fs writeFile after truncate doesn't write at beginning of the file

[kghost]

• Version: v11.3.0
• Platform: Win 10

Test case:

var fs = require('fs').promises;

async function bug() {
  await fs.writeFile('test', '012345678901234567890123456789');
  const fd = await fs.open('test', 'r+');
  await fd.readFile({ encoding: 'utf8' });
  await fd.truncate(0);
  await fd.writeFile('Extra data not at beginning');
  await fd.close();
}

bug();

This will result a file with lots of '\0' at beginning of the file.

And w/o the ability to seek, there is no way to write at the beginning of the file.

[thefourtheye]

truncate doesn't change the offset of the file descriptor. If you like to write from a specific position, I would recommend one of the filehandle.write variants.

[skyzyx]

#22554

[sam-github]

@kghost does https://github.com/nodejs/node/pull/27282/files clarify for you why this occurs?