stream/iter: merge() proactively drains fast sources while merged consumer is idle

[trivikr]

Version

26.2.0

Platform

macOS 26.5.0

Subsystem

stream

What steps will reproduce the bug?

import { merge } from 'node:stream/iter';
import { setTimeout as sleep } from 'node:timers/promises';

function source(n) {
  return {
    pulls: 0,
    async *[Symbol.asyncIterator]() {
      while (this.pulls < n) yield [Buffer.from(`${++this.pulls}`)];
    },
  };
}

const a = source(5);
const b = source(5);
const it = merge(a, b)[Symbol.asyncIterator]();

await it.next();
await sleep(20);

console.log({ a: a.pulls, b: b.pulls });
await it.return?.();

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

after only one await it.next() from the merged iterator, both sources should not be pulled to completion while the merged consumer is idle.

What do you see instead?

{ a: 5, b: 5 }

Additional information

No response