[REG 18.10.0->18.11.0] fs.promises.readFile from stdin is broken for slow input

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Version

18.11.0-18.19.0, 20.11.1

Platform

Linux

Subsystem

fs

What steps will reproduce the bug?

$ (echo -n '{'; sleep 1; echo -n '}') | docker run --rm -i node:18.10.0-alpine -e "const fs = require('fs/promises'); fs.readFile('/dev/stdin').then((data) => console.log(data.length))"
2
$ (echo -n '{'; sleep 1; echo -n '}') | docker run --rm -i node:18.11.0-alpine -e "const fs = require('fs/promises'); fs.readFile('/dev/stdin').then((data) => console.log(data.length))"
1

Reproducible also without docker:

$ (echo -n '{'; sleep 1; echo -n '}') | node -e "const fs = require('fs/promises'); fs.readFile('/dev/stdin').then((data) => console.log(data.length))"
1

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

Always.

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

It should read stdin until EOF is reached.

What do you see instead?

It stops too early.

Additional information

This was broken by 67fb765 (and its other versions in the other branches).

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@BridgeAR @danielleadams

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Reference issue:

• fs.promises.readFile is 40% slower than fs.readFile #37583