## Summary
- Fixed `controller.desiredSize` throwing `TypeError: null is not an
object` when the stream's internal `controlledReadableStream` has been
set to `null` during cleanup
- Added null check in `readableStreamDefaultControllerGetDesiredSize` to
return `null` when the stream reference is null, matching WHATWG Streams
spec behavior for detached/errored streams
## Root Cause
When piping streams (e.g., `fetch` with `ReadableStream` body), cleanup
code in `assignStreamIntoResumableSink` and `readStreamIntoSink` sets
`controlledReadableStream` to `null` for GC purposes. If the user's
`pull()` function is still running asynchronously when this happens,
accessing `controller.desiredSize` throws instead of returning `null`.
## Test plan
- [x] Added regression test `test/regression/issue/26377.test.ts`
- [x] Verified tests pass with `bun bd test
test/regression/issue/26377.test.ts`
Fixes#26377🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
