This is just one example of the general problem of fork()'ing with threads holding a lock. Given that locks come in different flavors and have different uses, they need individual handling. The best general solution I know of is "don't do that". One of the things I like about Haskell is that the type system gives me hints when I'm trying to do that.

That said, there are a number of issues raised by leaving FD's open across an exec, and that's rarely the right thing to do. So making it the default behavior is probably a good idea.