From 593b5bd77ea444ac01e8bb957c7997eeabe45072 Mon Sep 17 00:00:00 2001
From: Daniel Lockyer <hi@daniellockyer.com>
Date: Sat, 10 Oct 2026 08:27:36 +0200
Subject: [PATCH] fs: fix do_not_throw_error argument index in sync fstat

The synchronous branch of the fstat binding read its
do_not_throw_error flag from args[2], which is always undefined on
that branch, so the flag was ignored and the binding always threw.

Read the flag from args[3] as documented, and restore tryStatSync()
to closing the file descriptor it owns before rethrowing the fstat
error, which was its behaviour before the flag was introduced.

Refs: https://github.com/nodejs/node/pull/49868
Assisted-by: MiniMax M3.1 Flash Preview + Opus 5.5
Signed-off-by: Daniel Lockyer <hi@daniellockyer.com>
---
 lib/fs.js                                 | 10 +++++--
 src/node_file.cc                          |  2 +-
 test/parallel/test-fs-fstat-sync-error.js | 35 +++++++++++++++++++++++
 3 files changed, 43 insertions(+), 4 deletions(-)
 create mode 100644 test/parallel/test-fs-fstat-sync-error.js

diff --git a/lib/fs.js b/lib/fs.js
index 4436fa2df6e6..64aae5f2cdc6 100644
--- a/lib/fs.js
+++ b/lib/fs.js
@@ -516,9 +516,13 @@ function readFileAfterOneShot(err, buffer, fd, size, closeErr) {
 }
 
 function tryStatSync(fd, isUserFd) {
-  const stats = binding.fstat(fd, false, undefined, true /* shouldNotThrow */);
-  if (stats === undefined && !isUserFd) {
-    fs.closeSync(fd);
+  let threw = true;
+  let stats;
+  try {
+    stats = binding.fstat(fd, false, undefined, false);
+    threw = false;
+  } finally {
+    if (threw && !isUserFd) fs.closeSync(fd);
   }
   return stats;
 }
diff --git a/src/node_file.cc b/src/node_file.cc
index 0bc7511ccb54..9eac793bfa82 100644
--- a/src/node_file.cc
+++ b/src/node_file.cc
@@ -1287,7 +1287,7 @@ static void FStat(const FunctionCallbackInfo<Value>& args) {
     AsyncCall(
         env, req_wrap_async, args, "fstat", UTF8, AfterStat, uv_fs_fstat, fd);
   } else {  // fstat(fd, use_bigint, undefined, do_not_throw_error)
-    bool do_not_throw_error = args[2]->IsTrue();
+    bool do_not_throw_error = args[3]->IsTrue();
     const auto should_throw = [do_not_throw_error](int result) {
       return is_uv_error(result) && !do_not_throw_error;
     };
diff --git a/test/parallel/test-fs-fstat-sync-error.js b/test/parallel/test-fs-fstat-sync-error.js
new file mode 100644
index 000000000000..3b723f12c0de
--- /dev/null
+++ b/test/parallel/test-fs-fstat-sync-error.js
@@ -0,0 +1,35 @@
+// Flags: --expose-internals
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const { internalBinding } = require('internal/test/binding');
+
+const binding = internalBinding('fs');
+
+const badFd = fs.openSync(__filename, 'r');
+fs.closeSync(badFd);
+
+// The synchronous fstat binding honours its do_not_throw_error argument.
+assert.strictEqual(binding.fstat(badFd, false, undefined, true), undefined);
+assert.throws(() => binding.fstat(badFd, false, undefined, false), {
+  code: 'EBADF',
+  syscall: 'fstat',
+});
+
+// readFileSync() surfaces the fstat error and closes a file descriptor it
+// opened itself, but leaves a user-supplied one alone.
+fs.openSync = () => badFd;
+fs.closeSync = common.mustCall((fd) => {
+  assert.strictEqual(fd, badFd);
+});
+
+assert.throws(() => fs.readFileSync('dummy', 'latin1'), {
+  code: 'EBADF',
+  syscall: 'fstat',
+});
+assert.throws(() => fs.readFileSync(badFd, 'latin1'), {
+  code: 'EBADF',
+  syscall: 'fstat',
+});