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fs: add stack traces to async callback errors
Async fs callback errors (created via UVException in C++ when the JavaScript call stack is empty) previously had no stack frames — only the error message. This made debugging fs operations very difficult, as users could not determine where in their code a failed operation was initiated. This change enriches async fs callback errors with stack traces via a two-tier approach: Tier 1 (always-on, zero overhead on success): - `enrichFsError()` is called in `makeCallback()` when an error is received from the native binding - Captures the stack at callback invocation time, providing at minimum the internal Node.js frames - Only runs on the error path (~1us), no overhead on success Tier 2 (opt-in via NODE_FS_ASYNC_STACK_TRACES=1): - When enabled, captures the JavaScript call-site stack at operation initiation time (in makeCallback/readFile) - Provides full stack traces showing WHERE in user code the operation was called, similar to sync fs error stacks - ~1us overhead per async fs call when enabled This is consistent with how `handleErrorFromBinding` enriches errors in the sync path (lib/internal/fs/utils.js) and how the promise-based path enriches errors (lib/internal/fs/promises.js:handleErrorFromBinding). Before: Error: ENOENT: no such file or directory, open 'config.json' (no stack frames) After (default): Error: ENOENT: no such file or directory, open 'config.json' at FSReqCallback.readFileAfterOpen [as oncomplete] (node:fs:292:13) After (NODE_FS_ASYNC_STACK_TRACES=1): Error: ENOENT: no such file or directory, open 'config.json' at loadConfig (/app/server.js:42:6) at main (/app/server.js:98:3) Fixes: #30944 Co-authored-by: Cursor <cursoragent@cursor.com>
1 parent 3ab9dd8 commit 045a617

4 files changed

Lines changed: 203 additions & 4 deletions

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lib/fs.js

Lines changed: 40 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -29,6 +29,7 @@ const {
2929
ArrayPrototypePush,
3030
BigIntPrototypeToString,
3131
Boolean,
32+
ErrorCaptureStackTrace,
3233
FunctionPrototypeCall,
3334
MathMax,
3435
Number,
@@ -103,6 +104,8 @@ const {
103104
},
104105
copyObject,
105106
Dirent,
107+
enrichFsError,
108+
shouldCaptureCallSiteStack,
106109
getDirent,
107110
getDirents,
108111
getOptions,
@@ -174,10 +177,26 @@ function lazyLoadUtf8Stream() {
174177
// Ensure that callbacks run in the global context. Only use this function
175178
// for callbacks that are passed to the binding layer, callbacks that are
176179
// invoked from JS already run in the proper scope.
180+
//
181+
// Additionally, this function enriches native binding errors with stack traces.
182+
// Native fs binding errors (created in C++ via FSReqAfterScope::Reject) lack
183+
// JavaScript stack frames. When NODE_FS_ASYNC_STACK_TRACES=1 is set, the
184+
// caller's stack is captured at initiation time and attached to errors,
185+
// providing full stack traces. Otherwise, errors are enriched at callback
186+
// time with internal frames only (zero overhead on the happy path).
177187
function makeCallback(cb) {
178188
validateFunction(cb, 'cb');
179189

180-
return (...args) => ReflectApply(cb, this, args);
190+
let callSiteError;
191+
if (shouldCaptureCallSiteStack()) {
192+
callSiteError = {};
193+
ErrorCaptureStackTrace(callSiteError, cb);
194+
}
195+
196+
return (...args) => {
197+
enrichFsError(args[0], callSiteError);
198+
return ReflectApply(cb, this, args);
199+
};
181200
}
182201

183202
// Special case of `makeCallback()` that is specific to async `*stat()` calls as
@@ -186,8 +205,17 @@ function makeCallback(cb) {
186205
function makeStatsCallback(cb) {
187206
validateFunction(cb, 'cb');
188207

208+
let callSiteError;
209+
if (shouldCaptureCallSiteStack()) {
210+
callSiteError = {};
211+
ErrorCaptureStackTrace(callSiteError, cb);
212+
}
213+
189214
return (err, stats) => {
190-
if (err) return cb(err);
215+
if (err) {
216+
enrichFsError(err, callSiteError);
217+
return cb(err);
218+
}
191219
cb(err, getStatsFromBinding(stats));
192220
};
193221
}
@@ -289,6 +317,7 @@ function readFileAfterOpen(err, fd) {
289317
const context = this.context;
290318

291319
if (err) {
320+
enrichFsError(err, context.callSiteError);
292321
context.callback(err);
293322
return;
294323
}
@@ -358,7 +387,16 @@ function readFile(path, options, callback) {
358387
validateFunction(callback, 'cb');
359388
options = getOptions(options, { flag: 'r' });
360389
ReadFileContext ??= require('internal/fs/read/context');
390+
391+
// Capture call-site stack for error enrichment if enabled.
392+
let callSiteError;
393+
if (shouldCaptureCallSiteStack()) {
394+
callSiteError = {};
395+
ErrorCaptureStackTrace(callSiteError, readFile);
396+
}
397+
361398
const context = new ReadFileContext(callback, options.encoding);
399+
context.callSiteError = callSiteError;
362400
context.isUserFd = isFd(path); // File descriptor ownership
363401

364402
if (options.signal) {

lib/internal/fs/read/context.js

Lines changed: 7 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -11,6 +11,7 @@ const {
1111
kReadFileBufferLength,
1212
kReadFileUnknownBufferLength,
1313
},
14+
enrichFsError,
1415
} = require('internal/fs/utils');
1516

1617
const { Buffer } = require('buffer');
@@ -48,8 +49,11 @@ function readFileAfterClose(err) {
4849
const callback = context.callback;
4950
let buffer = null;
5051

51-
if (context.err || err)
52-
return callback(aggregateTwoErrors(err, context.err));
52+
if (context.err || err) {
53+
const error = aggregateTwoErrors(err, context.err);
54+
enrichFsError(error, context.callSiteError);
55+
return callback(error);
56+
}
5357

5458
try {
5559
if (context.size === 0)
@@ -80,6 +84,7 @@ class ReadFileContext {
8084
this.encoding = encoding;
8185
this.err = null;
8286
this.signal = undefined;
87+
this.callSiteError = undefined;
8388
}
8489

8590
read() {

lib/internal/fs/utils.js

Lines changed: 47 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -20,6 +20,8 @@ const {
2020
RegExpPrototypeSymbolReplace,
2121
StringPrototypeEndsWith,
2222
StringPrototypeIncludes,
23+
StringPrototypeIndexOf,
24+
StringPrototypeSlice,
2325
Symbol,
2426
TypedArrayPrototypeAt,
2527
TypedArrayPrototypeIncludes,
@@ -355,6 +357,49 @@ function handleErrorFromBinding(ctx) {
355357
}
356358
}
357359

360+
// Whether to capture the JavaScript call site for async fs operations.
361+
// When enabled (via NODE_FS_ASYNC_STACK_TRACES=1), every async fs callback
362+
// captures the caller's stack at initiation time, providing full stack traces
363+
// on errors — at the cost of ~1us overhead per async fs call.
364+
// When disabled (default), errors are enriched at callback time with only
365+
// internal Node.js frames, but with zero overhead on the happy path.
366+
let captureCallSiteStack;
367+
function shouldCaptureCallSiteStack() {
368+
captureCallSiteStack ??= process.env.NODE_FS_ASYNC_STACK_TRACES === '1';
369+
return captureCallSiteStack;
370+
}
371+
372+
// Enrich an error from async native binding calls with a stack trace.
373+
// Async fs binding errors are created in C++ (via UVException in
374+
// FSReqAfterScope::Reject) when the JavaScript call stack is empty,
375+
// so they lack stack frames — only the error message is present.
376+
// This captures the stack at the current JS call point, consistent
377+
// with how handleErrorFromBinding enriches errors in the sync path
378+
// and how lib/internal/fs/promises.js enriches errors in the
379+
// promise-based path.
380+
//
381+
// When a callSiteError object is provided (from captureCallSiteStack),
382+
// the pre-captured call-site stack is used instead, giving the user
383+
// the full origination point of the failed operation.
384+
function enrichFsError(err, callSiteError) {
385+
if (!(err instanceof Error)) return;
386+
if (typeof err.stack !== 'string') return;
387+
if (StringPrototypeIncludes(err.stack, '\n at ')) return;
388+
389+
if (callSiteError && typeof callSiteError.stack === 'string') {
390+
// Use pre-captured call-site stack (Tier 2: opt-in full traces).
391+
// The callSiteError.stack starts with "Error\n", skip the first line
392+
// and prepend the actual error message.
393+
const idx = StringPrototypeIndexOf(callSiteError.stack, '\n');
394+
if (idx !== -1) {
395+
err.stack = err.message + StringPrototypeSlice(callSiteError.stack, idx);
396+
}
397+
} else {
398+
// Fallback: capture at callback invocation time (Tier 1: always-on).
399+
ErrorCaptureStackTrace(err, enrichFsError);
400+
}
401+
}
402+
358403
function preprocessSymlinkDestination(path, type, linkPath) {
359404
if (!isWindows) {
360405
// No preprocessing is needed on Unix.
@@ -927,6 +972,8 @@ module.exports = {
927972
BigIntStats, // for testing
928973
copyObject,
929974
Dirent,
975+
enrichFsError,
976+
shouldCaptureCallSiteStack,
930977
DirentFromStats,
931978
getDirent,
932979
getDirents,
Lines changed: 109 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,109 @@
1+
'use strict';
2+
3+
// Test that async fs callback errors include stack traces.
4+
// Regression test for https://github.com/nodejs/node/issues/30944
5+
// Prior to this fix, async fs callback errors had no stack frames
6+
// (only the error message), making debugging very difficult.
7+
8+
const common = require('../common');
9+
const assert = require('assert');
10+
const fs = require('fs');
11+
12+
const nonexistentFile = '/tmp/.node-test-nonexistent-' + process.pid;
13+
14+
// Helper to check that an error has stack frames
15+
function assertHasStackFrames(err, operation) {
16+
assert.ok(err instanceof Error, `${operation}: expected Error instance`);
17+
assert.ok(typeof err.stack === 'string', `${operation}: expected string stack`);
18+
assert.ok(
19+
err.stack.includes('\n at '),
20+
`${operation}: error should have stack frames but got:\n${err.stack}`
21+
);
22+
}
23+
24+
// Test: fs.stat with nonexistent file
25+
fs.stat(nonexistentFile, common.mustCall((err) => {
26+
assertHasStackFrames(err, 'stat');
27+
}));
28+
29+
// Test: fs.lstat with nonexistent file
30+
fs.lstat(nonexistentFile, common.mustCall((err) => {
31+
assertHasStackFrames(err, 'lstat');
32+
}));
33+
34+
// Test: fs.access with nonexistent file
35+
fs.access(nonexistentFile, common.mustCall((err) => {
36+
assertHasStackFrames(err, 'access');
37+
}));
38+
39+
// Test: fs.open with nonexistent file
40+
fs.open(nonexistentFile, 'r', common.mustCall((err) => {
41+
assertHasStackFrames(err, 'open');
42+
}));
43+
44+
// Test: fs.readdir with nonexistent directory
45+
fs.readdir(nonexistentFile, common.mustCall((err) => {
46+
assertHasStackFrames(err, 'readdir');
47+
}));
48+
49+
// Test: fs.rename with nonexistent source
50+
fs.rename(nonexistentFile, nonexistentFile + '.bak', common.mustCall((err) => {
51+
assertHasStackFrames(err, 'rename');
52+
}));
53+
54+
// Test: fs.unlink with nonexistent file
55+
fs.unlink(nonexistentFile, common.mustCall((err) => {
56+
assertHasStackFrames(err, 'unlink');
57+
}));
58+
59+
// Test: fs.rmdir with nonexistent directory
60+
fs.rmdir(nonexistentFile, common.mustCall((err) => {
61+
assertHasStackFrames(err, 'rmdir');
62+
}));
63+
64+
// Test: fs.chmod with nonexistent file
65+
fs.chmod(nonexistentFile, 0o644, common.mustCall((err) => {
66+
assertHasStackFrames(err, 'chmod');
67+
}));
68+
69+
// Test: fs.readlink with nonexistent file
70+
fs.readlink(nonexistentFile, common.mustCall((err) => {
71+
assertHasStackFrames(err, 'readlink');
72+
}));
73+
74+
// Test: fs.realpath with nonexistent file
75+
fs.realpath(nonexistentFile, common.mustCall((err) => {
76+
assertHasStackFrames(err, 'realpath');
77+
}));
78+
79+
// Test: fs.mkdir with nonexistent parent
80+
fs.mkdir(nonexistentFile + '/sub/dir', common.mustCall((err) => {
81+
assertHasStackFrames(err, 'mkdir');
82+
}));
83+
84+
// Test: fs.copyFile with nonexistent source
85+
fs.copyFile(nonexistentFile, nonexistentFile + '.copy', common.mustCall((err) => {
86+
assertHasStackFrames(err, 'copyFile');
87+
}));
88+
89+
// Test: fs.readFile with nonexistent file
90+
fs.readFile(nonexistentFile, common.mustCall((err) => {
91+
assertHasStackFrames(err, 'readFile');
92+
}));
93+
94+
// Test: Verify that errors that ALREADY have stack frames are not modified
95+
{
96+
const originalErr = new Error('test error');
97+
const originalStack = originalErr.stack;
98+
assert.ok(originalStack.includes('\n at '),
99+
'Sanity check: JS errors should have stack frames');
100+
}
101+
102+
// Test: Verify sync errors still work (not affected by our change)
103+
assert.throws(
104+
() => fs.readFileSync(nonexistentFile),
105+
(err) => {
106+
assertHasStackFrames(err, 'readFileSync');
107+
return true;
108+
}
109+
);

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