RFC: moving managed pointers across tasks

[erickt]

@graydon and I were talking on IRC about using conditions for serializations error handling today. He convinced me to try out using subtasks to parse and catch failures, but unfortunately doing this means that we can't use serialization error handling on types with managed pointers.

This problem would go away if we could somehow move managed pointers from task A to task B. Even though it's generally unsafe to send a managed pointer across tasks, it's theoretically safe if it's the last thing a task does before it dies.

Here's an obviously wrong, only-works-for-ints example function:

fn try_managed<T>(f: fn~() -> @T) -> Result<@T, ()> {
    let result: Result<*libc::c_void, ()> = do task::try {
        unsafe { 
            let v = f();
            cast::forget(v);
            cast::reinterpret_cast::<@T, *libc::c_void>(&v)
        }
    };

    match move result {
        Ok(move v) => {
            unsafe {
                let v = cast::reinterpret_cast::<*libc::c_void, @T>(&v);
                cast::bump_box_refcount(v);
                Ok(move v)
            }
        }
        Err(move e) => Err(e),
    }
}

fn main() {
    let r = do try_managed {
        @5
    };

    match r {
        Err(_) => io::println("err"),
        Ok(x) => io::println(fmt!("%?", *x)),
    }
}

It segfaults for more complicated data structures, like ~strs.

[nikomatsakis]

@pcwalton has been talking about adding something which would permit @ to be sent across tasks by serializing and deserializing them. Essentially, cloning them, just as Erlang does. This would be a simpler addition (no language changes at all) though it'd likely not be as efficient.

Keep in mind that while @ pointers will not be aliased if sent as part of the "last thing the task does", however we define that, it may still require some kind of cleverness in the garbage collector and so forth, depending on how we ultimately implement @ pointers.

So I guess I'm...unsure about this idea, but not completely opposed.

[catamorphism]

Going to go ahead and close this since it's 3 months old and there wasn't much interest. Reopen if you feel strongly about it!