Move task notification out of the runtime

[brson]

To enable joining with tasks, they have a 'notification' feature where the task will send a message when it exits, regardless of success or failure. This currently involves a lot of work in the runtime, but with unwinding can all be done in Rust. Depends on #908.

[brson]

Actually, this doesn't depend on #908. Since task failure is catastrophic on Windows, it doesn't matter that we don't send the message during unwinding.

[nikomatsakis]

I am doing this via a resource and the code should be pushed soon.

[nikomatsakis]

turns out that this exposes a bug in the memory management around resources. I will commit an xfail-test shortly that reproduces the problem.

[nikomatsakis]

I am not sure if this is possible, actually. The problem is with task supervision: the correct behavior, from my point of view, is that you do not receive a "parent completed" notification until all supervised children have completed. This is because the parent may fail due to a supervised child having failed. If we put the notification message into a resource, it will fire a "succeeded!" msg as soon as the parent's task completes, but it may be that some child has not yet completed so the parent may later actually be killed. Therefore I am going to close the issue.

[brson]

Oof, good point. Surely there will (someday) be a viable way to do this in rust-land. Maybe by just joining on all children. Seems like we might be able to combine this with the (buggy) linked failure mechanism.

[brson]

OK, well actually, until this very moment I had always considered a parent that exited normally to be a success, even if the child later fails (the child will still manage to fail the grandparent, the scheduler etc., skipping the dead parent). But your description sounds more sane.

[brson]

I still have a desire to make this work. I'm going to reopen unless you're strongly opposed.

[nikomatsakis]

No, not strongly opposed.