stream.PassThrough not draining (from Node 16)

[davedoesdev]

Version

v16.13.0

Platform

Linux david-Latitude-E6440 5.13.0-21-generic #21-Ubuntu SMP Tue Oct 19 08:59:28 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

stream

What steps will reproduce the bug?

const { PassThrough } = require('stream');
const pt = new PassThrough({ highWaterMark: 0 });
pt.on('drain', () => console.log('drained'));
console.log(pt.write('hello'));
console.log(pt.writableLength, pt.writableNeedDrain);
console.log(pt.read().toString());
console.log(pt.writableLength, pt.writableNeedDrain);

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

Every time

What is the expected behavior?

On Node 14:

david@david-Latitude-E6440:/tmp$ node --version
v14.18.1
david@david-Latitude-E6440:/tmp$ node repro.js 
false
5 true
drained
hello
0 false

What do you see instead?

On Node 16:

david@david-Latitude-E6440:/tmp$ node --version
v16.13.0
david@david-Latitude-E6440:/tmp$ node repro.js 
false
5 true
hello
5 true

Additional information

No response

[davedoesdev]

I think this is due to the change to transforming the data on write instead of read.
Therefore the readable length gets to 5 when write() is called, i.e. before read(), and https://github.com/nodejs/node/blame/master/lib/internal/streams/readable.js#L474 doesn't trigger.
Whereas on Node 14, the length is 0 when read() is called so we see length less than watermark.
Perhaps a highWaterMark of 0 isn't going to work going forward?
FYI I'm using it for testing.