wait-group: Cancellation is unsafe for multiple uses of the same wait_group

[avdgrinten]

The cancellation code checks for counter > 0 to detect if a waiter is still part of the wait_group's linked list. However, that is broken if non-zero -> zero -> non-zero transitions occur.

libasync/include/async/wait-group.hpp

Lines 111 to 116 in 48dce66

	// Relaxed since non-zero -> zero transitions cannot happen while the mutex is held.
	if(wg_->ctr_.load(std::memory_order_relaxed) > 0) {
	cancelled = true;
	auto it = wg_->queue_.iterator_to(this);
	wg_->queue_.erase(it);
	}