Waiting for threadpool threads to drop

[PvdBerg1998]

I have some shared data in an Arc that I want to drop when my program exits.
However, when I drop the ThreadPool, this only sends a Close message to the worker threads but does not join them. This means I have no way to block until I know the threads/shared data is dropped.

Edit: Tokio's threadpool does not have this issue.

[PvdBerg1998]

I tried just saving all the thread handles and joining them in drop, but this doesn't work. This is probably because the WakeHandle gets passed down, which also contains an Arc. This should probably be a Weak though, since waking a future in a dropped executor makes no sense anyway.

[PvdBerg1998]

The behaviour I'm seeing is that futures are not dropped after the threadpool is dropped.

[PvdBerg1998]

// Let there be some shared data in an Arc
let shared_data = Arc::new(data);
// Let there be a future that has a strong reference to this data
let future = SomeFuture::new(shared_data.clone());
// Spawn it on the threadpool
threadpool.spawn(future);

// The threadpool Drop implementation has been patched to join every thread
drop(threadpool);
// Now all the threadpool threads are dropped

// So logically, we are the only owner of the shared data Arc now?
assert!(Arc::strong_count(&shared_data) == 1);
// PANIC!

// Replacing the assert with a while loop:
// Note that this actually returns after a very short time
while Arc::strong_count(&shared_data) != 1 {}
// Returns.

[PvdBerg1998]

I have created a P.o.C. threadpool that does not have this issue: https://github.com/PvdBerg1998/futures-executor