thread::scope 与 std 的 scope

pub fn scope<'env, F, R>(f: F) -> thread::Result<R>
where
    F: FnOnce(&Scope<'env>) -> R,
{
    struct AbortOnPanic;
    impl Drop for AbortOnPanic {
        fn drop(&mut self) {
            if thread::panicking() {
                std::process::abort();
            }
        }
    }

    let wg = WaitGroup::new();
    let scope = Scope::<'env> {
        handles: SharedVec::default(),
        wait_group: wg.clone(),
        _marker: PhantomData,
    };

    // Execute the scoped function, but catch any panics.
    let result = panic::catch_unwind(panic::AssertUnwindSafe(|| f(&scope)));

    // If an unwinding panic occurs before all threads are joined
    // promote it to an aborting panic to prevent any threads from escaping the scope.
    let guard = AbortOnPanic;

    // Wait until all nested scopes are dropped.
    drop(scope.wait_group);
    wg.wait();

    // Join all remaining spawned threads.
    let panics: Vec<_> = scope
        .handles
        .lock()
        .unwrap()
        // Filter handles that haven't been joined, join them, and collect errors.
        .drain(..)
        .filter_map(|handle| handle.lock().unwrap().take())
        .filter_map(|handle| handle.join().err())
        .collect();

    mem::forget(guard);

    // If `f` has panicked, resume unwinding.
    // If any of the child threads have panicked, return the panic errors.
    // Otherwise, everything is OK and return the result of `f`.
    match result {
        Err(err) => panic::resume_unwind(err),
        Ok(res) => {
            if panics.is_empty() {
                Ok(res)
            } else {
                Err(Box::new(panics))
            }
        }
    }
}

pub fn scope<'env, F, T>(f: F) -> T
where
    F: for<'scope> FnOnce(&'scope Scope<'scope, 'env>) -> T,
{
    // ...
    let result = catch_unwind(AssertUnwindSafe(|| f(&scope)));

    // Wait until all the threads are finished.
    while scope.data.num_running_threads.load(Ordering::Acquire) != 0 {
        // SAFETY: this is the main thread, the handle belongs to us.
        unsafe { scope.data.main_thread.park() };
    }

    // Throw any panic from `f`, or the return value of `f` if no thread panicked.
    match result {
        Err(e) => resume_unwind(e),
        Ok(_) if scope.data.a_thread_panicked.load(Ordering::Relaxed) => {
            panic!("a scoped thread panicked")
        }
        Ok(result) => result,
    }
}

let res = crossbeam::scope(|s| {
  let h = s.spawn(move |_| {
    if child_panics { panic!("child boom"); }
    7
  });
  if join_explicitly {
    println!("explicit join -> is_err={}", h.join().is_err());
  }
  "scope body done"
});

crossbeam no-panic: Ok("scope body done")
std       no-panic: returned "scope body done"
crossbeam child-panics: Err, is Vec<Box<dyn Any + Send>>=true, payloads=["child boom"]
std       child-panics: panicked with "a scoped thread panicked"
  [child-panics-joined] explicit join -> is_err=true
crossbeam child-panics-joined: Ok("scope body done")
  [child-panics-joined] explicit join -> is_err=true
std       child-panics-joined: returned "scope body done"