[Simn]

When a thread registers, RegisterCurrentThread takes a LocalAllocator from the pool in GetPooledAllocator, which holds gThreadStateChangeLock only for that step. AttachThread then calls AddLocal, which takes the lock again. In between, the allocator is neither in the pool nor in mLocalAllocs.

A collection resets every registered allocator at the end of Mark and every pooled one in ClearPooledAlloc. An allocator caught in that window gets neither reset. It keeps its holes, the collection may reclaim those blocks for other allocations, and the newly attached thread then allocates over live objects.

The fix calls Reset() at the start of AttachThread. That only drops the allocator's current holes, so the thread starts with a fresh allocation region.

This is easy to hit when threads attach and detach very often. hxcpp_luv_io's Work.queue registers the libuv worker thread with SetTopOfStack(&top, true) and unregisters it with UnregisterCurrentThread() for every work item. hxcoro's LuvDispatcher sends every dispatch through Work.queue. With that dispatcher, about 25% of hxcoro's C++ test runs on Windows crashed with access violations in unrelated objects (6 of 30). With this change, there were no access violations in 60 runs.

🤖 Generated with Claude Code