fs: do not call Dir read callback twice

[shani-singh1]

Dir.prototype.read(callback) invoked callback from inside a try block whose catch also calls callback:

try {
  this.#processReadResult(this.#path, result);
  const dirent = ArrayPrototypeShift(this.#bufferedEntries);
  if (this.#options.recursive && dirent.isDirectory()) {
    this.#readSyncRecursive(dirent);
  }
  callback(null, dirent);   // inside the try
} catch (error) {
  callback(error);          // fires again, with the user's own error
}

So an exception thrown by user code inside the callback is caught by the adjacent catch and handed straight back to the same callback, as if the directory read itself had failed. Two things go wrong: the callback runs twice, and a plain bug in user code is silently reported to the application as an I/O error with a nonsense message while the original stack is lost.

Reproduction

const fs = require('node:fs');
const os = require('node:os');
const path = require('node:path');

const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'dir-'));
fs.writeFileSync(path.join(tmp, 'a.txt'), '');
fs.writeFileSync(path.join(tmp, 'b.txt'), '');

let calls = 0;
fs.opendir(tmp, (err, dir) => {
  if (err) throw err;
  dir.read((err2, dirent) => {
    calls++;
    console.log(`callback #${calls}: err=${err2 && err2.message} dirent=${dirent && dirent.name}`);
    if (calls === 1) throw new Error('bug in user callback');
  });
});
setTimeout(() => console.log(`total invocations : ${calls} (expected 1)`), 400);

On v24.11.1:

callback #1: err=null dirent=a.txt
callback #2: err=bug in user callback dirent=undefined
total invocations : 2 (expected 1)

Every other fs callback lets the exception reach 'uncaughtException' instead. For comparison, fs.readdir(tmp, () => { throw new Error('boom'); }) reports uncaughtException : boom.

doc/api/fs.md describes one call per read, made after the read completes:

After the read is completed, the callback will be called with an {fs.Dirent}, or null if there are no more directory entries to read.

The second call happens after completion, carrying an error that did not come from the read. Node also documents double invocation as an error condition in its own right (ERR_MULTIPLE_CALLBACK, "A callback was called more than once").

Change

Compute the entry inside the try and invoke the callback after it, so only failures of the read itself are reported through the callback. Both the buffered path and the asynchronous FSReqCallback path had the same shape and both are fixed.

Verification

Running the real lib/internal/fs/dir.js (before and after) against this build's fs_dir binding:

SAME  read(cb) sequence             a.txt,b.txt,c.txt,sub
SAME  read(cb) recursive            a.txt,b.txt,c.txt,d.txt,sub
SAME  async iterator                a.txt,b.txt,c.txt,sub
SAME  async iterator recursive      a.txt,b.txt,c.txt,d.txt,sub

throwing callback:
  before: calls=2  ["err=null name=a.txt","err=user cb error name=undefined"]
  after : calls=1  ["err=null name=a.txt"]

The normal read paths are unchanged; only the throwing-callback case differs.