RFR: 8188055: (ref) Add Reference::refersTo predicate [v5]

[Kim Barrett]

> On Oct 20, 2020, at 2:09 AM, David Holmes <david.holmes at oracle.com> wrote:
> 
> On 20/10/2020 3:26 pm, Kim Barrett wrote:
>> On Tue, 20 Oct 2020 03:25:45 GMT, Mandy Chung <mchung at openjdk.org> wrote:
>>> @kimbarrett your reworded text is okay. I think "if it initially had some other referent value" can be dropped.
>>> 
>>> For a `Reference` constructed with a `null` referent, we can clarify in the spec that such reference object will never
>>> get cleared and enqueued. I suggest to file a separate issue to follow up.
>> I don't think that clause can be dropped, because of explicit clearing (by clear() or enqueue()) rather than by the
>> GC.  If the reference was constructed with a null referent, ref.refersTo(null) cannot tell whether ref.clear() has been
>> called.
> 
> I think that can be addressed by considering a Reference created with a null referent to be immediately cleared.

I think if it’s treated as immediately cleared then it should also be immediately enqueued.  But immediate
enqueue has problems; it can’t be done by the Reference constructor because the derived constructors
have not yet completed, and exposing the possibly incompletely constructed object to the queue processor
is problematic.  And if it can’t be done by the constructor, it’s not clear who would do it.

> But the more we discuss this the more I think allowing an initial null referent was a mistake in the first place. :(

I agree, but here we are.  Very hard to know what the compatibility impact of changing that would be.