Bug: can't use shortest path step

[peey]

janusgraph version: 0.5, using the cassandra/es configuration from janusgraph's official docker images

Steps to re-produce:

1. Start with a fresh instance of janusgraph
2. Create two vertices, with one having a directed edge to the other
3. In gremlin console, run g.withComputer().V().shortestPath() once, get results as expected
4. Run it again, and get an error

Error Stack:

jce-janusgraph     | java.lang.ClassCastException: org.apache.tinkerpop.gremlin.process.traversal.step.util.ImmutablePath cannot be cast to org.apache.tinkerpop.gremlin.structure.Vertex
jce-janusgraph     |    at org.apache.tinkerpop.gremlin.process.computer.search.path.ShortestPathVertexProgram.updateHaltedTraversers(ShortestPathVertexProgram.java:472)
jce-janusgraph     |    at org.apache.tinkerpop.gremlin.process.computer.search.path.ShortestPathVertexProgram.execute(ShortestPathVertexProgram.java:250)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.VertexProgramScanJob.process(VertexProgramScanJob.java:112)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.VertexJobConverter.process(VertexJobConverter.java:142)
jce-janusgraph     |    at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor$Processor.run(StandardScannerExecutor.java:291)
jce-janusgraph     | 363558 [Thread-244] ERROR org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor  - Exception processing row [232-  0-  0-  0-  0-  0-  0-128]:
jce-janusgraph     | java.lang.ClassCastException: org.apache.tinkerpop.gremlin.process.traversal.step.util.ImmutablePath cannot be cast to org.apache.tinkerpop.gremlin.structure.Vertex
jce-janusgraph     |    at org.apache.tinkerpop.gremlin.process.computer.search.path.ShortestPathVertexProgram.updateHaltedTraversers(ShortestPathVertexProgram.java:472)
jce-janusgraph     |    at org.apache.tinkerpop.gremlin.process.computer.search.path.ShortestPathVertexProgram.execute(ShortestPathVertexProgram.java:250)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.VertexProgramScanJob.process(VertexProgramScanJob.java:112)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.VertexJobConverter.process(VertexJobConverter.java:142)
jce-janusgraph     |    at org.janusgraph.diskstorage.keycolumnvalue.scan.StandardScannerExecutor$Processor.run(StandardScannerExecutor.java:291)
jce-janusgraph     | 363684 [gremlin-server-session-1] WARN  org.apache.tinkerpop.gremlin.server.op.AbstractEvalOpProcessor  - Exception processing a script on request [RequestMessage{, requestId=b1867572-11da-427b-9685-70222aad2c67, op='eval', processor='session', args={gremlin=g.withComputer().V().shortestPath(), session=694ac3ce-9953-47a5-8467-16ea4d66506e, manageTransaction=false, userAgent=Gremlin Console/3.4.6, batchSize=64}}].
jce-janusgraph     | org.janusgraph.core.JanusGraphException: Failed to process [2] vertices in vertex program iteration [5]. Computer is aborting.
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.FulgoraGraphComputer.executeOnNonPartitionedVertices(FulgoraGraphComputer.java:268)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.FulgoraGraphComputer.executeIterationOfJob(FulgoraGraphComputer.java:233)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.FulgoraGraphComputer.executeVertexProgram(FulgoraGraphComputer.java:214)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.FulgoraGraphComputer.submitAsync(FulgoraGraphComputer.java:193)
jce-janusgraph     |    at org.janusgraph.graphdb.olap.computer.FulgoraGraphComputer.lambda$submit$0(FulgoraGraphComputer.java:156)
jce-janusgraph     |    at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1604)
jce-janusgraph     |    at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1596)
jce-janusgraph     |    at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
jce-janusgraph     |    at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
jce-janusgraph     |    at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
jce-janusgraph     |    at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:175)

Additional Info: I believe that this happens because the ShortestPathVertexProgram OLAP uses the vertex property TraversalVertexProgram.HALTED_TRAVERSERS to store traversers (with ImmutablePath objects). I think that this property perhaps is ought to be cleared by JanusGraph once the query is over, but it isn't cleared. After step 3, you can still see this property existing on the vertices.

Please re-produce this bug and open a github issue if confirmed.

[peey]

Further, I believe that the following line might be responsible for suppressing/circumventing some tinkerpop compliance tests which may've highlighted this issue

https://github.com/JanusGraph/janusgraph/blame/master/janusgraph-core/src/main/java/org/janusgraph/graphdb/olap/computer/VertexMemoryHandler.java#L76