[crac] RFR: Ensure empty Reference Handler and Cleaners queues

[Anton Kozlov]

On 2/2/22 17:48, Alan Bateman wrote:> At a high level it should be okay to provide a JDK-internal way to await quiescent. You've added it as a public API which might be okay for the current exploration but I don't think it would be exposed in its current form. Once the method returns then there is no guarantee that the number of waiters hasn't changed, but I think you know that

I thought about blocking waiters regardless of References available in the Queue. This would leave threads in the quiescent state but References would pile up in the pendingReferenceQueue exposed by the VM. So I stopped on the method being a synchronization, rather than a blocking point.

I hoped to guarantee all Queues are empty by waiting a sufficient number of waiters for each Queue, in the order of Queues passing References between each other (for a single thread). But now even there, I see handling of a Reference later in the order may make another one pending, filling up a Queue that was supposed to be empty. For a strong guarantee that all Queues are empty, some sort of iteration may be required, that will check no Queue had a new reference since the last check.

I think a public API is needed as users may have the same problem as we do. But the current code does not support this (we need to allow user code after JDK Queues are emptied).

Interesting...

Thanks!
Anton