stream/iter: pipeToSync() ignores writeSync() backpressure and overcounts bytes

[trivikr]

Version

26.2.0

Platform

macOS 26.5.0

Subsystem

stream

What steps will reproduce the bug?

import { setTimeout as delay } from 'node:timers/promises';
import { pipeToSync, push } from 'node:stream/iter';

const decoder = new TextDecoder();
const { writer, readable } = push({ highWaterMark: 1 });

const total = pipeToSync(['a', 'b'], writer, { preventClose: true });

const iter = readable[Symbol.asyncIterator]();
const first = await iter.next();

const second = await Promise.race([
 iter.next().then((r) => r.done ? '<done>' : decoder.decode(r.value[0])),
 delay(20, '<no second chunk>'),
]);

await iter.return?.();

console.log(`pipeToSync returned: ${total}`);
console.log(`first buffered chunk: ${decoder.decode(first.value[0])}`);
console.log(`second buffered chunk: ${second}`);

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

Always

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

pipeToSync returned: 1
first buffered chunk: a
second buffered chunk: <no second chunk>

pipeToSync() should throw or otherwise signal that the second sync write was not accepted. It should not count bytes for a chunk when writeSync() or writevSync() returns false without accepting the data.

What do you see instead?

pipeToSync returned: 2
first buffered chunk: a
second buffered chunk: <no second chunk>

pipeToSync() reports that 2 bytes were written even though the strict push({ highWaterMark: 1 }) writer only accepted a. The failed writeSync('b') return value is ignored, so data is silently dropped and byte accounting is incorrect.

Additional information

No response

[trivikr]

The behavior proposed in the report should be clarified in the specification

Details WinterTC55/iter-streams#16