stream/iter: pipeTo retries accepted PushWriter writes under block backpressure

[trivikr]

Version

26.1.0

Platform

macOS 26.5.0

Subsystem

stream

What steps will reproduce the bug?

import { pipeTo, push, text } from "node:stream/iter";

const enc = new TextEncoder();

async function repro(source) {
 const { writer, readable } = push({
 highWaterMark: 1,
 backpressure: "block",
 });

 const piping = pipeTo(source(), writer);

 // Let pipeTo() fill the PushWriter queue before the reader drains it.
 await new Promise(setImmediate);

 const output = await text(readable);
 await piping;
 return output;
}

async function* writevSource() {
 yield [enc.encode("ab")];
 yield [enc.encode("c"), enc.encode("d")];
}

async function* writeSource() {
 yield [enc.encode("a")];
 yield [enc.encode("b")];
}

console.log(await repro(writevSource)); // actual: abcdcd, expected: abcd
console.log(await repro(writeSource)); // actual: abbb, expected: ab

Run with

node --experimental-stream-iter repro.js

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

Always

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

abcd
ab

What do you see instead?

abcdcd
abbb

Additional information

No response