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