`readableStream.tee()` doesn't work when using `process.nextTick(...)`

[szmarczak]

Version

v16.6.2

Platform

Linux solus 5.13.8-191.current #1 SMP PREEMPT Fri Aug 6 11:29:58 UTC 2021 x86_64 GNU/Linux

Subsystem

stream/web

What steps will reproduce the bug?

import {ReadableStream} from 'stream/web';

let controller;
let start = c => (controller = c);
let pull = () => {
 if (pull.called) return;
 pull.called = true;

 process.nextTick(() => {
 controller.enqueue(new Uint8Array([102, 111, 111, 98, 97, 114]))

 process.nextTick(() => {
 controller.close()
 });
 });
};

const [a, b] = new ReadableStream({start, pull}).tee();

for (const stream of [a, b]) {
 const chunks = [];

 for await (const chunk of stream) {
 chunks.push(chunk);
 }

 console.log('received:', Buffer.concat(chunks).toString());
}

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

Always.

What is the expected behavior?

received: foobar
received: foobar

What do you see instead?

received: foobar
received:

Additional information

Deno for reference: https://github.com/denoland/deno/blob/a0285e2eb88f6254f6494b0ecd1878db3a3b2a58/ext/web/06_streams.js#L1465-L1543

/cc @ronag @mcollina @jasnell

[ronag]

@jasnell @nodejs/streams

[szmarczak]

If to replace process.nextTick with queueMicrotask in the example then it works. Maybe a bug (race condition) in the spec?

Edit:

The spec uses queueMicrotask but process.nextTick results in race condition, so we need to controller.close() when all other web streams work is done - Promise.resolve() should be sufficient enough in the example.

[ronag]

@jasnell

[jasnell]

Yeah, I've got this queued up to look at this week. Just haven't yet had the opportunity.

[ronag]

@jasnell

[ronag]

@jasnell Sorry to bother. I think this is a rather bad bug. Any chance you will have time to look at it?

[ofirbarak]

Hi, I'll work on it

[jasnell]

Sorry I've been buried and haven't had the opportunity to dig in. It's not a bug in the spec, keep in mind that nextTick is a Node specific concept.

[ronag]

@szmarczak We had this issue on undici and there we don't really use nextTick in this context? i.e. we always wrap controller.close in a microtask to ensure this doesn't happen (https://github.com/nodejs/undici/blob/main/lib/fetch/util.js#L153-L155). Is it just related to nextTick?

[szmarczak]

Well Node.js streams use nextTick so that's where the incompatibility comes from. I'm thinking of the finished function exactly.

[RafaelGSS]

I've created a PR to address it. However, just to bring more context to the described issue:

import {ReadableStream} from 'stream/web';

let controller;
let start = c => (controller = c);
let pull = () => {
 if (pull.called) return;
 pull.called = true;

 process.nextTick(() => {
 controller.enqueue(new Uint8Array([102, 111, 111, 98, 97, 114]))

 process.nextTick(() => {
 controller.close()
 });
 });
};

const [a, b] = new ReadableStream({start, pull}).tee();

for (const stream of [a, b]) {
 const chunks = [];

 for await (const chunk of stream) {
 chunks.push(chunk);
 }

 console.log('received:', Buffer.concat(chunks).toString());
}

As mentioned by @szmarczak it returns:

received: foobar
received:

On ES Modules, however, when you change it to commonjs it works as expected:

- import{ReadableStream} from 'stream/web';
+ const{ReadableStream} = require('stream/web');

received: foobar
received: foobar

---

The #40901 aims to solve it