RFR: 8339687: Rearrange reachabilityFence()s in jdk.test.lib.util.ForceGC

[Brent Christian]

>From the bug description:
ForceGC would be improved by moving the Reference.reachabilityFence() calls for 'obj' and 'ref'.

Reference.reachabilityFence(obj) is currently placed after 'obj' has been set to null, so effectively does nothing. It should occur before obj = null;

For Reference.reachabilityFence(ref): 'ref' is a PhantomReference to 'obj', and is registered with 'queue'. ForceGC.waitFor() later remove()s the reference from the queue, as an indication that some GC and reference processing has taken place (hopefully causing the BooleanSupplier to return true).

The code expects the PhantomReference to be cleared and be put on the queue. But recall that a Reference refers to its queue, and not the other way around. If a Reference becomes unreachable and is garbage collected, it will never be enqueued.

I argue that the VM/GC could determine that 'ref' is not used by waitFor() and collect it before the call to queue.remove(). Moving Reference.reachabilityFence(ref) after the for() loop would prevent this scenario.

While this is only a very minor deficiency in ForceGC, I believe it would be good to ensure that the code behaves as expected.

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Commit messages:
 - rearrange reachabilityFence() calls

Changes: https://git.openjdk.org/jdk/pull/20898/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20898&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339687
  Stats: 4 lines in 1 file changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/20898.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20898/head:pull/20898

PR: https://git.openjdk.org/jdk/pull/20898