Readline does not appear to fully parse long stdin streams when printing

[rgrannell1]

• Version:
• Platform:
• Subsystem:

• node-version: 6.0.0
• platform: Linux ryan-S400CA 4.4.0-34-generic [streams] _flush not being called #53-Ubuntu SMP Wed Jul 27 16:06:39 UTC 2016 x86_64 x86_64 x86_64 GNU/Linux
• subsystem: Readline, stream-handling

Observed Behaviour

readline.createInterface({input: process.stdin}) does not print each line and then emit the close event in a program designed to do this; instead it prints a random number of lines from stdin before exiting silently.

I'm unsure whether this is due to a misconception I have about the test-program, or whether this is a genuine bug. If any more information is needed, leave a comment and I'll respond quickly.

Expected Behaviour

readline should print each line from stdin, and then emit the close event.

Data

The program below tries to print each line of War and Peace (roughly 65,000 lines long), which it receives from stdin. When not printing, it always manages to emit the close event before exiting. When printing, the program exists prematurely after printing roughly 1900 lines.

1925
1947
65325
1936
1947
1925
65325
1936
65325
65325
65325
1936
65325
1936
1947
1925

Test Proceedure

1. Download war and peace, as a text input file. Store it in ~/test-input.txt
2. Save the following program to ~/test-program.js:

"use strict"




const path      = require('path')
const fs        = require('fs')
const readline  = require('readline')

const inputPath = path.join(process.env['HOME'], 'test-input.txt')
const lines     = fs.readFileSync(inputPath).toString( ).split('\n')

var counts = {
    expected: lines.length,
    actual:   0
}





function performTest (shouldPrint) {

    readline.createInterface({input: process.stdin})
    .on('line', line => {

        if (shouldPrint) {
            console.log(line)
        }
        counts.actual++

    })
    .on('close', ( ) => {
        throw new Error(JSON.stringify(counts))
    })

}

performTest(false)

1. Run the following command from a unix OS (sorry about the useless use of cat, but its how I initially noticed the problem). Repeat several times:

cat ~/test-input.text | node ~/test-program.js

# EDIT: this behaves similarly 
node ~/test-program.js < ~/test-input.text 

        throw new Error(JSON.stringify(counts))
        ^

Error: {"expected":65326,"actual":65325}

You will repeatedly see an error showing the entire file was read.

1. Replace

performTest(false)

with

performTest(true)

and rerun the test several times. The results should vary; on my machine it either prints out a line
~1900 lines into the file, or reaches the end of the file.

[mscdex]

I think this may have to do with writes to a tty being asynchronous in some cases. This was changed in ab3306a, which as of this writing hasn't been included in a release yet.

[rgrannell1]

@mscdex Thanks for your response. Has this commit been added to Node.js yet?

[mscdex]

@rgrannell1 It was landed for v6.2.1 (and also in v7.0.0+). However, be aware that the environment flag has been removed for v8.0.0 and it is now set to always block for TTYs on all platforms.

[rgrannell1]

Thanks @mscdex, this test passes for me now