[Prototype] Node 18: let node-fibers run await continuations on a fiber (V8 microtask dispatch)

[jbaczuk-qualia]

Summary

R&D prototype: lets node-fibers run the code after an await (and .then callbacks) on a fiber when that code was registered inside a fiber. Today it resumes on the main stack, so a fiber-only call after await fails with Can't wait without a fiber:

Meteor.startup(async () => {                       // runs inside withFiber
  await OrdersAsync.countDocuments({});
  Orders.find().count();                           // stock node: "Can't wait without a fiber"
});

Based on znewsham/custom-v18 (f031bb860e, the source of nodejs-dev:f031bb860e, v18.16.1, V8 10.2). One commit, V8 only:

File	Change
deps/v8/include/v8-microtask-dispatch.h (new)	SetMicrotaskDispatchCallback / RunDispatchedMicrotask, plus unmangled v8_qualia_* C entry points so an addon can find them with dlsym and still load on unpatched node
deps/v8/src/builtins/builtins-microtask-queue-gen.cc	Before running a promise fulfill/reject reaction job whose continuation-preserved embedder data (CPED, captured when the reaction was registered) is not undefined, call Runtime::kDispatchMicrotask; if the embedder took the job, skip it. After a reaction job, restore the previous CPED instead of clearing it, so a job run on a fiber leaves the fiber's marker in place
deps/v8/src/runtime/runtime-promise.cc, runtime.h	Runtime_DispatchMicrotask: moves the job into a private single-job MicrotaskQueue (a GC root) and offers it to the embedder callback. RunDispatchedMicrotask runs that queue through the normal RunMicrotasks builtin, so context entry, CPED, promise hooks (async_hooks / AsyncLocalStorage) and exception reporting are unchanged

No effect unless an embedder registers a callback and something sets CPED (nothing in node 18 does). The fibers side is qualialabs/node-fibers#6 (and the Promise.await side, qualialabs/shared-packages#265).

Testing

Built in a dev-qualia:latest container (arm64, glibc of the qli5 image), with the fibers PR:

• Standalone checks (await then fiber-only call, nested awaits, .then, AsyncLocalStorage across the hop, Promise.all overlap, errors, fiberless code untouched, microtask order): 11/11 pass; FIBERS_AWAIT_DISPATCH=0 fails the expected 7 (same as stock node).
• node-fibers test/: identical results on stock vs patched node (pool.js/cleanup.js segfault on both, the known pthread coro_destroy bug).
• Local qli5 qualia boots on this build and the scratch startup hook above logs both counts.

Risk / open questions

• Prototype: not load-tested, no Lucky run yet. Every resume of fibered async code now costs a fiber switch (OS-thread handoff on CORO_PTHREAD).
• Patches the microtask runner builtin, so it has to be re-ported on each node/V8 upgrade (node 24 would also need a non-ACF CPED story; servers run --no-async-context-frame).
• PromiseResolveThenableJobTask is not dispatched (the reaction it creates carries the outer promise's CPED, so user continuations are still tagged).

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