Loader stalls MessagePort since v24 #63085

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1. Sahitya3105 commented on May 3, 2026

   

   Sahitya3105

   on May 3, 2026

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   Hi! I’m interested in working on this issue. Could you please assign it to me?
2. SudhansuBandha commented on May 14, 2026

   

   SudhansuBandha

   on May 14, 2026

   Last edited by SudhansuBandha

   Contributor

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   It looks like, the loader architeture for version newer than 24.11.1 does not support this async communication pattern inside resolve() hook.

   In newer Node.js versions, resolve() runs through a synchronous request/response flow between the main thread and the loader worker. While resolve() is running, the worker cannot process incoming MessagePort events needed to resolve the promise.

   Providing a synchronous loader.mjs resolves this issue @cinderflash

   let port
   
   // Store port for talking to main thread
   export async function initialize (...args) {
     port = args[0]
     port.start()
   
     // Listen for PONGs
     port.addEventListener('message', event => {
       console.log(event.data);
     })
   }
   
   export async function resolve(specifier, context, next) {
     port.postMessage('ping');
     return next(specifier, context)
   }
   
3. rochdev

   added a commit that references this issue on Jul 9, 2026

   fix(guardrails): forward abort telemetry synchronously to avoid a hun…

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   05a9f7c
4. dd-octo-sts

   added 2 commits that reference this issue on Jul 10, 2026

   fix(guardrails): forward abort telemetry synchronously to avoid a hun…

   ...

   f724484

   fix(guardrails): forward abort telemetry synchronously to avoid a hun…

   ...

   c79d4e0
5. juan-fernandez

   added 2 commits that reference this issue on Jul 10, 2026

   fix(guardrails): forward abort telemetry synchronously to avoid a hun…

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   b0d841a

   fix(guardrails): forward abort telemetry synchronously to avoid a hun…

   ...

   3194dc0
6. bitpshr commented on Aug 9, 2026

   

   bitpshr

   on Aug 9, 2026

   Contributor

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   Confirming this reproduces, and I narrowed down where it starts. The first bad version is v24.12.0: v24.11.1 works, and v24.12.0 through v26.5.0 all stall, consistently across repeated runs.

   It looks like the cause is #60380 ("esm: use sync loading/resolving on non-loader-hook thread"), which landed in 24.12.0. Resolution is now synchronous from the non-loader-hook thread, so the main thread sits in AtomicsWait while resolve() runs and never gets to service the MessagePort exchange the hook is waiting on. The hook waits for a reply only the blocked thread can send, so both sides deadlock.

   That also explains why there's no error. The worker only reports never-settle from its beforeExit handler in lib/internal/modules/esm/worker.js, and calling port.start() in the hook keeps a live handle on the worker's event loop, so beforeExit never fires. The process just hangs indefinitely instead of throwing ERR_ASYNC_LOADER_REQUEST_NEVER_SETTLED (I left it 40s with no output).

   Minimal repro:

   // loader.mjs
   let port;
   export async function initialize(p) { port = p; port.start(); }
   export async function resolve(specifier, context, next) {
     await new Promise((res) => {
       port.addEventListener('message', (e) => res(e.data), { once: true });
       port.postMessage('ping');
     });
     return next(specifier, context);
   }

   // index.mjs
   import { register } from 'node:module';
   import { MessageChannel } from 'node:worker_threads';
   const { port1, port2 } = new MessageChannel();
   port1.on('message', () => port1.postMessage('PONG'));
   port1.unref();
   register('./loader.mjs', { parentURL: import.meta.url, data: port2, transferList: [port2] });
   await import('./target.mjs');

   Happy to open a PR with whichever direction you'd prefer, whether that's documenting the limitation or making the hang surface an error instead.
7. bitpshr

   added a commit that references this issue on Oct 14, 2026

   doc: warn against awaiting replies in module hooks

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   05c5619
8. nodejs-github-bot

   added a commit that references this issue on Oct 16, 2026

   doc: warn against awaiting replies in module hooks

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   c0c8ce8
9. nodejs-github-bot

   closed this as completedin #65590on Oct 16, 2026