From cbe533bc59d2a32848967db17c960fa653b098b4 Mon Sep 17 00:00:00 2001
From: Matteo Collina <hello@matteocollina.com>
Date: Fri, 6 Feb 2026 09:03:13 +0100
Subject: [PATCH] fix: handle undefined __filename in bundled environments

When undici is bundled inside Node.js as an internal module, `__filename`
is not defined. This causes a ReferenceError when fetch() fails, which
swallows the original error and replaces it with the ReferenceError,
making `err.cause` undefined.

This fix:
1. Safely captures `__filename` at module load time, defaulting to
   `undefined` if not available
2. Uses the new stack trace augmentation when `__filename` is available
3. Falls back to the original `Error.captureStackTrace(err)` behavior
   when `__filename` is not available

Fixes failures in Node.js tests:
- test-permission-net-fetch.js
- test-tls-set-default-ca-certificates-append-fetch.mjs
- test-tls-set-default-ca-certificates-reset-fetch.mjs
---
 index-fetch.js | 10 +++++++++-
 index.js       | 10 +++++++++-
 2 files changed, 18 insertions(+), 2 deletions(-)

diff --git a/index-fetch.js b/index-fetch.js
index 0180b5865b9..3d1b811daa4 100644
--- a/index-fetch.js
+++ b/index-fetch.js
@@ -4,6 +4,10 @@ const { getGlobalDispatcher, setGlobalDispatcher } = require('./lib/global')
 const EnvHttpProxyAgent = require('./lib/dispatcher/env-http-proxy-agent')
 const fetchImpl = require('./lib/web/fetch').fetch
 
+// Capture __filename at module load time for stack trace augmentation.
+// This may be undefined when bundled in environments like Node.js internals.
+const currentFilename = typeof __filename !== 'undefined' ? __filename : undefined
+
 function appendFetchStackTrace (err, filename) {
   if (!err || typeof err !== 'object') {
     return
@@ -30,7 +34,11 @@ function appendFetchStackTrace (err, filename) {
 
 module.exports.fetch = function fetch (init, options = undefined) {
   return fetchImpl(init, options).catch(err => {
-    appendFetchStackTrace(err, __filename)
+    if (currentFilename) {
+      appendFetchStackTrace(err, currentFilename)
+    } else if (err && typeof err === 'object') {
+      Error.captureStackTrace(err, module.exports.fetch)
+    }
     throw err
   })
 }
diff --git a/index.js b/index.js
index 0dc12d62ecd..271c64e8ec3 100644
--- a/index.js
+++ b/index.js
@@ -121,6 +121,10 @@ module.exports.getGlobalDispatcher = getGlobalDispatcher
 
 const fetchImpl = require('./lib/web/fetch').fetch
 
+// Capture __filename at module load time for stack trace augmentation.
+// This may be undefined when bundled in environments like Node.js internals.
+const currentFilename = typeof __filename !== 'undefined' ? __filename : undefined
+
 function appendFetchStackTrace (err, filename) {
   if (!err || typeof err !== 'object') {
     return
@@ -147,7 +151,11 @@ function appendFetchStackTrace (err, filename) {
 
 module.exports.fetch = function fetch (init, options = undefined) {
   return fetchImpl(init, options).catch(err => {
-    appendFetchStackTrace(err, __filename)
+    if (currentFilename) {
+      appendFetchStackTrace(err, currentFilename)
+    } else if (err && typeof err === 'object') {
+      Error.captureStackTrace(err, module.exports.fetch)
+    }
     throw err
   })
 }