vfs: fs.writeFileSync(fd, ...) does not dispatch virtual file descriptors

[trivikr]

Version

main

Platform

macOS 26.5.0

Subsystem

vfs

What steps will reproduce the bug?

import fs from 'node:fs';
import os from 'node:os';
import path from 'node:path';
import vfs from 'node:vfs';

const mountPoint = fs.mkdtempSync(path.join(os.tmpdir(), 'vfs-mount-'));

const mounted = vfs
  .create({ emitExperimentalWarning: false })
  .mount(mountPoint);

try {
  const file = path.join(mountPoint, 'file.txt');

  fs.writeFileSync(file, 'abcdef');

  const fd = fs.openSync(file, 'r+');

  try {
    fs.writeFileSync(fd, 'XY');
    console.log(fs.readFileSync(file, 'utf8'));
  } catch (err) {
    console.log('threw:', err.code, err.syscall);
  } finally {
    fs.closeSync(fd);
  }
} finally {
  mounted.unmount();
  fs.rmSync(mountPoint, { recursive: true, force: true });
}

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

Always

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

XYcdef

Native fs.writeFileSync(fd, data) writes through the already-open descriptor at its current file position, so virtual fds should be dispatched the same way as fs.readFileSync(fd) and fs.readSync(fd, ...).

What do you see instead?

throws EBADF with syscall write.

Additional information

No response