pcbjam/docs/features/async/16-fiber-resume-guard.md

16 — Fiber resume guard (the production board-load trap, fixed)

Status: 2026-07-31 · red/green e2e tests/kicad/fiber-resume-park.spec.ts · lineage: 14 (open-settle gate), 15 (timer-park lever + the v0.1.20 decode), drift-trio #10b (fiber buffers, ghost beacons).

The bug

RuntimeError: index out of bounds / unreachable executed on prod board loads (editor.pcbjam.com, v0.1.12 → v0.1.20), runtime permanently poisoned afterwards. Survived three shipped fixes aimed at other reentrancy holes (open-settle gate, dispatch interlock, busy-guard-holds-interlock).

Root cause — two suspension protocols share one context. KiCad coroutines (TOOL_MANAGER tool loops, collab fiber bodies) run on emscripten fibers via the libcontext wasm port. A coroutine suspended by a real yield (jump_fcontext → emscripten_fiber_swap) has valid rewind state in its fiber struct. A coroutine whose body is asyncify-parked inside handleSleep (lib-bridge wait, emscripten_sleep, any EM_ASYNC_JS below a tool handler) is suspended in a way the fiber machinery — and TOOL_MANAGER — cannot see: its live state sits in the sleep's buffer, and the fiber struct still holds the stale data of its LAST real yield. Natively this state cannot exist (a coroutine cannot be suspended without yielding), so no upstream code guards it.

When the next event arrives during such a park, TOOL_MANAGER Resume()s the "waiting" tool → finishContextSwitch rewinds the STALE fiber suspension → binaryen's rewind-path mismatch unreachable, or replayed frames clobber the parked body's stack; the sleep's wake then rewinds into wreckage. Every later Asyncify entry (the GAL refresh timer's emscripten_async_call cascade was the usual first victim, which is why the symbolization kept pointing at timers) reads poisoned state → index out of bounds.

Trigger window in prod: right after open:settled, presync/realtime lib churn + first-paint activity dispatch events while tool bodies still park in lib waits. Fast loads crash too — the window needs only one event during one park.

The evidence chain

1. v0.1.20 shipped rate-limited [wx-asyncify] reporting in the handlesleep shim + the [wx-timer]/[wx-dispatch] channels. The crashing prod log (console-export-2026-7-31_22-33-27.log) showed NO concurrent-park, NO retry storms, NO depth erasure — every competing hypothesis ruled out on a live crash — and one overlapped-wake … over null 2 ms after settle: a fiber fingerprint (finishContextSwitch/fiber_swap are the only mid-flight sources of a null currData).
2. The trap-2 stack decoded mechanically against the shipped runtime: maybeStopUnwind → Fibers.trampoline → finishContextSwitch → doRewind → unreachable — a fiber swap-in rewinding stale data at the tail of an unwind.
3. The lever (wasm/bindings/fiber_park.h, kicadTestFiberPark*) stages the state machine deterministically: Call + yield (valid suspension) → legitimate resume → body parks in emscripten_sleep → Resume during the park. RED on the unguarded runtime: fiber-buffer/sleep-buffer cross-restores (aliased-wake-live 103710720 ⇄ 406683648), a jump-ghost beacon, and the parked body zombified (its wake lost — in prod, the next timer/tool entry turns this into the loud trap).

The fix

kicad/thirdparty/libcontext/libcontext.cpp (wasm port only): wasm_fcontext::swap_suspended — true only between a real fiber_swap suspension and the next swap-in (set at both swap sites; fresh contexts start true, the entry-point path needs no rewind). jump_fcontext REFUSES a target whose flag is false — returns 0, the null-INVOCATION_ARGS contract the jump-ghost path already established, with a rate-limited [collab-fcontext] jump-refused beacon. The refused dispatch is dropped; the parked body completes via its own wake, suspends properly, and the next dispatch lands normally.

Why refuse rather than wait: a waiting guard (park the resumer until the target suspends) deadlocks against the yield-back path — the target's yield needs to swap into the very context that is busy waiting.

Round 2 (2026-08-01) — v0.1.21 still trapped: the guard was laundered

The prod crash reproduced on v0.1.21 with ZERO jump-refused beacons: the fatal swap passed the swap_suspended check. Mechanism: when a fresh JS entry executes while g_current_context still points at a parked fiber (the port has no way to know a new JS turn began), jump_fcontext attributes the jump's old side to that parked fiber — fiber_swap then writes a fresh (foreign but valid-looking) suspension INTO the parked fiber's struct and re-marks it swap_suspended. The flag lies; the later Resume passes the C++ guard and rewinds garbage.

Layer 2 (attribution-proof, JS runtime): handlesleep.js wraps Fibers.finishContextSwitch and tracks truth at the emscripten-fiber layer:

• a fiber is validly suspended only when a real swap-out wrote its suspension (observable: fiber_swap leaves currData = oldFiber+20 when the trampoline runs);
• a fiber whose entered slice ends in a handleSleep park (currData holds a sleep buffer, no nextFiber) is internally parked — quarantined until a GENUINE swap-out, where genuine means its pending sleep has resolved (checked against the shim's __pendingSleepContexts); a laundering write while the sleep is still pending does not lift the quarantine;
• entering a quarantined or suspension-less fiber is REFUSED ([wx-asyncify] fiber-resume-refused beacon, currData cleared, ghost contract as usual).

Lever for the laundered scenario: kicadTestFiberParkStartSecond/PokeSecond (a second coroutine started while the first is parked = the misattributed jump), spec scenario 2 asserts the refusal beacon fires AND everything completes cleanly.

The white screen itself (fixed this round)

The prod logs also revealed why every fatal-overlay attempt failed: the last trap of each cascade lands inside a React EFFECT (an embind call via a react-query subscription), React unmounts the entire root, and the overlay + console die with the tree. Fix: WasmErrorBoundary inside WasmTool — all crash-capable children live inside it; the fatal screen (now an actual blue screen) and the console panel live OUTSIDE it and survive. All fatal promotions auto-open the console. Pinned by tests/web/fatal-overlay.spec.ts.

Verification

• fiber-resume-park.spec.ts red on unguarded build (phase-3 poll dies), green with the guard: mid-park poke refused + logged, body completes its park, second yield reached, post-yield resume works, no trap signatures, model walk functional afterwards.
• timer-park-repro.spec.ts, collab-load-fuzz.spec.ts, load-pcb.spec.ts stay green (guard must not refuse valid suspensions).
• Prod validation: watch for jump-refused beacons in the next crash-free Leonardo load — each one is a would-have-been crash.