[exfalso]

That's not my point. The issue is that when the blocking happens, with coroutines control is yielded to the scheduler which will now schedule other tasks. Those tasks may again request and block on resources. This is where the leak is coming from. A resource pool is one way to get around this, however this stops working if you have several kinds of resources.

On the other hand, with threads the IO block is a "proper" block. No new task will be scheduled, the thread will only continue when the IO operation finishes, providing a very natural backpressure mechanism that prevents overallocation/contention.