>>djha-s+(OP)
> (If they did, the unwinding would close the file handle and the other coroutine might then return to that stack frame, only to find the file handle already closed.)
The programming language is not supposed to keep state of OS level resources. You are. The gray beard in me would just say "Duh".
To my knowledge, python/C/C++/Java/Go doesn't do this automatically. You're responsible for that.