GH-2211 Fix DynamicModel upgrade flaky test failure

[abrokenjester]

GitHub issue resolved: #2211

This is a draft branch to address test failure in org.eclipse.rdf4j.model.DynamicModelConcurrentModificationAndUpgradeTest on Github CI. The test succeeds when ran on my local machine so it's flaky (which is not unusual with multithreaded stuff).

I am not immediately spotting a problem with either the test or the implementation, so opening this draft PR to see the problem (hoping the CI build at least consistently fails) and cooperate on a fix.

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PR Author Checklist:

• my pull request is self-contained
• I've added tests for the changes I made
• every commit message starts with the issue number (GH-xxxx) followed by a meaningful description of the change
• every commit has been signed off

Note: we merge all feature pull requests using squash and merge. See RDF4J git merge strategy for more details.

[abrokenjester]

Stacktrace for the thrown exception (when the test does succeed) is this:

java.lang.UnsupportedOperationException
	at java.util.Collections$UnmodifiableMap.put(Collections.java:1459)
	at org.eclipse.rdf4j.model.impl.DynamicModel.lambda$1(DynamicModel.java:268)
	at java.util.stream.ReferencePipeline$3$1.accept(ReferencePipeline.java:193)
	at java.util.Iterator.forEachRemaining(Iterator.java:116)
	at java.util.Spliterators$IteratorSpliterator.forEachRemaining(Spliterators.java:1801)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
	at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)

This is on trying to add a new statement to the private statements collection, a LinkedHashMap. I wonder if we're hitting a CPU caching issue (we're replacing the existing object reference with a new one when we mark it unmodifiable) , in which case perhaps just marking the field volatile could solve the problem.

[abrokenjester]

Grmbl, of course the build succeeds this time around :/ I'm going to push the volatile fix anyway, and then merge into my other issue branch to see if it solves the build failures in both cases.

[abrokenjester]

Both builds now succeed. This may just be luck of course, but I'm cautiously optimistic.

[hmottestad]

Build never failed for me on GitHub CI.

[abrokenjester]

Build never failed for me on GitHub CI.

Master branch is currently unstable. See https://github.com/eclipse/rdf4j/actions?query=workflow%3A%22master+status%22 . I know it only started failing after merging your Changeset improvements, but the test that fails has to do with the DynamicModel upgrade, so I am assuming it's intermittent and the first build just got lucky.

It also caused my PR build for a completely unrelated bug fix to fail by the way, so it's not just the one build, there's definitely something flaky.

[abrokenjester]

Unless you have objections I will merge this, and hope that it fixes the problem.

[hmottestad]

I don't want to use volatile there, for performance, and I think I've found something wrong with the latches in the test.