stream/iter: share() does not discard upstream results with "drop-newest" #64416

Version

main

Platform

macOS 26.5.2

Subsystem

stream

What steps will reproduce the bug?

import { share } from 'node:stream/iter';

async function* source() {
  for (let i = 0; i < 4; i++)
    yield [new TextEncoder().encode(`${i}`)];
}

const shared = share(source(), {
  highWaterMark: 2,
  backpressure: 'drop-newest',
});

const fast = shared.pull();
const slow = shared.pull();

const read = async (stream) => (await Array.fromAsync(stream))
  .flat()
  .map((chunk) => new TextDecoder().decode(chunk));

console.log('fast:', await read(fast));
console.log('bufferSize:', shared.bufferSize);
console.log('slow:', await read(slow));

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

Always

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

fast: [ '0', '1' ]
bufferSize: 2
slow: [ '0', '1' ]

Once the two-slot buffer is full, upstream results '2' and '3' are discarded under "drop-newest".
Neither consumer receives them, and the buffer never exceeds highWaterMark.

From §13.2.2 — “Share buffering and backpressure”

With "drop-newest", the upstream pull result is discarded.

What do you see instead?

fast: [ '0', '1', '2', '3' ]
bufferSize: 4
slow: [ '0', '1', '2', '3' ]

The buffer grows to four entries despite highWaterMark: 2, and the stalled consumer eventually receives every batch

Additional information

No response