openAsBlob only looks at nanoseconds part of mtime to detect modifications #67283

Version

v28.0.0-pre

Platform

Darwin

Subsystem

fs

What steps will reproduce the bug?

On a filesystem with low-precision timestamps (e.g. FAT), openAsBlob a file, then edit the file but keep the size, then read from the blob. This should give NotReadableError because the file has changed, but it works because is_modified only checks tv_nsec, which is always 0 on FAT, and ignores tv_sec.

Runnable repro (from Claude), requires you to pass a path to a writable directory on a FAT volume:

'use strict';

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

if (process.argv.length !== 3) {
  console.error('must pass a path to a directory on a FAT volume');
  process.exit(1);
}

const dir = fs.mkdtempSync(path.join(process.argv[2], 'blob-mtime-'));
const file = path.join(dir, 'x');
fs.writeFileSync(file, 'from-a');

(async () => {
  const blob = await fs.openAsBlob(file);
  console.log('original:      ', await blob.text());
  await new Promise((r) => setTimeout(r, 2500));  // Past FAT's 2s granularity.
  fs.writeFileSync(file, 'EDITED');  // Same size.
  console.log('mtimeNs:       ', fs.statSync(file, { bigint: true }).mtimeNs);
  try {
    console.log('after rewrite: ', await blob.text());
  } catch (e) {
    console.log('after rewrite: ', e.name);
  }
  fs.rmSync(dir, { recursive: true });
})();

On MacOS you can make and mount a FAT volume like this:

hdiutil create -size 16m -fs MS-DOS -volname BLOBFAT -o fat.dmg
hdiutil attach -nobrowse fat.dmg      # mounts at /Volumes/BLOBFAT

then clean up with

hdiutil detach /Volumes/BLOBFAT
rm fat.dmg

when you're done.

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

Always.

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

Should throw NotReadableError, like it would on a filesystem with high-precision timestamps.

What do you see instead?

"EDITED"

Additional information

This should at least check st_mtim.tv_sec, probably also both of those fields for st_ctim, arguably also st_ino and st_dev.