Change panic handling in scoped thread

[taiki-e]

• Panic instead of returning result
• Do not own join handles in main thread

TODO: if we don't do #844 (comment), we probably shouldn't change the signature of scope function.

Fixes #816
Closes #724

[taiki-e]

BTW, in addition to this, I'm considering porting the rust-lang/rust#94559's changes to crossbeam.

[m-ou-se]

This implementation introduces a soundness bug, since the WaitGroup is dropped before the result of a thread (which might still borrow things) is dropped:

use std::{thread::sleep, time::Duration};

struct PrintOnDrop<'a>(&'a str);

impl Drop for PrintOnDrop<'_> {
    fn drop(&mut self) {
        sleep(Duration::from_millis(100));
        println!("{}", self.0);
    }
}

fn main() {
    let a = "hello".to_string();
    let r = &a;
    crossbeam::scope(|s| {
        s.spawn(move |_| PrintOnDrop(r));
    });
    drop(a);
    sleep(Duration::from_millis(200));
}

$ cargo r -q
;]Æǟ

[taiki-e]

@m-ou-se Thanks! I pushed a fix for that bug

EDIT: The fix is:

• If the closure is already finished when the handle is dropped, drop the result.
• If the closure is not yet finished when the handle is dropped, drop the closure result without storing it to result.
• Add WaitGroup to the handle.

The last one is needed to properly handle cases where handles are not dropped:

-     s.spawn(move |_| X(actually_finished));
+     s.spawn(move |s| mem::forget(s.spawn(move |_| X(actually_finished))));

[taiki-e]

bors r+

[taiki-e]

bors r-

miri failed due to thread leaks: https://github.com/crossbeam-rs/crossbeam/runs/7479452681?check_suite_focus=true

[bors]

Canceled.