🐛 BUG: Server Sent Events (SSE) causes Error: The script will never generate a response.

[shmuli9]

Which Cloudflare product(s) does this pertain to?

Wrangler, Miniflare, Pages

What versions are you using?

3.99.0 [wrangler], 3.20241218.0 [miniflare],

What operating system and version are you using?

Ubuntu [WSL]

Please provide a link to a minimal reproduction

No response

Describe the Bug

I have implemented server sent events (SSE) in a Remix/React Router v7 app using remix-utils and node native, EventEmitter:

import { EventEmitter } from "node:events";

import { eventStream } from "remix-utils/sse/server";
import type { Route } from "./+types/seed.logs";

export const seedEmitter = new EventEmitter();

export async function loader({ request }: Route.LoaderArgs) {
  let batchedLogs: string[] = [];
  let timeoutId: NodeJS.Timeout;

  return eventStream(request.signal, function setup(send, abort) {
    function handleLog(message: string) {
      batchedLogs.push(message);

      // Clear existing timeout
      if (timeoutId) clearTimeout(timeoutId);

      // Send batched logs every 250ms
      timeoutId = setTimeout(() => {
        if (batchedLogs.length > 0) {
          send({ event: "log", data: JSON.stringify(batchedLogs) });
          batchedLogs = [];
        }
      }, 250);
    }

    seedEmitter.on("log", handleLog);

    return function cleanup() {
      seedEmitter.off("log", handleLog);
      if (timeoutId) clearTimeout(timeoutId);
      abort();
    };
  });
}

The event emitter is shared and can be triggered elsewhere in the app. the handleLog function queues up messages for 250ms
When running this under node (eg react-router dev) it works perfectly - events stream to UI as expected (using useEventSource from remix-utils)
When building and running under workerd (eg wrangler pages dev) I get the errors:

[wrangler:inf] GET /admin/seed/logs 200 OK (13ms)
✘ [ERROR] Uncaught (in response) Error: The script will never generate a response.

✘ [ERROR] A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.

✘ [ERROR] workerd/server/server.c++:3898: error: Uncaught exception: workerd/io/io-context.c++:1209: failed: remote.jsg.Error: The script will never generate a response.

  stack:
  /home/user/code/remix-project/node_modules/.pnpm/@[email protected]/node_modules/@cloudflare/workerd-linux-64/bin/workerd@286cb40

This seems like a bug to me as EventEmitter and ReadableStream/streaming responses are officially supported (and working in remix/react-router7 generally as streaming promises works as expected).

I have tried adding a ctx.waitUntil in the functions handler, but this doesnt seem to do anything here

For completeness, this is my [[path]].ts file:

import { createPagesFunctionHandler } from "@react-router/cloudflare";

import { getLogger } from "../app/services/logger.server";
// eslint-disable-next-line @typescript-eslint/ban-ts-comment
// @ts-ignore - the server build file is generated by `remix vite:build`
// eslint-disable-next-line import/no-unresolved
import * as build from "../build/server";
import { getLoadContext } from "../load-context";

export const onRequest = async ({
  request,
  ...ctx
}: EventContext<unknown, string, unknown>) => {
  const logger = getLogger({
    request,
    context: { cloudflare: { ctx, env: ctx.env } },
  });

  const response = createPagesFunctionHandler({
    // @ts-expect-error - the types are not correct
    build,
    getLoadContext,
  })({ request, ...ctx });

  ctx.waitUntil(logger.flush());

  return response;
};

Any help/guidance is welcome. This works so nicely... when it works

Please provide any relevant error logs

[wrangler:inf] GET /admin/seed/logs 200 OK (13ms)
✘ [ERROR] Uncaught (in response) Error: The script will never generate a response.

✘ [ERROR] A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.

✘ [ERROR] workerd/server/server.c++:3898: error: Uncaught exception: workerd/io/io-context.c++:1209: failed: remote.jsg.Error: The script will never generate a response.

  stack:
  /home/user/code/remix-project/node_modules/.pnpm/@[email protected]/node_modules/@cloudflare/workerd-linux-64/bin/workerd@286cb40