# CHECKPOINT-FIBERS — Stream B1 (fibers + Io + M:1 scheduler) Companion to [PLAN-FIBERS.md](PLAN-FIBERS.md). Update after every step (one step at a time, per the cadence rule). New corpus category: `18xx` concurrency. ## Last completed step **B1.4c — REAL fd-readiness blocking via kqueue (macOS).** `library/modules/std/sched.sx` now lets a fiber park on a file descriptor and the run loop block on `kevent` until the kernel reports it ready. Reuses the existing verified `library/modules/std/net/kqueue.sx` bindings (`Kevent` (32 bytes), `kqueue`/`kevent`/`kq_apply`/`kq_wait` + the `EVFILT_READ`/`EV_ADD`/`EV_ENABLE`/`EV_ONESHOT` constants) rather than re-deriving the FFI — sched.sx imports it as `kqb`. Added to `Scheduler`: - `kq: i32` (LAZY — `-1` in `init`, opened by the first `block_on_fd`, so a pure-compute / virtual-timer scheduler never opens a kqueue fd; leaks one fd at exit once opened, same class as the documented spawn-env / go-Task leaks — no deinit yet); - `io_waiters: List(IoWaiter)` (`IoWaiter :: struct { fd: i32; fiber: *Fiber; }`, grown through `own_allocator` per the long-lived-container rule); - `block_on_fd(self, fd, want_read)` — lazily opens `kq`, arms a one-shot `EVFILT_READ` registration, records an `IoWaiter{fd, current}`, then `suspend_self()`. Guards a null `current` (loud abort, like `sleep`); `want_read=false` (write-readiness) is not wired yet → loud abort rather than silently arming a read filter. - Run-loop: after the ready queue drains, **Mode 1 (virtual time)** fires the earliest pending timer (takes precedence — a program uses `sleep` OR fds, documented non-unification limitation); **Mode 2 (real fd)** — if `io_waiters` is non-empty, BLOCK on `kq_wait(kq, evbuf, MAXEV=16, -1)` (null timeout), then for each fired event match `ev.ident` back to its waiter, evict it, and `wake` the fiber; **else** break. Orphan-deadlock check unchanged in spirit but now correct: an fd waiter is NOT an orphan (while `io_waiters.len > 0` the loop blocks on kqueue rather than reaching the check), and a genuine no-timer/no-fd suspend still aborts loudly (verified with a probe: exit 134). - `wake` now also evicts a pending fd-waiter (`cancel_io_waiter_for`, mirror of `cancel_timer_for`) — same UAF reasoning: a fiber woken by another path must not leave a stale `IoWaiter` pointing at a reaped `*Fiber`. The kqueue registration is `EV_ONESHOT` so we never `EV_DELETE` (a never-fired one-shot lingers harmlessly; the drain ignores an unmatched ident; closing the fd auto-removes it). - DE-RISK probe (run first, no scheduler): confirmed `size_of(Kevent) == 32`, the pipe roundtrip (`kq_wait` returned 1 with `out.ident == read_fd`, `out.filter == -1` (EVFILT_READ), `out.data == 1` byte readable) — the struct layout reads back the fd correctly. - Locked by `examples/concurrency/1816-concurrency-fiber-io-pipe.sx`: a `pipe`; a reader fiber spawned FIRST blocks on the empty read end, then a writer fiber writes `a b c` → the run loop blocks on kqueue, wakes the reader, which reads the 3 bytes. Output `log: wrote read 3 [97 98 99]` / `n_suspended: 0` (the "wrote" before "read" ordering proves the reader actually blocked then woke via kqueue readiness). `.build` `{ "target": "macos" }` (matches host arch → runs end-to-end; ir-only on a mismatch, like 1814/1815 — no `.ir` snapshot needed since it runs here). The example declares its own `read`/`write`/`close` externs with the CANONICAL signatures std already binds (`(i32,[*]u8,usize)->isize` / `(i32)->i32`) — a divergent re-binding is rejected by the extern dedupe. - **Adversarial review (worker) of the run-loop change — found 2 CRITICALs:** - **(1) two fibers on the SAME fd → lost wakeup + permanent hang.** macOS `EV_ADD` for an existing `(ident, filter)` REPLACES the registration (doesn't stack), so two waiters share one registration: the fd fires once, one wakes, the other is stranded in `io_waiters` and the next `kq_wait(-1)` blocks forever. FIXED: `block_on_fd` now enforces one-waiter-per-fd with a loud abort (the model already assumed it). Verified: dup-fd → `sched: block_on_fd: fd N already has a waiter`, not a hang. - **(2) an fd-waiter on a never-ready fd hangs instead of the timer path's loud abort.** Re-examined: this is CORRECT event-loop semantics — blocking on I/O until ready (possibly forever, like a server idling on a socket) is the point; the scheduler cannot know an fd will never become ready, so it must keep waiting. NOT a scheduler deadlock. Fixed the MISLEADING comment that implied the orphan check covers fd-waiters: it does not, by design (it covers only pure `suspend_self` parks). No code change — the "hang" is a caller-side logic issue (waiting on input that never arrives), not a bug to abort on. - Review CLEARED: the IoWaiter UAF parity (early-wake evicts the waiter; a lingering one-shot that later fires hits no match → clean no-op), ident width/sign, `kq_wait` EINTR/error handling, timer-vs-io precedence (timer wins; no hang). All probed safe. - Suite GREEN **754/0** (incl. the dup-fd guard, no new example needed — the abort is host-fragile to pin like 1809's guard-firing). Next: **B1.5** (end-to-end M:1 validation under the deterministic timers / fd readiness); a linux epoll twin of `block_on_fd` (mirror via `std/net/epoll`, the OS-neutral facade is `std.event`) is future work. ### Earlier — B1.4b — deterministic VIRTUAL-TIME timer scheduling (the KEYSTONE) — landed + adversarially reviewed (caught a CRITICAL UAF, fixed). `library/modules/std/sched.sx` gained a virtual clock + sleep timers so fibers schedule in reproducible simulated time (no real clock): `clock_ms` (advances ONLY as timers fire), a `timers: List(Timer)` (insertion-order, linear min-scan, FIFO tiebreak), `now_ms()`, `sleep(ms)` (arm `{clock_ms+ms, current}` + `suspend_self`), and a timer-driven `run` (drain ready → fire earliest timer → advance clock → wake → repeat; orphan-deadlock check preserved for a genuine no-timer suspend). Locked by `1814` (5 fibers sleep 30/10/20/15/15 → wake order B@10, D@15, E@15 (FIFO), C@20, A@30 — deadline order, not spawn order; `now_ms()` reads each virtual deadline; final clock 30). §8.1.3 calibration note in the header: the deterministic wake ORDER equals what real `sleep`s produce, reproducing blocking semantics' observable ordering without real time. The deterministic-sim `Io` is realized at the scheduler level (`sleep`/`now_ms`/timer-`run`), not as an erased `Io`-protocol impl (same erasure reason as FiberIo). - **Adversarial review (worker) of the run-loop change: found a CRITICAL use-after-free** — a fiber that armed a `sleep` timer but was woken EARLY by another path (a manual/`Task` `wake`) ran to completion + was reaped (stack `munmap`'d, `Fiber` freed) while its `Timer` still held a dangling `*Fiber`; a later fire would `wake` freed memory (silent-corruption: "passes" only because the freed slot coincidentally read `state != .suspended`). FIXED: `wake` now evicts the woken fiber's pending timer (`cancel_timer_for`) — every re-ready path funnels through `wake` (the timer-fire in `run` already removed the fired timer, so it's a harmless re-scan there), so no stale timer can outlive its fiber. Regression `1815-concurrency-fiber-timer-early-wake.sx` (early wake → `clock: 0`, the stale 100ms timer evicted, not fired). Review CLEARED: `n_suspended` accounting, orphan-deadlock false-positives, timer-list integrity (re-arm during fire), clock monotonicity, termination — all traced/probed safe. - Suite GREEN (count below). Next: **B1.4c** (event-loop `Io` — real fd readiness, kqueue/epoll). ### Earlier — B1.4a — a truly-SUSPENDING fiber-task async layer (`go`/`wait`/`cancel`) landed + adversarially reviewed; cleared two more compiler blockers en route. `library/modules/std/sched.sx` now carries `Task($R)` + `Scheduler.go(work) -> *Task($R)` + `wait`/`cancel` (a `ufcs` layer over the M:1 scheduler). `s.go(work)` runs the nullary thunk `work` as a REAL fiber; `t.wait()` SUSPENDS the caller until it completes (vs io.sx's blocking `context.io.async`, which runs inline). Locked by `examples/concurrency/1813-concurrency-fiber-async-suspend.sx`: two tasks interleave (A yields mid-body so B runs first → `1 2 3`), awaited values `42`/`100`, and a canceled task's `wait` raises `.Canceled` → `or -99` → `sequence: 1 2 3 42 100 -99`. - **Design: a NULLARY thunk, not `async(worker, ..args)`.** A comptime variadic pack can't cross a deferred (fiber) boundary — `..args` captured into a closure re-expands from the spawner's now-gone locals (issue 0156 Part 2). So `go` takes `work: Closure() -> $R`; the user captures inputs in the lambda at the call site (the `go func(){…}()` idiom). **Self-contained in sched.sx** (NOT io.sx): io.sx importing sched.sx duplicates the `_fib_tramp` global asm when a program also imports sched.sx directly (global asm emits per import-path) — so the Io-protocol `spawn_raw`/`suspend_raw`/`ready` hooks stay reserved for the future M:N model; M:1 uses `go`/`wait` directly. Heap `*Task` (must outlive `go`'s frame; leak documented). `TaskErr` is LOCAL (the `!` failable detection doesn't see through io.sx's `IoErr` re-export alias). - **Two compiler blockers hit + FIXED (user-authorized in-session):** - **issue 0156 Part 1** — a single-type generic `$R` (parsed as `comptime_pack_ref`) used as a type-arg (`Box($R)`, `size_of(Box($R))`) inside a pack-fn body hit a missing arm in `resolveTypeWithBindings` → `.unresolved` → LLVM panic. Fix: mirror `resolveTypeArg`'s `comptime_pack_ref` arm (look up `type_bindings`, else a loud diagnostic). Regression `examples/generics/0216-generics-typearg-in-pack-fn-body.sx`. (Part 2 — deferred `..` spread crashes — reframed OPEN/non-blocking, `issues/0156`.) - **issue 0157** — a user generic `ufcs` method whose name collides with a stdlib re-export (`cancel` on `*Task` vs io.sx's `cancel` on `*Future`) resolved via last-wins `fn_ast_map` with NO receiver filtering → wrong overload → `$R` unbound → LLVM panic. Fix (`src/ir/lower/call.zig` `selectUfcsGenericByReceiver`): every generic-ufcs dispatch enumerates ALL module authors (`module_decls`), keeps receiver-binding ones, picks the most receiver-SPECIFIC (concrete > bare `$T`), dedups re-exports, and flags a genuine 2-specific tie as a deterministic "ambiguous — qualify" diagnostic (never a silent order-dependent pick). Regression `examples/generics/0217-generics-ufcs-method-name-collides-stdlib.sx`. - **Adversarial review (worker) of the 0157 fix + Task layer.** Caught the determinism CRITICAL (fixed: always-run selection + specificity + ambiguity), `wait`-outside-a-fiber null-deref (fixed: loud guard in `suspend_self`/`yield_now`), and cancel-doesn't-skip-work (fixed: worker skips `work()` if already canceled). Lost-wakeup / cancel-after-complete / reap traced safe. Also simplified `1812` (`**Fiber` shared handle → a `Sh.parked` field; output identical). - Suite GREEN 751/0 (749 + 1813 + 0217). Next: **B1.4b** (deterministic-sim `Io`). ### Earlier — B1.5a — the M:1 cooperative fiber scheduler CORE — landed + adversarially reviewed The hand-bootstrapped ping-pong (1807-1810) is now a reusable scheduler API in pure sx: `library/modules/std/sched.sx` — a generic `Fiber` (`body: Closure() -> void`) + `Scheduler` with `init`/`spawn`/`yield_now`/`suspend_self`/`wake`/`run` over the proven `swap_context` on guarded `mmap` stacks. The ONE generic dispatch (`fib_dispatch`, reached from the `_fib_tramp` trampoline) runs ANY stored closure body on a fresh stack — replacing the fixed `bl _fib_body`. Reaping `munmap`s the stack + frees the heap `Fiber` on completion; an intrusive FIFO gives round-robin order. - **Foundational design de-risked by probe before building:** a fiber can store + call a `Closure() -> void` on its fresh stack via the generic dispatch; outputs flow OUT through pointers captured in the closure (capture-by-value does NOT write back — pushed onto the user). - **Hit + FIXED a blocker compiler bug — issue 0154** (user-authorized in-session fix). `null` / `---` assigned to a struct field picked up a leaked enclosing `target_type` (the function's RETURN type, set for the whole body at decl.zig:2691) and built a WHOLE-STRUCT-typed null → an oversized `zeroinitializer` store through the field's GEP that overran the field's slot and clobbered the saved x29/x30, so the fn `ret`'d to 0x0. This was EXACTLY the `Scheduler.init()` by-value-return shape (`sched_ctx: [13]u64` before `current: *Fiber`). Fix: added `.null_literal, .undef_literal` to the `needs_target` switch in `lowerAssignment` (`src/ir/lower/stmt.zig`) so the field's type is used. Repro → regression test `examples/types/0193-types-sret-array-before-pointer.sx`; `issues/0154-*.md` RESOLVED. - **Adversarial review (worker): asm/bootstrap/lifetime SOUND** (the headline closure-env-lifetime fear was disproven — envs are heap-promoted, survive the spawn frame). Found **1 CRITICAL** + robustness gaps, ALL hardened: (CRITICAL) `wake` re-enqueued an already-queued fiber → FIFO corruption/segfault → now GUARDED on `.suspended` (spurious/double/stale wake = safe no-op); orphan-suspend leak/deadlock → `n_suspended` accounting + a loud `run()`-drain diagnostic+abort; `mmap` `MAP_FAILED` (=-1, not null) / `mprotect` / Fiber-OOM → loud bails (per §8.1.1 the guard is mandatory); the per-fiber closure-env leak (sx exposes no env-free) → documented as a KNOWN LIMITATION (bounded by spawn count; invisible under the default GPA). - **Locked two `18xx` examples** (aarch64-macos `.build`-pinned, ir-only on a mismatch): `1811-concurrency-fiber-scheduler.sx` (3 fibers round-robin via `yield_now` → ordering contract `sequence: 0 1 2 0 1 2 0 1 2`, all `.done`) + `1812-concurrency-fiber-suspend-wake.sx` (park via `suspend_self`, resumed by another fiber's `wake`, + the spurious-wake no-op — the CRITICAL-fix regression → `log: 10 20 21 11` / `suspended-left: 0`). - **Filed issue 0155 (NON-blocking, NOT fixed)** — found incidentally in the review: indexing a scalar pointer (`pc[0]`, `pc: *i64`) panics codegen (`.unresolved` reaching LLVM emission). The scheduler uses array-field indexing + `.*`, never this, so it's filed for its own session. - Suite GREEN **748/0** (746 base + 1811 + 1812 + 0193 regression). Next: **B1.4a** (FiberIo — wire `Io.spawn_raw`/`suspend_raw`/`ready` onto the scheduler so `async`/`await` truly suspend). ### Earlier — B1.3b-1 — the x86_64 / Win64 `swap_context` sibling — VALIDATED on real hardware The context switch is now proven on a SECOND architecture + ABI. A Win64 `swap_context` saves the COMPLETE Win64 callee-saved set — 8 GP (rbx, rbp, rdi, rsi, r12-r15) + rsp **and xmm6-xmm15** (10 XMM, 128-bit via `movups` — Win64 has callee-saved XMM, unlike SysV/aarch64) — plus a Win64 `scribble_verify` (32-byte shadow + 16-align at each `call`, COFF symbols, rsp-carried return addr). Locked by `examples/1810-concurrency-fiber-switch-win64.sx` (pinned `x86_64-windows-gnu`, ir-only here): the 2-fiber mutual scribble printed **`0 0 P`** when built `--target x86_64-windows-gnu --self-contained` and **run on a Windows 7 x64 VM (UTM)** — every GP + XMM callee-saved survived. **Adversarially reviewed before the VM run** (worker emitted the real `.s` and verified every `call` alignment, the 264-byte frame offsets, the rsp/return-addr round-trip, swap ordering, and COFF naming against the Win64 ABI — no critical/minor bugs). The build→VM→run loop was set up this session (cross-build needs `--self-contained`; output via the Win32 `WriteFile` boundary, the 1660 pattern). Suite green. Note: this is the GOOD-swap-only mutual scribble (self-validating by construction; the in-process negative control was dropped to avoid an sx fn-ptr-convention rabbit hole — the detection of this exact logic was negative-controlled on aarch64 in 1808). The SysV/Linux x86_64 sibling (different reg set: no callee-saved XMM, args rdi/rsi) remains for a Linux x86_64 host. ### Earlier — B1.3b-2 — mmap guard-page stacks (commit `dd532ab`) Fiber stacks are `mmap`'d with a `PROT_NONE` GUARD PAGE at the low end (§8.1.1: a fixed stack without a guard silently corrupts neighbors on overflow). `mmap` the `[guard | usable]` region, `mprotect` the low 16KB page `PROT_NONE`; SP descends into the guard and faults loudly at the boundary instead of corrupting a neighbor. Locked by `examples/1809-concurrency-fiber-guard-stack.sx` (aarch64-macos-pinned): `guard armed: 1` (`mprotect`→0) + `sum: 20100` (a fiber runs real recursion on the guarded stack + yields). - **Guard FIRING validated** (manually, not corpus-pinned — a deliberate overflow crash is host-fragile): a fiber recursing past its 128KB stack faults with `Bus error` at the guard page (`region+GUARD`); the sx crash handler turns it into exit 134. Documented in the example header. - **x86_64 sibling:** was deferred here (couldn't run x86_64 on this arm64 host), then DONE as Win64 once a Windows 7 x64 VM became available — see B1.3b-1 above (`examples/1810`, `0 0 P`). ### Earlier — B1.3a-2 — the context-switch STRESS GATE (design §10.7) — DONE + adversarially reviewed The explicit every-callee-saved-register scribble that B1.3a-1 owed. `swap_context` now saves the COMPLETE AAPCS64 callee-saved set — integer x19-x28 + fp/lr + sp AND FP **d8-d15** (per §6.1.2 only the low 64 bits of v8-v15 are callee-saved, so `d8-d15` is exactly sufficient; x18 is Apple's reserved platform reg, untouched). A naked `scribble_verify(self_ctx, peer, base)` loads a unique sentinel into all 18 callee-saved regs, yields, and on resume counts the ones that didn't survive (honoring its own caller ABI via a 176-byte frame that saves+restores the caller's callee-saved; base reloaded from the frame post-swap; the original lr round-trips through the swap). The gate is a **2-fiber MUTUAL scribble** (A and B scribble DISTINCT sentinels into the same physical regs, so each survives only if `swap_context` saved+restored it — a lone fiber yielding to an idle peer would NOT exercise preservation). Locked by `examples/1808-concurrency-fiber-switch-stress.sx` (aarch64-pinned): `A mismatches: 0` / `B mismatches: 0`. - **Validity proven by NEGATIVE controls:** dropping the d8-d15 save/restore → 8/8 mismatches (exactly the FP regs); dropping x27/x28 → 2/2. The gate genuinely catches a broken switch. - **Adversarial review (worker, per the plan): no CRITICAL bugs.** Verified the callee-saved set is complete + correct, all frame offsets/16-alignment, the lr/sp dance, and swap read-ordering against AAPCS64. Applied its one recommendation: `boot` now zeroes the FP ctx slots [13..20] so a first switch-to loads 0 (not garbage) into d8-d15. Residual gaps it flagged (all spec-correct for a call-boundary swap, documented in the example header): NZCV/FPSR not swapped; **FPCR** (rounding mode — thread-global, bleeds across fibers if changed) and **TPIDR_EL0/TLS** (errno, allocator thread-caches — shared by same-thread fibers) not swapped; fp=0 bootstrap blocks unwind/signal walking past a fiber trampoline. These bite at the N×M:1 / signals stages, not the single-thread switch. - Suite green **734/0**, master clean. WIP probes: `.sx-tmp/scribble2.sx` (+ `_broken`/`_gp`). ### Earlier — B1.3a-1 — the foundational stackful context switch (commit `b234b7d`) Pure sx over `abi(.naked)`: naked `swap_context` (GP-only 13-slot save) + by-hand fiber bootstrap (SP = `alloc_bytes` stack top, LR = global-asm trampoline, x19 = `*Fiber`). Locked by `examples/1807-concurrency-fiber-context-switch.sx`: 2-fiber ping-pong (`rounds: 6` / `canary fails: 0`) + 64-frame deep recursion (`frames verified: 64` / `depth fails: 0`). Indirect register/stack survival; 1808 supersedes its switch with the complete GP+FP save area + the explicit gate. ### Earlier — B1.2 COMPLETE — the async surface works end-to-end All three surface blockers (0151, 0152, 0153) FIXED + committed; async examples landed + green. - **0151 fixed** (`362674f`): generic `$T` infers through generic-struct / pointer / UFCS-pack params. Regression `0214` + `0215`. - **0152 fixed** (`e5586f6`): `Atomic(bool)` load/store byte-promoted to `i8` in the codegen emitters. Regression `1705`. - **0153 fixed** (`68c1991`): `inferGenericReturnType` now pins return-type resolution to the fn's DEFINING module (mirroring `monomorphizeFunction`), so a re-exported value-failable's `!E` resolves to the real `.error_set` TypeId — the failable channel survives the re-export alias. Regression `1058-errors-reexport-value-failable-channel.sx`. - **Async examples landed:** `examples/1805-concurrency-io-blocking-async.sx` (`context.io.async((a,b)->i64 => a+b, 40, 2).await() or {…}` → `sum: 42` / `double: 42` / `clock ok`) + `examples/1806-concurrency-io-cancel.sx` (`f.cancel()` → `await` raises `.Canceled` → `or` default; `ok: 7` / `canceled: -99`). Both green, snapshots captured. ### Earlier — the three B1.2 surface fixes (committed) Generic `$T` inference, `Atomic(bool)` byte-promotion, and re-export failable-channel pin — details below. - **0151 fix (committed):** four gaps closed on the inference + UFCS-dispatch path — (1) `extractTypeParam`/`matchTypeParam(Static)` got a `parameterized_type_expr` arm (recover the arg instance's recorded per-param bindings via `struct_instance_bindings` + the template's ordered `type_params`, recurse positionally; this also fixes `*Box($T)` — it recurses into its `Box($T)` pointee); (2) the `pointer_type_expr` arm now falls through to match the pointee against a non-pointer arg (auto-address-of: a `*Box($T)` param accepts a by-value `Box($T)`, e.g. a UFCS receiver `b.m()`); (3) `ExprTyper.inferType` got a `.lambda` arm building the closure type from the lambda's annotations (the UFCS binder types args from the raw AST before they're lowered, so it can now bind `Closure(..) -> $R` from the worker's declared return type); (4) a pack UFCS target routes through the SAME `lowerPackFnCall` the direct call uses, with the receiver spliced in as `args[0]`. - Regression tests: `examples/0214-generics-ufcs-closure-return-pack.sx` (direct + UFCS closure-return pack) + `examples/0215-generics-infer-through-pointer.sx` (by-value / pointer / multi-param / nested / UFCS-auto-ref struct-head inference). Issue 0151 marked RESOLVED; repro moved into the suite. ### Earlier — B1.2 (Io capability) — LANDED + adversarially reviewed Commits `a1b14f0` (lock) + `45d869d` (Io capability) + `3eeb965` (issue 0151 lock). - **LANDED + review-confirmed correct** (commit 45d869d): `Io :: protocol #inline` (spawn_raw/suspend_raw/ready/poll/now_ms/arm_timer) + `io` field on `Context` (`{allocator; data; io}`, io LAST); BOTH `__sx_default_context` materializers (protocol.zig + comptime_vm.zig) build an identical CBlockingIo→Io vtable (review verified byte-for-byte agreement; `context.io.now_ms()` dispatches at runtime AND comptime); the `push Context.{…}` omitted-field-**inherits-ambient** fix (review: correct, right fix, no bad blast radius); `library/modules/std/io.sx` (`Future($R)`, `CBlockingIo`, `async`/`await`/`cancel`); the `!`-protocol-impl-lint suppression; 37 `.ir` regens (review: pure layout/type-table, no error text, zero .exit/.stdout/.stderr change). - **BLOCKED — async surface non-functional:** `await`/`cancel` take `*Future($R)` and are **uncallable in EVERY form** (not just UFCS) — sx can't infer a generic `$T` from a pointer-wrapped arg (`*Future($R)`). `async(...)` (create) works via explicit call and produces a correct `.ready` Future, but you can't `await` it. Root bug = **issue 0151 (WIDENED)**: infer `$T` from `*T`-wrapped params + closure-return-via-pack + UFCS dispatch. Minimal repro: `unbox :: (b: *Box($T)) -> $T` fails to infer `T`. - **No async example in the corpus** (1805 was removed because it needs the blocked surface) → the green suite does NOT cover async. Restore `1805` (async/await) + add `1806` (cancel) once 0151 is fixed. ### Earlier — B1.1 (per-fiber `context` root) — DONE. Zero compiler change (confirmed by probe). The fiber-spawn context convention works end-to-end with ordinary language features: - `snap := context` captures the spawner's `Context` as a value; - the snapshot is stored in a struct (the stand-in `Fiber`); - a trampoline running under a *different* ambient context installs the fiber's stored root with `push f.root { … }`, and the body reads the snapshot — not the trampoline's ambient context — because `context` is an implicit slot-0 `*Context` param (call-carried, rides the callee's own stack) and `push` allocates on the caller frame (no global, no TLS). - Locked by `examples/1804-concurrency-context-snapshot.sx`: prints `fiber root: 42` (the installed snapshot wins over ambient 99) + `ambient after: 99` (the `push` scope restores the ambient context on exit). No fiber runtime yet (that's B1.3) — this proves the plumbing it will build on. No `.build` pin (pure sx, host-independent). - **Probe result:** the design doc's "lower as swappable indirection, never raw TLS" guarded a non-problem — context was already param-carried, never TLS. No path re-reads `__sx_default_context` mid-stack, so there is **no compiler obligation** here. - `zig build && zig build test` green: **726 ran, 0 failed**. ### Earlier — B1.0 (`abi(.naked)` codegen) — complete Replaced the emit bail with real LLVM `naked` emission: - `emit_llvm` declaration pass: for `func.is_naked`, add the LLVM `naked` + `noinline` + `nounwind` attributes and **skip** the `frame-pointer=all` attribute (incompatible with a frameless function). Pass 2 now emits the `.naked` body normally — `naked` makes the backend emit it verbatim (the inline asm + its own `ret`) with no prologue/epilogue. - IR shape (verified): `; Function Attrs: naked noinline nounwind` / `define internal i64 @answer() #0 { entry: call void asm sideeffect "…ret…", ""() unreachable }` / `attributes #0 = { naked noinline nounwind }`. The caller invokes it as an ordinary `() -> i64` call (`.naked` is `call_conv == .default`). - `examples/1800-concurrency-naked-asm.sx` — now GREEN, aarch64-pinned (`.build {"target": "macos"}`): runs end-to-end → **exit 42** on this host, ir-only on a mismatch; `.ir` snapshot captured. - `examples/1801-concurrency-naked-generic.sx` (renamed from `-bail`) — the generic `.naked` now emits a correct naked `answer__i64` (exit 42), proving generic.zig produces a naked body, not a framed one. aarch64-pinned. - `examples/1802-concurrency-naked-asm-x86.sx` — x86_64 cross sibling (`.build {"target": "x86_64-linux"}`, ir-only here): `.ir` locks `naked` + `movl $42, %eax` / `ret`. - Unit test `emit: abi(.naked) function gets the naked attribute (no frame-pointer)` in `emit_llvm.test.zig` (asserts `naked` present, `frame-pointer` absent). - **B1.0c (review-hardening):** a param-bearing `.naked` fn emitted invalid LLVM (loud verifier error "cannot use argument of naked function") because the param-alloca loop wasn't gated. Fixed forward (this *enables* the B1.3 context-switch use case rather than rejecting it): gated the param-alloca loop on `fd.abi != .naked` in decl.zig (both paths) + generic.zig; a naked fn's args stay in registers (read by asm), declared-but-unused in LLVM. Locked by `examples/1803-concurrency-naked-asm-param.sx` (`add(a,b)` → x0+x1 → 42). - `zig build && zig build test` green: **725 ran, 0 failed** + unit tests. ### Earlier — B1.0a (lock + review hardening) Plumbed `Function.is_naked` (set from `fd.abi == .naked` at both decl sites + generic.zig + pack.zig); `funcWantsImplicitCtx` skips `.naked` (no synthetic ctx, like `.c`); all body-lowering paths bypass `lowerValueBody` for `.naked` (asm body + `unreachable` cap — no sx return); `emit_llvm` Pass 2 bailed loudly (since flipped to real emission). Adversarial review caught the generic/pack `is_naked` gap (a generic `.naked` silently shipped a framed body); closed + locked. The review's `.naked`-lambda CRITICAL was a false positive (unparseable — `isLambda` breaks on the `abi` keyword). ## Current state **B1.4c COMPLETE — real fd-readiness blocking via kqueue (macOS) exists.** `library/modules/std/sched.sx` now carries: the M:1 scheduler core (B1.5a: `spawn`/`yield_now`/`suspend_self`/`wake`/`run`), the suspending fiber-task async (B1.4a: `Task($R)`/`go`/`wait`/`cancel`), deterministic timers (B1.4b: `clock_ms` virtual clock, `timers` list, `now_ms`/`sleep`, timer-driven `run`), AND real fd readiness (B1.4c: lazy `kq`, `io_waiters` list, `block_on_fd`, a kqueue-blocking run-loop Mode 2 that wakes the fiber whose fd fired). It reuses the verified `std/net/kqueue.sx` bindings (imported as `kqb`) rather than re-deriving the FFI. Fibers can now block on either virtual `sleep(ms)` OR a real fd; both park paths are balanced through `wake` (which evicts a stale timer AND a stale fd-waiter, the UAF guard). Locked by `1811` (round-robin), `1812` (suspend/wake), `1813` (async go/wait/cancel), `1814` (sim-timer deadline ordering), `1815` (timer early-wake eviction), `1816` (pipe fd block→kqueue-wake→read). Suite GREEN **754/0**. The remaining B1 work: **B1.5** end-to-end M:1 validation under the deterministic timers / fd readiness; a **linux epoll twin** of `block_on_fd` (mirror via `std/net/epoll`; the OS-neutral facade is `std.event`) is future work — B1.4c wired the **macOS kqueue** path only. NOTE: the suspending async + deterministic timers live as `sched.*` methods (M:1, receiver-driven), NOT routed through the erased `context.io` (which would force sched.sx into every std consumer + duplicate the `_fib_tramp` global asm); the `Io` protocol's `spawn_raw`/`suspend_raw`/`ready`/`arm_timer`/`poll` remain reserved for the M:N evolution / when a program wants the capability-threaded form. ### Earlier — B1.5a COMPLETE — the M:1 scheduler CORE exists `library/modules/std/sched.sx` drives N fibers (generic `Closure() -> void` bodies) cooperatively over the proven `swap_context`, on guarded `mmap` stacks: `spawn` / `yield_now` (round-robin) / `suspend_self` + `wake` (off-queue park/resume) / `run` (drives to drain, reaps on `.done`). Adversarially reviewed + hardened (wake guarded, loud mmap/mprotect/OOM/deadlock bails, env-leak documented). Locked by `1811` (round-robin ordering contract) + `1812` (suspend/wake park-resume + spurious-wake guard). Suite GREEN **748/0**. The remaining B1.4 work wires this scheduler under the `Io` capability: **B1.4a (FiberIo)** makes `context.io` route `spawn_raw`/`suspend_raw`/`ready` onto the `Scheduler` so `async`/`await` truly SUSPEND (today's `CBlockingIo` runs the worker to completion inline); **B1.4b** the deterministic-sim `Io` (virtual clock + timer queue, calibrated against blocking — the KEYSTONE test harness); **B1.4c** the event-loop `Io` (kqueue/epoll). Then **B1.5** is the end-to-end M:1 validation under the deterministic `Io`. ### Earlier — B1.2 COMPLETE The full async surface (Io capability on Context + `async`/`await`/`cancel` + blocking `CBlockingIo`) works end-to-end. Master GREEN (732/0), installed `sx` clean. All four B1.2 surface bugs resolved or deferred: - **0151 fixed** (`362674f`): generic `$T` through generic-struct / pointer / UFCS-pack params. Regression `0214` + `0215`. - **0152 fixed** (`e5586f6`): `Atomic(bool)` byte-promoted to `i8` in the load/store emitters. Regression `1705`. - **0153 fixed** (`68c1991`): `inferGenericReturnType` pins return-type resolution to the fn's defining module, so a re-exported value-failable keeps its `!` channel. Regression `1058`. - Issue **0150** (`void` struct field → SIGTRAP) DEFERRED — only `Future(void)` / `timeout`, which are B1.4. The async examples are landed + green: `1805` (`async`/`await` + `now_ms` → `sum: 42` / `double: 42` / `clock ok`) + `1806` (`cancel` → `await` raises `.Canceled` → `or` default). The `18xx` concurrency category now covers naked-asm (1800-1803), context-snapshot (1804), and the async surface (1805-1806). ### B1.2 Io capability — what is LANDED + verified (commit 45d869d) - `Io :: protocol #inline { spawn_raw; suspend_raw -> !; ready; poll; now_ms; arm_timer; }` in `core.sx` next to `Allocator`, with `SpawnOpts{ pin: PinTarget }` + `ParkToken{ handle }`. Six methods, each justified by a downstream consumer (B1.3-B1.5). - `Context :: struct { allocator; data; io: Io; }` — `io` appended LAST so `allocator` stays index 0 (the `call.zig:1229` hardcode) and `data` keeps index 1 (minimal VM-fallback churn). - Both `__sx_default_context` materializers updated in lockstep + verified: `protocol.zig` `emitDefaultContextGlobal` (extended `ctx_fields` 2→3, built the `CBlockingIo→Io` inline 7-word vtable `{null-ctx, fn0..fn5}` via `getOrCreateThunks("Io","CBlockingIo")`) and `comptime_vm.zig` `materializeDefaultContext` fallback (wrote the 6 thunk func-refs at `io_base = addr + 4*ps`, offset `+ (i+1)*ps`). The global path auto-followed the 3-field Context type. **`context.io.now_ms()` printed `clock ok` live — the capability threads + the vtable dispatches correctly.** - Stateless `CBlockingIo :: struct {}` + `impl Io for CBlockingIo` (mirror of `CAllocator`): blocking semantics — `spawn_raw`/`ready`/`poll`/`arm_timer` no-op/0, `now_ms` → `time.mono_ms()`. - **push-inherit-omitted fix** (`stmt.zig` `lowerPush`): a `push Context.{...}` now SEEDS the new slot from the ambient context (load+store), then overwrites ONLY the literal's named fields — so omitted fields (now incl. `io`) are INHERITED, never zero-inited to a null vtable. Eliminates the omitted-field footgun globally (zero per-site churn across the 17 partial-literal sites). This is the correct capability-bag semantics; it compiled clean. - **`!`-protocol-method warning fix** (`error_analysis.zig` + a new `Lowering.impl_method_names` set populated in `protocols.zig` `registerImplBlock`): a protocol impl method may be declared `!` by contract (e.g. `Io.suspend_raw`) yet never raise; the "declared `!` but never errors — drop the `!`" hint is a false positive for impl methods, now suppressed for them. Status of the blockers that originally stopped B1.2: - **issue 0151 — FIXED this session** (generic `$T` through generic-struct / pointer / UFCS-pack params). `async`/`await`/`cancel` are callable. See "Last completed step". - **issue 0152 — NEW, the current blocker** (`Atomic(bool)` → sub-byte i1 atomic; LLVM reject). Blocks the async examples via `Future.canceled: Atomic(bool)`. Filed; codegen-level fix. - **issue 0150** — `void` struct field SIGTRAP; only `Future(void)`/`timeout` (B1.4). DEFERRED. Per the IMPASSABLE STOP rule: 0151 fix shipped (suite green 728/0), 0152 filed, STOPPED. Resume B1.2's async examples once 0152 lands. ### Earlier — B1.0 + B1.1 complete Stream A (atomics) is feature-complete (✅). Stream B1: **B1.0 + B1.1 complete.** The two compiler-floor preconditions for the fiber runtime are in place: (1) `abi(.naked)` emits a real LLVM `naked` function end-to-end (decl, generic, pack paths) — the context-switch substrate; (2) per-fiber `context` root needs **no compiler change** — the spawn convention (snapshot `context`, store, `push` it from the trampoline) is pure library sx. No fibers/Io/scheduler code yet. Grounded floor facts: - `context` is an implicit slot-0 `*Context` param + `push Context` is a stack `alloca` ⇒ **fiber-local for free** (confirmed by the B1.1 probe — never TLS, never re-read from the `__sx_default_context` global mid-stack). A spawn passes the snapshot as the fiber-entry fn's slot-0 ctx via `push f.root { entry(args) }`. Locked by `1804-...-context-snapshot`. - Inline asm works end-to-end (lower→emit→JIT, aarch64 + x86_64) — the `.naked` body reuses it. - **`.naked` with PARAMS works** (B1.0c, the B1.3 substrate): the param-alloca loop is gated on `fd.abi != .naked` in decl.zig (both paths) + generic.zig — a naked fn's args stay in ABI registers (read by the asm body), declared-but-unused in LLVM (verifier-legal). Example `1803-concurrency-naked-asm-param.sx` (`add(a,b)` reads x0/x1). **Unsupported (loud, not silent):** a `.naked` *variadic-pack* fn (pack.zig's param loop is intertwined with comptime-param/`#insert` handling, and a naked fn can't read a runtime-sized pack from registers anyway) → loud LLVM-verifier error for that nonsensical construct. Acceptable boundary; a sharper sx diagnostic for it is a candidate polish, not a blocker. ## Next step **→ B1.5 — end-to-end M:1 validation under the deterministic timers / fd readiness.** B1.4c (real fd-readiness blocking via kqueue, `sched.block_on_fd` + the kqueue-blocking run-loop Mode 2) is done — the macOS event-loop path exists. Build an `18xx` example that exercises the full M:1 story together (multiple fibers, a mix of `sleep`/`go`/`wait` and `block_on_fd`, reaping, the orphan-deadlock guard). The §10.7 gate (1808) + guarded-stack (1809) + Win64 (1810) + scheduler/async/timers/fd (1811-1816) must keep passing throughout. **Deferred (future B1.4c sibling): the linux epoll twin of `block_on_fd`.** B1.4c wired the **macOS kqueue** path only (the host is aarch64-macOS). The linux mirror would register interest via `std/net/epoll` and the OS-neutral facade is `std.event` — keep the two as separate run modes inside `run`, branching on the platform, exactly as the timer-vs-fd modes are kept separate now. Documented non-unification: virtual-time timers and real kqueue timeouts are NOT merged — `run` fires a pending timer before ever blocking on kqueue (a program uses `sleep` OR fds); a true "fd-or-real-timeout" wants a kqueue `EVFILT_TIMER`, future work. Design note carried forward: an event-loop `Io` needs a current-`Scheduler` handle. `sched.*` methods thread it via `self`/the `Task`; if B1.4c wants the capability-threaded `context.io` form it'll need an ambient current-scheduler accessor in sched.sx (still deferred — the `sched.*`-method form suffices). The `Io` protocol's `poll`/`arm_timer` map onto this when/if that wiring is built. **Side thread (optional, low priority): the SysV/Linux x86_64 sibling.** A THIRD switch variant for `x86_64-linux`: SysV callee-saved = rbx, rbp, r12-r15 + rsp (6 GP + sp; **no** callee-saved XMM, unlike Win64) — a 7-slot ctx, args rdi/rsi/rdx, the rsp-carried return addr. Needs a Linux x86_64 host (or a working cross-run) to RUN + the mutual-scribble gate. Not blocking — the switch is already validated on two arch/ABI pairs. **Deferred (do NOT block on these):** issue **0150** (`void` struct field SIGTRAP) — only `Future(void)`/`timeout` (B1.4). The **`::` callable-parameter feature** (named-fn async workers `async(read_a, conn)`) — WIP at `.sx-tmp/wip-callable-params/patch.diff` (parser done, inference incomplete); a dedicated effort; lambda workers are the idiom meanwhile. `Context` layout settled: `{ allocator; data; io; }` (allocator index 0 fixed by `call.zig:1229`, io last). Io protocol + materializers + push-inherit are LANDED + reviewed. ## Known issues / capability gaps - **issue 0157 (OPEN, BLOCKING B1.4a)** — a user-defined generic ufcs method whose NAME collides with a stdlib re-export (`cancel`, re-exported by `std.sx` from `io.sx` as `ufcs (f: *Future($R))`), called via UFCS on a different generic struct (`*Task($R)`), leaves `$R` unresolved → `.unresolved` reaches LLVM emission → panic (`src/backend/llvm/types.zig:196`). Renaming → works; the non-UFCS call form already diagnoses `cannot infer generic type parameter 'R'`, so the UFCS path skips that diagnostic. Surfaced by `cancel :: ufcs (t: *Task($R))` in `std/sched.sx`. Minimal repro (no fibers/closures): `issues/0157-ufcs-generic-method-name-collides-stdlib-unresolved.{md,sx}`. - **✅ issue 0154 — FIXED** (`null`/`---` to a struct field over-stored a whole-struct null when the function's return type leaked as `target_type`, corrupting the frame → `ret` to 0x0; surfaced building `Scheduler.init()`'s by-value return). Fix: `.null_literal`/`.undef_literal` added to `needs_target` in `lowerAssignment` (`src/ir/lower/stmt.zig`). Regression: `examples/types/0193`. - **issue 0155 (OPEN, NON-blocking)** — indexing a scalar pointer (`pc[0]`, `pc: *i64`) panics codegen (`.unresolved` reaching LLVM emission, `src/backend/llvm/types.zig:196`). Found in the B1.5a review; the scheduler doesn't use it (array-field index + `.*` only). Filed for its own session: `issues/0155-scalar-pointer-index-llvm-panic.{md,sx}`. - **✅ issue 0157 — FIXED** (B1.4a) — a user generic `ufcs` method whose name collides with a stdlib re-export resolved via last-wins `fn_ast_map` with no receiver filtering → wrong overload → `$R` unbound → LLVM panic. Fix: `selectUfcsGenericByReceiver` (`src/ir/lower/call.zig`) — most receiver-specific binding author across ALL module authors, deterministic, ambiguity-diagnosing. Regression: `examples/generics/0217`. - **✅ issue 0156 Part 1 — FIXED** (B1.4a) — single-type generic `$R` as a type-arg in a pack-fn body (`Box($R)`/`size_of(Box($R))`) → `.unresolved` → panic. Fix: `comptime_pack_ref` arm in `resolveTypeWithBindings`. Regression: `examples/generics/0216`. - **Part 2 (OPEN, NON-blocking)** — a deferred `..` spread (a comptime pack captured into a closure, or a tuple `..t` spread) crashes instead of working/diagnosing. The fiber async layer avoids it by design (nullary thunks), so it's filed for its own session: `issues/0156`. - **Heap leaks in the fiber runtime (documented limitations, NOT bugs):** `spawn`'s closure env + `go`'s heap `Task` are never freed (sx exposes no closure-env free; Task ownership is deferred). Bounded by spawn/go count, invisible under the default GPA. Revisit for a long-running arena-backed scheduler. - **✅ issue 0153 — FIXED** (re-exported generic value-failable `($R, !E)` kept its `!` channel: `inferGenericReturnType` now pins return-type resolution to the fn's defining module). Regression: `examples/1058`. Was the LAST B1.2 surface blocker. - **✅ issue 0152 — FIXED** (`Atomic(bool)` sub-byte i1 atomic → byte-promoted to i8 in the load/store emitters). Regression: `examples/1705`. Unblocked `Future.canceled`. - **✅ issue 0151 — FIXED** (generic `$T` through generic-struct / pointer / UFCS-pack params). Regression: `examples/0214` + `0215`. Was the original B1.2 surface blocker. - **issue 0150** (deferred) — a `void` struct field crashes the compiler (unsized-type SIGTRAP in LLVM `getTypeSizeInBits`). Blocks `Future(void)` → `timeout` (B1.4). Repro: `issues/0150-...`. - (Note: **issue 0149**, filed by another session against an earlier dirty binary, was a manifestation of the pre-fix 0151 — now moot.) - **Orthogonal (not a B1 blocker):** default VALUES for comptime params don't bind on generic-struct methods (free-fn defaults DO work) — inherited from Stream A. Only matters if a B2 lib type wants a defaulted comptime param; atomics/fibers require explicit, so unaffected. - **Issue 0144 (open, independent):** calling an unrecognized bodiless `#builtin` silently returns 0 / exit 0 — a silent-fallback footgun in the generic builtin-call path. Filed; leave for its own fix session unless prioritized. Not a B1 blocker. - **Deferred design gap (documented):** the B1.4 event-loop `Io` does not yet cooperate with a platform UI run loop (CFRunLoop/NSRunLoop/ALooper); pinning gives thread-affinity, not run-loop integration — a §6 app-target concern, out of B1 scope. ## Decisions (Stream B1 specifics; surface locked in design §4 / §4.6) - **The async runtime is sx LIBRARY code.** The compiler provides only: the general primitives (inline asm ✅, `abi(.naked)` naked [B1.0], atomics ✅) + fiber-safe codegen (`context` already fiber-local — B1.1). Schedulers, fibers, channels, futures, `Io` vtables, `mmap` stacks are all sx. - **`abi(.naked)` is the real spelling of the design's `callconv(.naked)`** — postfix slot, `name :: (sig) -> Ret abi(.naked) { asm { … }; }`. B1.0 = carry it into IR + emit LLVM `naked` + skip prologue/ctx (mirror the existing `.c` skip), NOT extend the enum (it's already there, just inert). - **`.naked` ≠ `.c`:** a `.c` epilogue would restore SP from the wrong stack across a context switch (SP-in ≠ SP-out by design). `.naked` = no prologue/epilogue/frame; the asm emits its own `ret`. This is why the switch must be `.naked`. - **Naming:** sx-facing name is **`naked`** (keyword `abi(.naked)`, field `is_naked`, the diagnostic), matching LLVM's `naked` attribute and the industry term (Zig/Rust/GCC/Clang). The ABI variant was renamed `.pure → .naked` (user direction): "pure" universally means *side-effect-free*, the opposite of a register-clobbering context switch. - **B1.0 snapshot scope:** a `.naked` body is raw per-arch asm; LLVM's `naked` attr text is arch-invariant. **B1.0a** = one host example locked to the emit bail (host-independent — fires before instruction selection; no `.build` pin). **B1.0b** = pin aarch64 + add an x86_64 cross sibling (`.build` target-gated, ir-only on mismatch), like the asm corpus split. The `.ir` proves the `naked` attr + asm emitted, NOT register-save correctness (that's B1.3's stress harness). - **B1.1 — per-fiber context is library-only (CONFIRMED by probe):** push frames are stack-`alloca`'d and the implicit ctx rides slot 0, so the spawn convention — snapshot `context`, store it, `push f.root { entry(args) }` from the trampoline — installs the fiber's root with no compiler change. Verified: the body reads the snapshot over a different ambient context, and `push` restores ambient on exit (`1804-...-context-snapshot`). The design doc's "never raw TLS" guarded a non-problem (context was never TLS). - **Test keystones (design §10):** the **B1.3 switch-stress harness** gates the context-switch (the one piece the deterministic `Io` can't test — §8.1.1, §10.7); the **B1.4 deterministic-sim `Io`** (calibrated against blocking `Io` — §8.1.3) gates all scheduling tests. Both must exist + be calibrated before the async tests they gate are trusted. `18xx` asserts program-emitted ordering contracts, not raw interleaving. ## Log - **B1.4c — real fd-readiness blocking via kqueue (macOS).** De-risked first with a no-scheduler probe (confirmed `size_of(Kevent)==32` and the pipe→kevent roundtrip: `kq_wait` returned 1, `out.ident == read_fd`, `out.filter == -1`, `out.data == 1` — the struct layout reads the fd back correctly). Then added to `library/modules/std/sched.sx` (importing the existing verified `std/net/kqueue.sx` as `kqb` rather than re-deriving the FFI): a lazy `kq: i32` (-1 until first use), `io_waiters: List(IoWaiter)`, `block_on_fd(fd, want_read)` (arm one-shot `EVFILT_READ`, record waiter, `suspend_self`), a run-loop Mode 2 (block on `kq_wait(kq, evbuf, MAXEV=16, -1)` when only fd waiters remain, wake the fiber whose fd fired), and `wake` now also evicts a stale fd-waiter (`cancel_io_waiter_for`, the same UAF guard as `cancel_timer_for`). Timers keep precedence over fds (documented non-unification). Orphan-deadlock check still fires for a genuine no-timer/no-fd suspend (probed: exit 134). Locked by `1816-concurrency-fiber-io-pipe.sx` (reader blocks on empty pipe → writer writes `a b c` → kqueue wakes reader → reads 3 bytes; `log: wrote read 3 [97 98 99]`, `n_suspended: 0`), `.build` `{ "target": "macos" }`, runs end-to-end on host. The example's `read`/`write`/`close` externs use the canonical signatures std already binds (extern-dedupe rejects a divergent re-binding). Suite GREEN **754/0**. Next: B1.5 (end-to-end M:1 validation); linux epoll twin deferred. - **carve** — wrote PLAN-FIBERS.md + CHECKPOINT-FIBERS.md. Grounded the B1 compiler floor: `ABI.naked` inert (type_resolver.zig:237), IR `Function` has no naked flag (inst.zig:605), attribute API pattern (emit_llvm.zig:1339 nounwind), `.c` ctx-skip precedent (decl.zig:515), `push Context` stack-alloca + slot-0 implicit ctx (stmt.zig:1263, lower.zig:259), `__sx_default_context` root (decl.zig:2667/2815), inline-asm corpus (1645/1651). Corrected the design's `callconv(.naked)` → real `abi(.naked)` spelling and the B1.0 snapshot story. B1.1 grounded as likely library-only. Baseline green (721/0). - **B1.0a** — plumbed `Function.is_naked` (set from `fd.abi == .naked` at both decl sites); `funcWantsImplicitCtx` skips `.naked` (no implicit ctx, like `.c`); both body-lowering paths bypass `lowerValueBody` for `.naked` (asm body + `unreachable` cap — no sx return); `emit_llvm` Pass 2 bails loudly on `func.is_naked`. `examples/1800-concurrency-naked-asm.sx` locked to the bail (exit 1 + diagnostic). Suite green (722/0). (ABI variant later renamed `.pure → .naked` — see the Naming decision above — so all `is_*`/`abi(.*)`/example names here read `naked`.) - **B1.0a review-hardening** — adversarial review found generic/pack Function-creation paths left `is_naked` false (silent framed body for a generic `.naked` instance — returned 42 but corrupted the stack). Fixed generic.zig + pack.zig (set `is_naked` + asm-only `unreachable` cap); locked by `examples/1801-concurrency-naked-generic-bail.sx`. The review's `.naked`- lambda CRITICAL was a false positive (unparseable — `isLambda` breaks on `abi`). Suite green (723/0). - **B1.0b** — real `naked` emission: emit_llvm declaration pass adds LLVM `naked`/`noinline`/ `nounwind` + skips `frame-pointer` for `func.is_naked`; Pass 2 emits the body verbatim (no prologue). `1800` green aarch64-pinned (exit 42 + `.ir`); renamed `1801` → `-generic` (generic `.naked` emits a naked body, exit 42); added x86_64 sibling `1802` (ir-only, `.ir` locks `naked` + `movl $42, %eax`). Unit test asserts `naked` present + `frame-pointer` absent. Suite green (724/0). - **B1.0c** — review-hardening: param-bearing `.naked` emitted invalid LLVM (loud verifier error). Gated the param-alloca loop on `fd.abi != .naked` (decl.zig both paths + generic.zig) — naked args stay in registers, read by the asm body (the B1.3 context-switch shape). Locked by `examples/1803-concurrency-naked-asm-param.sx`. Pack `.naked` left unsupported (loud, nonsensical). **B1.0 complete.** Suite green (725/0). - **rename** — ABI variant `.pure → .naked` (keyword, `Function.is_naked`, diagnostics, examples 1800-1803 `*-pure-* → *-naked-*`, docs). "pure" universally means side-effect-free — wrong for a register-clobbering switch; "naked" matches LLVM/Zig/Rust/GCC/Clang. Pure cosmetics, no semantic change. Suite green (725/0). - **B1.1** — per-fiber `context` root: **zero compiler change** (probe-confirmed). The spawn convention (snapshot `context` → store in a struct → `push f.root { entry() }` from the trampoline) installs the fiber's root via the implicit slot-0 `*Context` param; the body reads the snapshot, not the trampoline's ambient ctx, and the `push` scope restores ambient on exit. Locked by `examples/1804-concurrency-context-snapshot.sx` (prints `fiber root: 42` / `ambient after: 99`). Suite green (726/0). **Next: B1.2 (Io interface + context.io).** - **B1.2 (BLOCKED)** — built the full `Io` capability (protocol on `Context`, stateless `CBlockingIo` blocking default, both `__sx_default_context` materializers, push-inherit-omitted fix, `!`-impl-method warning fix) and VERIFIED the core works live (`context.io.now_ms()` → `clock ok`). Two independent compiler bugs blocked the `async`/`await`/`timeout` layer: **0150** (`void` struct field → unsized SIGTRAP, blocks `Future(void)`) and **0151** (type-var from a fn-ptr param's return type not bound in the body, blocks `async`'s `Future(R)`). Both filed with standalone repros + investigation prompts. Per the STOP rule: reverted ALL B1.2 working changes (master green again, 726/0; the dirty binary had broken the photo project — see the now-moot 0149), saved WIP to `.sx-tmp/b12-wip/`, STOPPED. Resume after 0150 + 0151. - **0151 FIXED** — generic inference now binds `$T` through a generic-struct param head, a pointer (`*Box($T)`, incl. UFCS auto-ref), and a closure-return-via-pack on the UFCS path. Four gaps closed: `parameterized_type_expr` arm in `extractTypeParam`/`matchTypeParam(Static)` (recovers the arg instance's recorded per-param bindings, recurses positionally); pointer arm falls through to match a value arg (auto-address-of); `ExprTyper.inferType` `.lambda` arm (closure type from annotations — UFCS types args from raw AST pre-lowering); pack UFCS target routes through `lowerPackFnCall` with the receiver spliced in as `args[0]`. Issue 0151 marked RESOLVED; repro → `examples/0214-generics-ufcs-closure-return-pack.sx`; widened cases → `examples/0215-generics-infer-through-pointer.sx`. Suite green 728/0. The now-callable async surface immediately exposed a SEPARATE codegen bug — **issue 0152** (`Atomic(bool)` → sub-byte i1 atomic, LLVM reject; `Future.canceled` hits it). Filed with standalone repro + fix prompt. Per the STOP rule: shipped the 0151 fix, filed 0152, STOPPED. Resume the async examples (1805/1806) after 0152. - **0152 FIXED** — the atomic load/store emitters (`src/backend/llvm/ops.zig`) byte-promote a sub-byte (`bool`→`i1`) access to its `i8` storage type and `trunc`/`zext` the value at the boundary (new `atomicByteType` helper). rmw/cmpxchg left as-is (a `bool` rmw/CAS is rejected at the sx level — integer-only — so a sub-byte element never reaches them; comments record this). Regression `examples/1705-atomics-bool-byte-promoted.sx` (load/store round-trip). Issue 0152 marked RESOLVED. Suite green 729/0. With `Atomic(bool)` working, the async surface exposed the TRUE remaining blocker — **issue 0153**: a re-exported generic value-failable `($R, !E)` loses its `!` channel at the call site (the earlier "secondary `or` PHI" symptom was this, NOT an `Atomic` cascade — confirmed it persists after 0152). Narrowed to the generic+re-export co-requirement (non-generic re-export OK; direct generic import OK; only the combination drops `!`). Root cause: the monomorphized return-type's error-set, reached via the re-export alias, resolves to a non-`.error_set` TypeId, so `errorChannelOf` (`lower/error.zig:148`) misses the channel. Filed `issues/0153-...` with a minimal co-located 2-file repro + a single-file stdlib-`await` repro + investigation prompt. Per the STOP rule: shipped the 0152 fix, filed 0153, STOPPED. Resume the async examples after 0153. - **0153 FIXED → B1.2 COMPLETE** — `inferGenericReturnType` (`src/ir/generics.zig`) resolved the return-type AST in the CALL-SITE module, so a re-exported error set (`LE :: lib.LE`) resolved to a non-`.error_set` alias and the planned call-result was a plain tuple (channel lost). Fix: pin the source to `fd.body.source_file` around the return-type resolution, exactly as `monomorphizeFunction` does — the `!E` now resolves to the real `.error_set`. One-function change; full suite green (732/0), no regression. Issue 0153 RESOLVED; repro → `examples/1058-errors-reexport-value-failable-channel.sx` (+ companion `lib.sx`). With the channel preserved, landed the async examples: **`1805`** (`async`/`await` + `now_ms` → `sum: 42` / `double: 42` / `clock ok`) + **`1806`** (`cancel` → `await` raises `.Canceled` → `or` default; `ok: 7` / `canceled: -99`). **B1.2 (Io capability + M:1 async surface) is COMPLETE.** Next: B1.3 (fiber runtime) on the `.naked` context-switch substrate. - **B1.3a-1 — context switch works.** Implemented the stackful switch in pure sx over `abi(.naked)`: `swap_context(from, to)` (save callee-saved x19-x28 + fp/lr + sp into `*from`, load from `*to`, `ret` onto `to`'s stack) + by-hand fiber bootstrap (SP = top of an `alloc_bytes` stack, LR = a `.global _fib_tramp` global-asm trampoline that does `mov x0, x19; bl _fib_body`, x19 = `*Fiber`). Proven via a probe (main↔fiber), then locked by `examples/1807-concurrency-fiber-context-switch.sx` (aarch64-pinned): a 2-fiber ping-pong (`rounds: 6`, `canary fails: 0` — a per-fiber stack canary survives every switch) + a 64-frame deep recursive chain suspended at the bottom and resumed (`frames verified: 64` / `depth fails: 0`). The `bl _fib_body` reaches the sx body via `export "fib_body"` (the 1655 asm→sx pattern); runs under JIT, ir-only on a non-arm host (`.ir` captured — `swap_context` shows `naked noinline nounwind`). Suite green 733/0. **Honest scope:** indirect register/stack survival only; the EXPLICIT every-callee-saved + FP scribble (§10.7) is B1.3a-2, still owed. Next: B1.3a-2. - **B1.3a-2 — the §10.7 stress gate, adversarially reviewed.** Extended `swap_context` to the COMPLETE AAPCS64 callee-saved set (added FP d8-d15 → 21-slot ctx) and wrote a naked `scribble_verify` that loads a unique sentinel into all 18 callee-saved regs, yields, and counts non-survivors on resume (176-byte frame saves/restores the caller's callee-saved + base; lr round-trips the swap). The gate is a 2-fiber MUTUAL scribble (each clobbers the other's regs, so survival ⇒ the switch saved+restored them). Locked by `examples/1808-concurrency-fiber-switch-stress.sx` (`A/B mismatches: 0`). Validity proven by negative controls (drop d8-d15 → 8/8; drop x27/x28 → 2/2). **Spawned an adversarial-review worker (per the plan + user request): NO critical bugs** — callee-saved set complete (x18 rightly excluded; d8-d15 suffices per §6.1.2), offsets/alignment/lr-sp dance all verified. Applied its one rec: `boot` zeroes FP ctx slots so first-entry loads 0, not garbage. Honest residual gaps (spec-correct for a call-boundary swap; in the example header): FPCR/FPSR/NZCV + TPIDR/TLS not swapped, fp=0 blocks unwind — relevant at N×M:1 / signals, not here. Suite green 734/0. Next: B1.3b (x86_64 sibling + mmap guard-page stacks). - **B1.3b — mmap guard-page stacks (x86_64 sibling deferred).** Fiber stacks now `mmap` a `[guard | usable]` region and `mprotect` the low 16KB page `PROT_NONE`, so a stack overflow faults at the guard boundary instead of silently corrupting a neighbor (§8.1.1). Locked by `examples/1809-concurrency-fiber-guard-stack.sx` (aarch64-macos-pinned): `guard armed: 1` (`mprotect`→0) + `sum: 20100` (a fiber runs real recursion on the guarded stack + yields). Guard FIRING validated manually (overflow → `Bus error` at `region+GUARD`, exit 134 via the sx crash handler) — not corpus-pinned because a deliberate-overflow crash is host-fragile (and a mere "child faulted" fork test wouldn't prove the BOUNDARY catch). The x86_64 `swap_context` sibling was DEFERRED: `--target x86_64-macos` mislinks on this arm64 host and `x86_64-linux` can't run here, so it could only ship un-run/un-negative-controlled — which §10.7 forbids for the highest-risk asm. SysV target notes (rbx/rbp/r12-r15/rsp, no callee-saved XMM, rsp-carried return addr) recorded in Next step. Suite green **735/0**. Next: x86_64 sibling (needs an x86_64 host) OR B1.4 (`Io` impls / scheduler) on the proven aarch64 substrate. - **B1.3b-1 — x86_64 / Win64 switch sibling VALIDATED on real hardware.** The user provided a Windows 7 x64 VM (UTM), so the x86_64 switch became RUNNABLE (as Win64). Validated the cross-build→VM→run loop (`--target x86_64-windows-gnu --self-contained` → PE32+; output via the Win32 `WriteFile` boundary, the 1660 pattern). Wrote a Win64 `swap_context` (8 GP rbx/rbp/rdi/ rsi/r12-r15 + rsp + **xmm6-xmm15** via `movups` — Win64 has callee-saved XMM) + a Win64 `scribble_verify` (264-byte frame, 32-byte shadow + 16-align at each `call`, COFF symbols, rsp-carried return addr) driving the 2-fiber mutual scribble. **Adversarially reviewed (worker emitted the real `.s`, verified every alignment/offset/round-trip against the Win64 ABI — no critical/minor bugs), THEN run on the VM → `0 0 P`** (all 8 GP + 10 XMM callee-saved survived). Locked by `examples/1810-concurrency-fiber-switch-win64.sx` (pinned `x86_64-windows-gnu`, ir-only on this host; the VM run is the runtime-correctness provenance). Good-swap-only (the in-process negative control was dropped to avoid an sx fn-ptr-convention rabbit hole; the detection of this exact logic was negative-controlled on aarch64 in 1808). Suite green **736/0**. The B1.3 context switch is now proven on TWO arch/ABI pairs. Next: **B1.4** (Io impls / M:1 scheduler) on the proven substrate. (Side thread: the SysV/Linux x86_64 sibling, when a Linux x86_64 host is available.) - **B1.5a — M:1 scheduler CORE + a fixed blocker bug.** Built `library/modules/std/sched.sx`: a generic `Fiber`/`Scheduler` over `swap_context` on guarded `mmap` stacks. `spawn` heap-allocs a fiber, bootstraps its ctx, enqueues it; the ONE generic dispatch (`fib_dispatch` via `_fib_tramp`) runs ANY stored `Closure() -> void` on a fresh stack (replacing the fixed `bl _fib_body`); `yield_now` round-robins, `suspend_self`/`wake` park/resume off-queue, `run` drives to drain + reaps `.done` fibers (`munmap` + free). **De-risked first by probe** (closure-on-fiber + output via captured pointer). **Hit blocker bug 0154** (user-authorized fix): `null`/`---` to a struct field over-stored a whole-struct null when the fn return type leaked as `target_type`, corrupting the frame (`ret` 0x0) — exactly the `Scheduler.init()` by-value-return shape. Fixed in `stmt.zig` (`needs_target` += `null`/`undef` literals); regression `examples/types/0193`; `0154` RESOLVED. **Adversarial review:** asm/bootstrap/lifetime sound (env-lifetime fear disproven — heap-promoted); 1 CRITICAL (`wake` re-enqueue → FIFO segfault) + robustness gaps ALL hardened (wake guarded on `.suspended`, `n_suspended` deadlock diagnostic+abort, loud mmap/mprotect/OOM bails, env-leak documented). Locked `1811` (round-robin `0 1 2 ×3`) + `1812` (suspend/wake + spurious-wake guard, `log: 10 20 21 11`). Filed NON-blocking `0155` (scalar-pointer index panics codegen — review incidental, unused by sched). Suite GREEN **748/0**. Next: **B1.4a** (FiberIo). - **B1.4a (truly-suspending fiber-task async, nullary-thunk design) — BLOCKED on issue 0157.** Implemented the async layer SELF-CONTAINED in `library/modules/std/sched.sx` (kept its lone `#import "modules/std.sx"` to avoid the duplicate-`_fib_tramp` trap): `TaskState`, a LOCAL `TaskErr :: error { Canceled }` (the re-exported `IoErr` alias is NOT seen through by the `raise`/failable-type check — verified), `Task($R)`, and `go`/`wait`/`cancel` ufcs. Design is the validated nullary-thunk (`.sx-tmp/pnullary.sx` → `log: 1 2 3 42 100`): `work` is a `Closure() -> $R`, user captures inputs at the call site, NO `..args` crosses the fiber boundary (deliberately sidesteps 0156). `go`+`wait` run correctly; both wake-orderings traced. Wrote the example `examples/concurrency/1813-concurrency-fiber-async-suspend.sx` (+ `{ "target": "macos" }` `.build`) but its `cancel` ufcs surfaced a NEW compiler bug — issue **0157**: a user generic ufcs whose name collides with a stdlib re-export (`cancel` from io.sx) is mis-resolved on UFCS call over a different generic struct, leaving `$R` unresolved → LLVM panic. Bisected to a minimal no-fiber repro (name is the sole trigger; non-UFCS form diagnoses correctly). Example NOT seeded into the corpus (no `.exit` marker) — do NOT regen its goldens until 0157 lands. Per the STOP rule: filed `issues/0157-*.{md,sx}`, marked state BLOCKED, paused. - **B1.4a COMPLETE (this session) — suspending fiber-task async + two compiler fixes.** Built the `Task($R)` + `go`/`wait`/`cancel` layer in `sched.sx` (nullary-thunk design; self-contained to avoid the `_fib_tramp` duplicate-symbol trap). Locked `1813` (`sequence: 1 2 3 42 100 -99`). FIXED the two blockers the worker had filed: **0156 Part 1** (`comptime_pack_ref` arm in `resolveTypeWithBindings`; regression `0216`) and **0157** (receiver-driven UFCS overload selection `selectUfcsGenericByReceiver`; regression `0217`). Adversarial review of the 0157 fix + Task layer found a determinism CRITICAL (always-run selection + specificity + ambiguity diagnostic), a `wait`-outside-fiber null-deref (loud guard), and cancel-not-skipping-work (skip if pre-canceled) — all fixed. Simplified `1812` (`**Fiber` → `Sh.parked`). 0156 Part 2 reframed OPEN/non-blocking. Suite GREEN **751/0**. Next: B1.4b (deterministic-sim `Io`, the KEYSTONE). - **B1.4b COMPLETE (this session) — deterministic virtual-time timers + a CRITICAL UAF fix.** Added `clock_ms`/`timers`/`now_ms`/`sleep` + a timer-driven `run` to `sched.sx` (worker-built): fibers sleep in reproducible simulated time, waking in deadline order (FIFO tiebreak). Locked `1814` (5 fibers, wake order B@10/D@15/E@15/C@20/A@30). Adversarial review of the run-loop change found a CRITICAL use-after-free — a fiber woken EARLY (manual/Task `wake`) before its `sleep` timer fired was reaped while its `Timer` kept a dangling `*Fiber`; a later fire dereferenced freed memory (silent "pass" only by luck). Fixed: `wake` evicts the fiber's pending timer (`cancel_timer_for`); regression `1815` (early wake → `clock: 0`, stale timer never fires). Review cleared n_suspended accounting, deadlock false-positives, timer-list integrity, clock monotonicity, termination. Suite GREEN **753/0**. Next: B1.4c (event-loop `Io`, kqueue/epoll).