[clavigne]

> I'm not talking about whether the scheduler can block the task on IO. This is trivial.

The whole point of coroutines is that they are trivial to block and resume, right? So the way to deal with limited resources... is to block on them.

So when you need a connection, you await until one from the pool becomes available. From the point of view of the consumer it's very natural because grabbing a connection is just a standard async call, and returning it to the pool can be done the same way you would usually close it. If anything, it's a lot easier to manage them like this because unlike in a non async program you don't have to worry too much about blocking progress.

[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.