8132984: incorrect type for Reference.discovered

[kimbarrett]

Please review this change which fixes the type of the private
Reference.discovered field. It was Reference, but that's wrong because
it can be any Reference object.

I've changed it to Reference and let that flow through, updating some other variables that were previously somewhat incorrectly typed (usually with an Object type parameter). The interesting change is to the ReferenceQueue.enqueue parameter, which is now also Reference.

This ultimately end up with a provably safe and correct, but uncheckable,
cast in ReferenceQueue.enqueue.

An alternative might be to use a raw type for the discovered field, but I
think that ends up with more @SuppressWarnings of various flavors. I think
the unbounded wildcard approach is clearer and cleaner.

Note that all of the pending list handling, including the discovered field,
could be moved into a non-public, non-generic, sealed(?) base class of
Reference. The pending list handling has nothing to do with the generic
parameter T.

Testing:
mach5 tier1 and tier4 (tier4 is where vmTestbase_vm_gc_ref tests are run)

/label core-libs
/summary Use unbounded wildcard placeholders and final safe but unchecked cast.

---

Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• Change must be properly reviewed

Issue

• JDK-8132984: incorrect type for Reference.discovered

Reviewers

• Roman Kennke (@rkennke - Reviewer) ⚠️ Review applies to b95f514
• Peter Levart (@plevart - Reviewer) ⚠️ Review applies to b95f514
• Roger Riggs (@RogerRiggs - Reviewer) ⚠️ Review applies to b95f514
• Mandy Chung (@mlchung - Reviewer) ⚠️ Review applies to b95f514

Download

$ git fetch https://git.openjdk.java.net/jdk pull/1897/head:pull/1897
$ git checkout pull/1897

[bridgekeeper]

👋 Welcome back kbarrett! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

@kimbarrett
The core-libs label was successfully added.

[openjdk]

@kimbarrett Setting summary to Use unbounded wildcard placeholders and final safe but unchecked cast.

[mlbridge]

Webrevs

• 03: Full - Incremental (cd66bff)
• 02: Full - Incremental (b95f514)
• 01: Full - Incremental (80415b7)
• 00: Full (ff250a1)

[plevart]

Hi Kim,

If you introduce a private method in Reference:

    private void enqueueFromPending() {
        var q = queue;
        if (q != ReferenceQueue.NULL) q.enqueue(this);
    }

...and use it Reference.processPendingReferences while loop like this:

            if (ref instanceof Cleaner) {
                ...
            } else {
                ref.enqueueFromPending();
            }

Then you can keep the signature of ReferenceQueue.enqueue(Reference<? extends T> r) and no unchecked casts are needed there.

But what you have is OK and much better than what was before.

[mlbridge]

Mailing list message from Kim Barrett on core-libs-dev:

On Jan 18, 2021, at 12:28 PM, Peter Levart <plevart at openjdk.java.net> wrote:
If you introduce a private method in Reference:

private void enqueueFromPending() {
var q = queue;
if (q != ReferenceQueue.NULL) q.enqueue(this);
}

...and use it Reference.processPendingReferences while loop like this:

       if \(ref instanceof Cleaner\) \{
           \.\.\.
       \} else \{
           ref\.enqueueFromPending\(\)\;
       \}

Then you can keep the signature of `ReferenceQueue.enqueue(Reference<? extends T> r)` and no unchecked casts are needed there.

Nice! And this reverts all changes to ReferenceQueue.java

But what you have is OK and much better than what was before.

Thanks, but I?m going to take your improvement. I?ll update the PR once I?ve re-run some tests.

[rkennke]

Looks good to me!

[openjdk]

@kimbarrett This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8132984: incorrect type for Reference.discovered

Use unbounded wildcard placeholders, plus a new helper to get back to the Reference<T> domain.

Reviewed-by: rkennke, plevart, rriggs, mchung

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[plevart]

This looks good.

[mlchung]

Looks good.

[kimbarrett]

Thanks @plevart , @rkennke , @RogerRiggs , and @mlchung for reviews.

[kimbarrett]

/summary Use unbounded wildcard placeholders, plus a new helper to get back to the Reference domain.

[openjdk]

@kimbarrett Updating existing summary to Use unbounded wildcard placeholders, plus a new helper to get back to the Reference<T> domain.

[kimbarrett]

/integrate

[openjdk]

@kimbarrett Pushed as commit 33dcc00.

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