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fibers: event-loop Io — real fd readiness via kqueue (B1.4c)

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A fiber can block on a file descriptor and the run loop blocks on
kevent until the kernel reports it ready. Reuses the existing
std/net/kqueue.sx bindings. Scheduler gains a lazy kq fd + an
io_waiters list; block_on_fd arms a one-shot EVFILT_READ registration,
records an IoWaiter, and suspends. Run-loop Mode 2: when the ready
queue drains and no timer is pending, block on kq_wait(-1), match each
fired ident to its waiter, evict it, wake the fiber. wake evicts a
pending fd-waiter (cancel_io_waiter_for) so no stale IoWaiter outlives
a reaped fiber.

Adversarial review found two CRITICALs: (1) two fibers on the same fd
share one kqueue registration (macOS EV_ADD replaces), so one is lost
and the loop hangs -- fixed by enforcing one-waiter-per-fd with a loud
abort; (2) an fd-waiter on a never-ready fd 'hangs' -- reclassified as
correct event-loop semantics (a server idling on a socket), with the
misleading orphan-check comment corrected. UAF parity, ident width,
EINTR handling, timer/io precedence all probed safe.

Example: 1816 (pipe roundtrip -- reader blocks, writer writes, reader
wakes via kqueue). macOS only; linux epoll twin deferred. Suite green 754/0.
This commit is contained in:
agra
2026-06-21 19:39:16 +03:00
parent 62ffea0663
commit 1b0d640f73
8 changed files with 433 additions and 40 deletions
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current/PLAN-FIBERS.md
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@@ -8,8 +8,8 @@
> `sched.go`/`wait`/`cancel` over `Task($R)`, nullary-thunk) ✅** (adversarially reviewed; fixed
> blockers 0156-Part1 + 0157 en route; locked `1813`).
> **B1.4b (deterministic virtual-time timers — sched.sleep/now_ms/timer-run) ✅** (reviewed; fixed a CRITICAL timer-vs-early-wake UAF; locked 1814/1815).
> **→ NOW: B1.4c** — the event-loop `Io` (kqueue/epoll, real fd readiness). Then B1.5 (end-to-end
> M:1). Detailed progress in [CHECKPOINT-FIBERS.md](CHECKPOINT-FIBERS.md). NOTE: suspending async +
> **B1.4c (event-loop — real fd readiness via kqueue: `block_on_fd` + run-loop Mode 2) ✅** (reviewed; fixed a CRITICAL same-fd lost-wakeup hang; locked 1816). macOS only — linux epoll twin deferred.
> **→ NOW: B1.5** — end-to-end M:1 validation under the deterministic timers / fd readiness. Detailed progress in [CHECKPOINT-FIBERS.md](CHECKPOINT-FIBERS.md). NOTE: suspending async +
> deterministic timers live as `sched.*` methods (M:1), NOT routed through the erased `context.io` (avoids forcing sched.sx into every std consumer + the `_fib_tramp` dup-symbol
> trap); the `Io` protocol's `spawn_raw`/`suspend_raw`/`ready` stay reserved for M:N. Deferred:
> issue 0150 (`Future(void)`/`timeout`); 0156-Part2 (deferred `..` spread); the `::` callable-param