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fix(asyncify): consume-once root suspensions — replaces the wake-window deferral

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The deferral (9ca2ac1) modeled the wrong condition and taxed every parked
fiber completion with a macrotask hop: under CI load that stretched
three-client apply chains and flaked drift-trio S4 twice consecutively
(26/26 green locally under stress) — retracted.

The actual fatal state, readable in all four prod stacks once seen: a SECOND
rewind of the same root suspension. Root suspends once per fiber_swap out of
it; two parked fibers completing against one root suspension epoch (a tool
fiber + a collab fiber both waking around open:settled) each drive
finishContextSwitch(root) — the second rewinds already-consumed data →
"unreachable executed" → poisoned runtime, with the wake-side "index out of
bounds" as the sibling symptom.

Cure: stop exempting root from the validity check the shim already keeps.
First consumption proceeds synchronously — zero added latency anywhere; the
second is refused ([wx-asyncify] "root suspension already consumed") — the
yielded fiber stays properly suspended and resumable, root continues via its
real pending resume, libcontext's ghost-epoch contract enforced one layer
lower. Root remains exempt only from the internally-parked quarantine (its
yield park is routine).

.ci-cache-epoch 4→5 (the epoch-4 cache holds the retracted deferral shim).

Local: fiber 2/2 + timer 1/1, firefox sweep 20 passed, drift-trio-scenarios
kicad-chromium 26/26 under 3-worker stress, web fatal+follow 2/2.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019SE4o46Lnq3hF574FFq8x4
This commit is contained in:
Gergő Törcsvári 2026-08-01 15:53:18 +02:00
commit ae33a100c2
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26
docs/features/async/16-fiber-resume-guard.md
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@ -127,12 +127,26 @@ the fiber's `finishContextSwitch` rewind of `main+20` — replaying frames
over live state. The 8 ms-earlier "index out of bounds" is the wake side of
the same collision.
Root entry is legal and constant in healthy flow; ONLY the wake-window
overlap is fatal. **Layer 3: serialize, don't refuse.** The shim marks the
synchronous wake window (`Asyncify.__inSleepWake` around `wakeUp()`);
`finishContextSwitch(root)` inside that window is DEFERRED one macrotask
(`[wx-asyncify] root-entry-deferred` beacon) and re-fired via the trampoline
once the wake has settled. Ordering change only — nothing is dropped.
**Layer 3, first attempt (wake-window deferral) — RETRACTED same day:**
deferring every root entry inside a sleep-wake window taxed EVERY parked
fiber completion with a macrotask hop; under CI load that stretched
three-client apply chains and flaked drift-trio S4 twice in a row (green
26/26 locally). It also modeled the wrong condition.
**Layer 3, final: consume-once root suspensions.** The actual fatal state is
a SECOND rewind of the same root suspension: root suspends once per
`fiber_swap` out of it, but two parked fibers completing against one root
suspension epoch (a tool fiber + a collab fiber both waking around
`open:settled`) each trigger `finishContextSwitch(root)` — the second rewinds
already-consumed data → "unreachable executed". The shim already records
every suspension (including root's); the fix is simply to stop exempting
root from the validity check: first consumption proceeds synchronously (zero
added latency anywhere), the second is refused
(`[wx-asyncify] fiber-resume-refused: root suspension already consumed`) —
the yielded fiber stays properly suspended and resumable, root continues via
its real pending resume, same contract as libcontext's ghost epochs enforced
one layer lower. Root stays exempt ONLY from the internally-parked
quarantine (its yield park is routine — the 19-red lesson).
## The white screen, round 3 (fixed at the DOM level)