awaiting between creating a readline interface, and using async iterator will cause iterator to sometimes skip.

[Tatsujinichi]

• Version:v12.13.0
• Platform:Linux myuser 4.15.0-74-generic Proposal: Introduce CI builds to GitHub PR process #84-Ubuntu SMP Thu Dec 19 08:06:28 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
• Subsystem:
  readline.createInterface
  asyncIterator

What steps will reproduce the bug?

if you perform something like
https://github.com/nodejs/node/blob/2a7432dadec08bbe7063d84f1aa4a6396807305c/test/parallel/test-readline-async-iterators.js

async function testSimple() {
  for (const fileContent of testContents) {
    fs.writeFileSync(filename, fileContent);

    const readable = fs.createReadStream(filename);
    const rli = readline.createInterface({
      input: readable,
      crlfDelay: Infinity
    });

    const iteratedLines = [];
    for await (const k of rli) {
      iteratedLines.push(k);
    }

    const expectedLines = fileContent.split('\n');
    if (expectedLines[expectedLines.length - 1] === '') {
      expectedLines.pop();
    }
    assert.deepStrictEqual(iteratedLines, expectedLines);
    assert.strictEqual(iteratedLines.join(''), fileContent.replace(/\n/gm, ''));
  }
}

If you add some kind of await xxx() between creating the interface and iterating, the iterator will miss lines. In my case I input 100k lines from a file, then output those same lines to a new file. several thousand lines will go missing.

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

100%
Need to add some await async between creating the interface and using for await

What is the expected behavior?

not to miss iterations

What do you see instead?

Additional information

[devsnek]

readline isn't a stream, if you don't attach a handler you will miss events. that being said, exposing an async iterable doesn't make that very clear.

[BridgeAR]

As @devsnek outlined there's nothing actually wrong with the behavior. We might want to improve our docs when it comes to that?

[preyunk]

@devsnek @BridgeAR I would like to take up this issue, could you guide me through the process for achieving the same as I am a first time contributor.

[jfriend00]

Similar problem shown here: https://stackoverflow.com/questions/62885667/why-does-this-readline-async-iterator-not-work-properly.

I see only this output in the console:

before for() loop
finished
finally
done

The for await (const line1 of rl1) loop never goes into the for loop - it just skips right over it:

const fs = require('fs');
const readline = require('readline');
const { once } = require('events');

async function test(file1, file2) {
    try {
        const stream1 = fs.createReadStream(file1);
        await once(stream1, 'open');
        const rl1 = readline.createInterface({input: stream1, crlfDelay: Infinity});

        const stream2 = fs.createReadStream(file2);
        await once(stream2, 'open');
        const rl2 = readline.createInterface({input: stream2, crlfDelay: Infinity});

        console.log('before for() loop');
        for await (const line1 of rl1) {
            console.log(line1);
        }
        console.log('finished');
    } finally {
        console.log('finally');
    }
}

test("data/numbers.txt", "data/letters.txt").then(() => {
    console.log(`done`);
}).catch(err => {
    console.log('Got rejected promise:', err);
})