## Summary
- Fixes#20321 - spawnSync crashes with RangeError when stdio is set to
process.stderr
- Handles file descriptors in stdio array correctly by treating them as
non-captured output
## Problem
When `spawnSync` is called with `process.stderr` or `process.stdout` in
the stdio array, Bun.spawnSync returns the file descriptor number (e.g.,
2 for stderr) instead of a buffer or null. This causes a RangeError when
the code tries to call `toString(encoding)` on the number, since
`Number.prototype.toString()` expects a radix between 2 and 36, not an
encoding string.
This was blocking AWS CDK usage with Bun, as CDK internally uses
`spawnSync` with `stdio: ['ignore', process.stderr, 'inherit']`.
## Solution
Check if stdout/stderr from Bun.spawnSync are numbers (file descriptors)
and treat them as null (no captured output) instead of trying to convert
them to strings.
This aligns with Node.js's behavior where in
`lib/internal/child_process.js` (lines 1051-1055), when a stdio option
is a number or has an `fd` property, it's treated as a file descriptor:
```javascript
} else if (typeof stdio === 'number' || typeof stdio.fd === 'number') {
ArrayPrototypePush(acc, {
type: 'fd',
fd: typeof stdio === 'number' ? stdio : stdio.fd,
});
```
And when stdio is a stream object (like process.stderr), Node.js
extracts the fd from it (lines 1056-1067) and uses it as a file
descriptor, which means the output isn't captured in the result.
## Test plan
Added comprehensive regression tests in
`test/regression/issue/20321.test.ts` that cover:
- process.stderr as stdout
- process.stdout as stderr
- All process streams in stdio array
- Mixed stdio options
- Direct file descriptor numbers
- The exact AWS CDK use case
All tests pass with the fix.
🤖 Generated with [Claude Code](https://claude.ai/code)
---------
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
