FS promises does not work well with big files

[aduh95]

• Version: v10.0.0-nightly20180318d74184c2fa
• Platform: Ubuntu Linux
• Subsystem: fs/promises

When dealing with big files, the beginning of the file is cut. I was not able to reproduce this with small files.

$ wget -O emojis.json https://api.github.com/emojis
$ node -p "require('fs').readFile('./emojis.json',(err,buffer)=>console.log(buffer.toString('utf8', 0, 100)))"
undefined
{
  "+1": "https://assets-cdn.github.com/images/icons/emoji/unicode/1f44d.png?v8",
  "-1": "https://
$ node -p "require('fs/promises').readFile('./emojis.json').then(buffer=>console.log(buffer.toString('utf8', 0, 100)))"
Promise { <pending> }
(node:13241) ExperimentalWarning: The fs/promises API is experimental
arge_square": "https://assets-cdn.github.com/images/icons/emoji/unicode/2b1c.png?v8",
  "white_mediu

I made a script to compare different approches, and fs/promises seems to be the only one impacted by thus bug.

const { promisify } = require("util");

const fs = require("fs");
const fsPromises = require("fs/promises");

const promisifiedRead = promisify(fs.readFile);

const JSON_FILE = process.argv[2];

fs.readFile(JSON_FILE, (err, buffer) => {
  try {
    JSON.parse(buffer);
    console.log("FS OK");
  } catch (e) {
    console.error("FS Failed");
  }
});
fsPromises.readFile(JSON_FILE).then(buffer => {
  try {
    JSON.parse(buffer);
    console.log("FS/promises OK");
  } catch (e) {
    console.error("FS/promises Failed");
  }
});
promisifiedRead(JSON_FILE).then(buffer => {
  try {
    JSON.parse(buffer);
    console.log("FS promisified OK");
  } catch (e) {
    console.error("FS promisified Failed");
  }
});

I tried with the following:

$  wget -O emojis.json https://api.github.com/emojis
$ node json.js ./emoji.json
(node:11797) ExperimentalWarning: The fs/promises API is experimental
FS/promises Failed
FS OK
FS promisified OK

I used the following script to find the thershold where the promises API starts to work:

const fs = require("fs");
const fsPromises = require("fs/promises");

const JSON_FILE = process.argv[2];

const readAndRemoveOneLine = () =>
  fs.readFile(JSON_FILE, (err, buffer) => {
    const data = JSON.parse(buffer);
    delete data[Object.keys(data)[0]];
    fs.writeFile(
      JSON_FILE,
      JSON.stringify(data),
      err =>
        err
          ? console.error(err)
          : fsPromises
              .readFile(JSON_FILE)
              .then(
                buffer =>
                  JSON.parse(buffer) &&
                  console.log("API works for that size:", buffer.length)
              )
              .catch(readAndRemoveOneLine)
    );
  });

readAndRemoveOneLine();

And the API seems to work fine for files smaller than 16 kiB.

$ wget -O emojis.json https://api.github.com/emojis
$ node threshold.js emojis.json
(node:12609) ExperimentalWarning: The fs/promises API is experimental
API works for that size: 16368

[aduh95]

Fixed by #19643, landed in 11819c.