Dan LaRocque created TINKERPOP-1397:
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Summary: StarVertex self edge has buggy interaction with graph
filters
Key: TINKERPOP-1397
URL: https://issues.apache.org/jira/browse/TINKERPOP-1397
Project: TinkerPop
Issue Type: Bug
Reporter: Dan LaRocque
When StarVertex adds a self-loop, it adds an instance of concrete type
{{StarOutEdge}} to both its {{outEdges}} map and its {{inEdges}} map.
https://github.com/apache/tinkerpop/blob/8f7218d53a31cf41f4a0269d64ac1c27dfc0907a/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/star/StarGraph.java#L321-L330
(line 329 adds a {{StarOutEdge}} to the {{inEdges}} map)
Having a {{StarOutEdge}} in the {{inEdges}} map mostly doesn't matter.
However, this does matter in the {{applyGraphFilter}} method. This method uses
{{instanceof StarOutEdge}} to guess whether an edge's original direction was in
or out:
https://github.com/apache/tinkerpop/blob/8f7218d53a31cf41f4a0269d64ac1c27dfc0907a/gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/structure/util/star/StarGraph.java#L470
If you trace {{applyGraphFilter}} through, you see that a {{StarVertex}} with a
self-loop can pop out of {{applyGraphFilter}} with two out edges corresponding
to the self loop and no in edges. This leads to weird traversal results. One
way to trigger this is to write a {{GraphFilterAware}} {{InputRDD}} that
produces {{StarVertex}} instances and then run a {{g.V()....inE("somelabel")}},
so that TP pushes down the "somelabel" constraint, which is how I got here.
I think {{addEdge}} should continue putting a {{StarOutEdge}} into
{{outEdges}}, like it already does. However, it should put a {{StarInEdge}}
into {{inEdges}}. This would increase the object overhead associated with
StarVertices that have self loops (two edge objs instead of one). If we wanted
to be obsessive about optimizing this case we could probably pervert the
{{inEdge}}/{{outEdge}} datastructure contents to do it, but IMHO that's not
worth it.
The change I'm suggesting is a one-liner. I'll make a PR soon.
I don't know all of the versions this affects, but I do know it affects 3.2.1.
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