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https://issues.apache.org/jira/browse/TINKERPOP-3071?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18096881#comment-18096881
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Cole Greer commented on TINKERPOP-3071:
---------------------------------------
I'm closing this JIRA with code changes given the patch in Java 9
(https://bugs.openjdk.org/browse/JDK-8170733), and the fact that Java 8 support
has been discontinued as of 3.8.0.
> barrier() reliant on undefined HashSet behavior
> -----------------------------------------------
>
> Key: TINKERPOP-3071
> URL: https://issues.apache.org/jira/browse/TINKERPOP-3071
> Project: TinkerPop
> Issue Type: Improvement
> Components: process
> Affects Versions: 3.6.7, 3.7.2
> Reporter: Cole Greer
> Priority: Major
>
> Certain traversals can force the barrier step into relying on undefined
> behavior in Java. The barrier step operates by storing all incoming
> traversers into a HashSet, and only once the input is exhausted, will it
> release traversers to the next step. The contract of HashSet requires that
> users do not modify objects in any way which will change their hash code
> while it is stored in the HashSet. It is undefined how a HashSet will behave
> if the hash codes of a contained object changes.
> Some very specific types of traversals can force barrier() into such
> behavior. Consider the following traversal:
> {code:java}
> g.V().as("label").aggregate(local,"x").select("x").barrier().select("label")
> {code}
> The aggregate step will attach a BulkSet to each traverser, as "x". It is key
> here that each traverser is given a reference to the same BulkSet. The
> execution of this traverser will go as follows:
> 1. The first traverser moves through the traversal, is given the BulkSet
> (currently size 1) as "x"
> 2. The first traverser is stored in the barrier. At this time,
> hashcode(Traverser1) = hashcode(BulkSetSize1).
> 3. The second traverser moves through the traversal, as it passes through the
> aggregate step, it is added to the BulkSet, the BulkSet now has a size of 2.
> 4. The hashcode of Traverser1 has now changed. Now, hashcode(Traverser1) =
> hashcode(BulkSetSize2).
> What happens next will depend on the version of Java being used.
> Currently in Java 8, the HashSet can be iterated and Traverser1 can be read,
> but it can never be deleted as any attempt to find Traverser1 in the set by
> it's current hash code will fail. The result of this is that the iterator
> will get stuck on the first element. Repeated calls to .next() on the HashSet
> iterator will always return Traverser1. This causes the traversal to get
> stuck in a loop until it runs out of resources.
> In Java 9 and up, HashSet has been
> [patched|https://bugs.openjdk.org/browse/JDK-8170733] such that elements with
> changed hash codes can be deleted, which allows the above traversal to run as
> expected. We are still using HashSet wrong, but we aren't getting punished
> for it in Java 9+.
> There isn't a clear obvious solution for such traversals. The fix with the
> lowest impact that I'm currently aware of would be for barrier step to "box"
> each incoming traverser in some wrapper which computes the hash code of the
> internal object once, and stores it. Successive calls to hashcode() on the
> box, will always return the cached value regardless of if the internal object
> has been modified. This would prevent the hashcode from changing (from the
> perspective of the HashSet) while the traversers are being stored in the
> barrier.
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