Headers already sent when errors are thrown from AsyncIterator

[bhamon]

I've setup a basic test subscription that throws inside the async iterator:

  type Subscription {
    test: Date!
  }

The subscribe function:

import timers from 'node:timers/promises';
function resolverTest() {
  return (async function* () {
    const interval = timers.setInterval(1000);
    for await (const _ of interval) {
      yield {test: new Date()};
      throw new Error('test');
    }
  })();
}

Server setup:
I've copied it from the fastify TS file. The recipe from the website doc is not in sync and produce the same error but it was easier to debug with this one.

import Fastify from 'fastify';
import { createHandler } from 'graphql-sse/lib/use/fastify';

const handler = createHandler({ schema });
const fastify = Fastify();
fastify.all('/graphql/stream', async (req, reply) => {
  try {
    await handler(req, reply);
  } catch (err) {
    console.error(err);
    reply.code(500).send();
  }
});

fastify.listen({ port: 4000 });
console.log('Listening to port 4000');

When I subscribe to test the server crashes with the following error:
Error [ERR_HTTP_HEADERS_SENT]: Cannot write headers after they are sent to the client

It comes from the reply.code(500).send(). If I remove this line the client hangs indefinitely.

Would it make sense to forward the error into the classic errors property of the response before closing the stream?
Or is there another way to signal the error now that the stream is already open?
Or I am missing something?

[enisdenjo]

Sorry for the delayed answer. The issue actually stems from graphql-js not handling errors thrown during iteration. I'd prefer having this handled by the user (or graphql-js) instead.

I've added a recipe that showcases how you may achieve this by augmenting the subscribe function, check it out here.

[bhamon]

No need to apology, we are all busy. Instead, thanks a lot for your time and for this amazing library.
I'll use your recipe in the meantime and wait for a proper support from graphql-js.