Producer reads one chunk per Redis round trip while a client is rejoined

[nikolailehbrink]

Version: resumable-stream 2.2.13 (redis 6.3.0, Upstash)

Problem

In createNewResumableStream, the producer's read loop awaits a publish to every rejoined listener before it reads the next chunk:

for (const listenerId of listenerChannels) {
  promises.push(ctx.publisher.publish(`${ctx.keyPrefix}:chunk:${listenerId}`, value));
}
await Promise.all(promises);
read();

Once a client rejoins with resumeExistingStream, the producer reads at most one chunk per Redis round trip. The source stream is usually a tee of an AI SDK UI message stream, so backpressure slows the whole pipeline: generation, onFinish persistence, and reacting to an abort signal. With about 55 ms round trip time (a laptop talking to a regional Upstash database), a rejoined reply streamed about 350 characters in 3 seconds instead of about 2000, and a stop requested meanwhile only took effect after the buffered backlog was drained.

Stale listeners (#48, #49) make this worse, because every chunk then waits on publishes to channels nobody reads anymore.

Expected

Reading the source does not wait for publishes to listeners. One publisher connection already keeps publishes in order, so awaiting each publish is not needed for ordering.

Suggested fix

Publish without awaiting and handle rejections:

for (const listenerId of listenerChannels) {
  ctx.publisher.publish(`${ctx.keyPrefix}:chunk:${listenerId}`, value).catch((e) => {
    if (isDebug()) console.error(e);
  });
}
read();

We apply this as a patch on 2.2.13 and are happy to open a PR.