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

[Alan Bateman]

On 01/02/2022 09:11, Anton Kozlov wrote:
> Cross-posting RFR from CRaC Project. The change touches Reference 
> class, so I
> would be glad to receive any feedback from core-libs-dev.
>
> In CRaC project, java code participates in the preparation of the 
> platform
> state that can be safely stored to the image. The image can be 
> attempted at any
> time, so the image may capture unprocessed References. Recently I 
> found cases
> when objects became unreachable during preparation for the checkpoint, 
> and
> their associated clean-up actions to close external resources (which 
> we don't
> allow open when the image is stored). So it's become necessary to 
> ensure as
> many References as possible are processed before the image is created. 
> As a
> nice additional feature, restored java instances won't start with the 
> same
> Reference processing.
>
> With the change, the image is not created until VM's queue of pending
> j.l.References are drained, and then, as an example, each 
> j.l.ref.Cleaner queue
> is drained, only then the VM is called to prepare the image. More 
> Reference
> handling threads will be changed like Cleaner's ones. I'm looking for 
> possible
> problems or general comments about this approach.

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.

-Alan.