streamSSE() throws an error on close

[hond-hexagon]

What version of Hono are you using?

3.12.0

What runtime/platform is your app running on?

Cloudflare Workers

What steps can reproduce the bug?

I have this piece of code:

let sseId = 0;

app.get('/stream-sse', async (c) => {
  return streamSSE(c, async (stream) => {
    for (let i = 0; i < 5; i++) {
      await new Promise((resolve) => setTimeout(resolve, 200));

      const jsonValue = JSON.stringify({ progress: i });
      await stream.writeSSE({ data: jsonValue, id: String(sseId++) });
    }

    console.log('finished?');
  });
});

Once the loop is over and the Response should close, it throws an error of an uncaught exception which over time throws much severe error in the Cloudflare worker.

I tried to replicate an SSE stream with a plain Worker and everything works as expected:

let nonce = 0;

export default {
  async fetch(): Promise<Response> {
    const encoder = new TextEncoder();

    const readableStream = new ReadableStream({
      async pull(controller) {
        for (let i = 0; i < 5; i++) {
          await new Promise((resolve) => setTimeout(resolve, 200));

          const jsonValue = JSON.stringify({ progress: i });
          const sseValue = [`data: ${jsonValue}`, `id: ${nonce++}`].join('\n') + '\n\n';
          const encodedValue = encoder.encode(sseValue);
          controller.enqueue(encodedValue);
        }

        controller.close();
      },
    });

    return new Response(readableStream, {
      headers: {
        'Transfer-Encoding': 'chunked',
        'Content-Type': 'text/event-stream',
        'Cache-Control': 'no-cache',
        Connection: 'keep-alive',
      },
    });
  },
};

After digging into hono source code, it seems that flipping preventClose to false in stream.ts solves the issue, a response is closed and there is no uncaught exception. However, I'm not sure what implication this change could have.

What is the expected behavior?

Response should properly close once streamSSE() callback is finished.

What do you see instead?

[wrangler:inf] GET /stream-sse 200 OK (22ms)
✘ [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 [object Object]

✘ [ERROR] Uncaught (in response) Error: The script will never generate a response.

Additional information

It behaves the same with locally running wrangler or deployed to Cloudflare.

[hond-hexagon]

It might be related to #1902 but this issue originates from a server not from a client.

[yusukebe]

Hi @hond-hexagon

Thanks for raising the issue.

@sor4chi or @watany-dev Could you see this issue?

[yusukebe]

This has been closed by #2529 . Thanks!