ConcurrentModificationException when unloading & reloading queries into Query Results View

[eclipse-viatra-bot]

| --- | --- | | Bugzilla Link | 535270 | | Status | NEW | | Importance | P3 normal | | Reported | May 29, 2018 07:46 EDT | | Modified | Nov 23, 2020 08:30 EDT | | Version | 2.0.0 | | See also | Gerrit change https://git.eclipse.org/r/123720, Git commit 76f48798 | | Reporter | Gabor Bergmann |

Description

When using the "Remove loaded queries" button on the Query Results View to unload and then reload queries from an open vql editor, the following exception is thrown:

!ENTRY org.eclipse.core.jobs 4 2 2018-05-29 13:35:19.763\ !MESSAGE An internal error occurred during: "Loading queries into the Query Result View".\ !STACK 0\ java.util.ConcurrentModificationException\ at java.util.HashMap$HashIterator.nextNode(HashMap.java:1437)\ at java.util.HashMap$KeyIterator.next(HashMap.java:1461)\ at com.google.common.collect.AbstractMapBasedMultimap$WrappedCollection$WrappedIterator.next(AbstractMapBasedMultimap.java:486)\ at org.eclipse.viatra.query.patternlanguage.emf.specification.SpecificationBuilder.forgetSpecificationTransitively(SpecificationBuilder.java:357)\ at org.eclipse.viatra.query.patternlanguage.emf.specification.SpecificationBuilder.forgetSpecificationTransitively(SpecificationBuilder.java:373)\ at org.eclipse.viatra.query.tooling.ui.queryresult.QueryResultTreeInput.removeMatcher(QueryResultTreeInput.java:295)\ at org.eclipse.viatra.query.tooling.ui.queryresult.QueryResultTreeInput.removeMatcher(QueryResultTreeInput.java:276)\ at org.eclipse.viatra.query.tooling.ui.queryresult.QueryResultTreeInput.loadQueries(QueryResultTreeInput.java:317)\ at org.eclipse.viatra.query.tooling.ui.queryresult.QueryResultView$1.run(QueryResultView.java:145)\ at org.eclipse.core.internal.jobs.Worker.run(Worker.java:56)

Likely cause: forgetSpecificationTransitively() recursively calls itself within an iteration over dependantQueries, but modifies said collection at the end of each nested call.

[eclipse-viatra-bot]

May 31, 2018 05:32

New Gerrit change created: https://git.eclipse.org/r/123720

[eclipse-viatra-bot]

By Zoltan Ujhelyi on May 31, 2018 05:46

I have tried to either reproduce or find the issue by code review, but was not too lucky. I also think that the case for the ConcurrentModification is not so simple as you suggest, as the method changes the dependantQueries, but iterates over sets contained in dependantQueries. Furthermore, after a specification is loaded, the stored set is immutable. To test this, I have added tests this functionality in the related change, and it has worked correctly.

The only way I can imagine the issue you have seen happening is when the SpecificationBuilder adds and removes queries in parallel. However, that should not be possible even though the patterns are loaded by a background thread for the following reason:

• There is a single background thread that loads the specifications. In other words, when you press the load patterns button, the patterns to load are added to a queue that is processed by said background thread.
• Unloading patterns does not happen in the said background thread, but before it can start, a synchronization step is called that is designed to avoid the situation mentioned above. While the unloading lasts, the UI thread is blocked so you shouldn't be able to load more patterns.\ \ In other words, I have no real idea what is the issue here, so any details you could provide would be very helpful.

[eclipse-viatra-bot]

May 31, 2018 10:02

Gerrit change https://git.eclipse.org/r/123720 was merged to [master].\ Commit: http://git.eclipse.org/c/viatra/org.eclipse.viatra.git/commit/?id=76f48798dd3e058dea7daec090dc1fe8818d5d4b

[eclipse-viatra-bot]

By Zoltan Ujhelyi on Jun 04, 2018 10:40

We had a long session with Gábor Bergmann today and we could not reproduce the original issue anymore. Given the original issue is not easy to trigger, but any potential fix might break other, more common use cases, I suggest postponing the fix to 2.1 (and possibly backporting it to a 2.0.x bugfix build).

[eclipse-viatra-bot]

By Zoltan Ujhelyi on Nov 20, 2019 05:51

Postponing issues to the next release.

[eclipse-viatra-bot]

By Zoltan Ujhelyi on May 18, 2020 13:04

Postponing issues that will not be solved for version 2.5.

[eclipse-viatra-bot]

By Zoltan Ujhelyi on Nov 23, 2020 08:30

Mass postponing of issues to the 2.6 timeframe.