Fetch call causes exceptions when process.exit is called shortly after it

[art-alexeyenko]

Bug Description

On Windows platforms, fetch will cause an exception (Assertion failed..) to be thrown on process.exit() call, if it's called shortly after a fetch() execution. This could affect fetch calls within CLI logic. Why it happens:

1. Native fetch is undici. On a successful request undici returns the
   socket to a keep-alive pool (default keepAliveTimeout = 4s) instead of
   closing it. The fetch controller does not abort/close on success.
2. The pooled socket keeps libuv handles alive (a TCP handle + a uv_async_t/timer).
3. process.exit(0) tears the event loop down immediately. libuv closes handles;
   if undici's async handle is mid-teardown, libuv double-closes it and asserts.
   Windows' src/win/async.c is strict about this; POSIX usually isn't.

Reproduction

See: https://github.com/art-alexeyenko/node-fetch-handles

Steps to reproduce (if not using the script above):

0. Use the reproduction repo on a Windows platform
1. Run a mock server (npm run server)
2. Run the repro (npm run repro). This will:
   a. Perform a fetch request to the local endpoint
   b. Execute process.exit(0)

Expected Behavior

No exception at the end.

Actual Behavior

Assertion failed: !(handle->flags & UV_HANDLE_CLOSING), file src\win\async.c, line 94 error occurs upon the process.exit call.

Logs & Screenshots

Sample log output

npm run repro

> undici-exit-crash-repro@1.0.0 repro
> node repro.mjs

Sent 2 requests
Assertion failed: !(handle->flags & UV_HANDLE_CLOSING), file src\win\async.c, line 94

Environment

• OS: Windows
• Node.js version: >=24
• undici version: 7.29.0

[metcoder95]

This is more a Node's related issue. It will be better if you can open it there