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v16.5.0 change in async writable._final() behaviour #39535

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@tchetwin

Description

@tchetwin

Version

16.5.0

Platform

Microsoft Windows NT 10.0.19041.0 x64

Subsystem

stream

What steps will reproduce the bug?

As a result of #39329, a program that works in v16.4.2 no longer works in v16.5.0.
The issue occurs when an async writable._final() implementation explicitly calls the callback argument.
In v16.5.0 the callback-call and async-resolve combined result in [ERR_MULTIPLE_CALLBACK]: Callback called multiple times in some circumstances.

import { Duplex, Readable } from "stream";
import { setTimeout } from "timers/promises";

const slowDuplex = new Duplex({
    read(_size) {
        setTimeout(1000).then(() => {
            this.push("abc\n");
            this.push(null);
        })
    },
    async final(cb) {
        // both this `cb()` and the `_final()` Promise resolving
        // will trigger `onFinish` under the hood.
        cb();
    },
});

slowDuplex.on("error", (err) => {
    console.log(`"error" fired\n  ${err}`);
});

Readable.from([]).pipe(slowDuplex).pipe(process.stdout);

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

Always

What is the expected behavior?

$ nvm run 16.4.2 repro.mjs
Running node v16.4.2 (npm v7.18.1)
abc

What do you see instead?

$ nvm run 16.5.0 repro.mjs
Running node v16.5.0 (npm v7.19.1)
"error" fired
  Error [ERR_MULTIPLE_CALLBACK]: Callback called multiple times

Additional information

Documentation

I don't believe the _final() documentation sets expectations about the behaviour when it is async (or a Promise returner). The source code before and after the change in question seems to point towards it being an intended means of implementing the _final().

(Unless I've missed a part of the documentation): Users equipped only with this documentation and without looking at the implementation may be relying on the previous behaviour.

Types

The DefinitelyTyped types could potentially encode this expectation with an overload for void-returners having the argument, and Promise-returners not having the argument.

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