Luite Stegeman authored Mar 05, 2026 and Marge Bot committed Mar 27, 2026

In stg_block_async{_void}, a stack slot was reserved for
an StgAsyncIOResult. This slot would be filled by the IO
manager upon completion of the async call.

However, if the blocked thread was interrupted by an async
exception, we would end up in an invalid state:

 - If the blocked computation was never re-entered, the
   StgAsyncIOResult would never be freed.
 - If the blocked computation was re-entered, the thread would
   find an unitialized stack slot for the StgAsyncIOResult,
   leading to a crash reading its fields, or freeing the pointer.

We fix this by removing the StgAsyncIOResult altogether and writing
the result directly to the stack.

Fixes #26341