset in THREE places** — `Slot::vacant()`, `reclaim_slot()` (runtime.rs), and the slot-init block in `spawn_under` (scheduler.rs). Any new Slot field must be reset in all three (monitors was). - **Pid = (index, generation);** stale handles caught by generation mismatch in `slot()/slot_mut()`. The monitor `NoProc` path relies on this. - **No `select`, no unified per-process mailbox.** Why the supervisor uses the single `supervisor_channel` funnel rather than N monitor channels, and why trap_exit/selective-receive delivery need an explicit channel decision. - **Cooperative-only**: preemption and (future) cancellation both depend on the actor reaching `check!()`/yield/alloc/blocking points. - `run()` is single-thread (`Config::exact(1)`); tests rely on deterministic single-thread ordering (parent runs until it parks). Multi-thread via `runtime::init(Config…)`. ## Workflow expectations (from the human) - TDD: write the failing test first, then implement. - Commit incrementally with conventional-commit messages; keep each commit a reviewable unit (they diff in their IDE and are the filter to the codebase). - Run the full suite before each commit; check perf when a change touches Slot/scheduler/spawn/finalize hot paths.