Subgraph() step not working via GraphBinary

The subgraph() step returns a Graph instance via GraphBinary. This however results in an exception for JanusGraph (see stack trace below).

I first found this with our productive setup which uses CQL + ES, but could also reproduce it with the vanilla Docker image using Berkeleyje + Lucene by just adding two vertices and an edge between them and then executing the traversal (see steps to reproduce).

While this sounds similar to #2843, I think that it's a different issue because there the problem is that no serializer for the type can be found. With GraphBinary here we're getting a completely different exception and the stack trace already shows that the correct serializer for the graph type was used: GraphSerializer.

For confirmed bugs, please report:

Version: 0.6.1
Storage Backend: berkeleyje / cql
Mixed Index Backend: lucene / elasticsearch
Link to discussed bug:
Expected Behavior: The subgraph should be returned to the client.
Current Behavior: See the exception below.
Steps to Reproduce: Configure GraphBinary in the client and execute a traversal that returns a graph, e.g,, g.E().subgraph('sg').cap('sg').next().

Stack Trace (if you have one)

org.apache.tinkerpop.gremlin.server.op.AbstractEvalOpProcessor  - The result [[tinkergraph[vertices:2 edges:1]]] in the request 80f14092-8ace-410b-bde9-46d4bd857f3b could not be serialized and returned.
java.lang.IllegalStateException
        at com.google.common.base.Preconditions.checkState(Preconditions.java:492)
        at org.janusgraph.graphdb.relations.RelationIdentifier.getInVertexId(RelationIdentifier.java:63)
        at org.janusgraph.graphdb.tinkerpop.io.binary.RelationIdentifierGraphBinarySerializer.writeNonNullableValue(RelationIdentifierGraphBinarySerializer.java:43)
        at org.janusgraph.graphdb.tinkerpop.io.binary.RelationIdentifierGraphBinarySerializer.writeNonNullableValue(RelationIdentifierGraphBinarySerializer.java:24)
        at org.janusgraph.graphdb.tinkerpop.io.binary.JanusGraphTypeSerializer.writeValue(JanusGraphTypeSerializer.java:89)
        at org.janusgraph.graphdb.tinkerpop.io.binary.JanusGraphTypeSerializer.write(JanusGraphTypeSerializer.java:72)
        at org.apache.tinkerpop.gremlin.structure.io.binary.GraphBinaryWriter.write(GraphBinaryWriter.java:98)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.GraphSerializer.writeVertex(GraphSerializer.java:138)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.GraphSerializer.writeValue(GraphSerializer.java:120)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.GraphSerializer.writeValue(GraphSerializer.java:45)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.SimpleTypeSerializer.writeValue(SimpleTypeSerializer.java:91)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.SimpleTypeSerializer.write(SimpleTypeSerializer.java:73)
        at org.apache.tinkerpop.gremlin.structure.io.binary.GraphBinaryWriter.write(GraphBinaryWriter.java:112)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.CollectionSerializer.writeValue(CollectionSerializer.java:52)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.ListSerializer.writeValue(ListSerializer.java:44)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.ListSerializer.writeValue(ListSerializer.java:29)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.SimpleTypeSerializer.writeValue(SimpleTypeSerializer.java:91)
        at org.apache.tinkerpop.gremlin.structure.io.binary.types.SimpleTypeSerializer.write(SimpleTypeSerializer.java:73)
        at org.apache.tinkerpop.gremlin.structure.io.binary.GraphBinaryWriter.write(GraphBinaryWriter.java:112)
        at org.apache.tinkerpop.gremlin.driver.ser.binary.ResponseMessageSerializer.writeValue(ResponseMessageSerializer.java:84)
        at org.apache.tinkerpop.gremlin.driver.ser.GraphBinaryMessageSerializerV1.serializeResponseAsBinary(GraphBinaryMessageSerializerV1.java:150)
        at org.apache.tinkerpop.gremlin.server.op.AbstractOpProcessor.makeFrame(AbstractOpProcessor.java:291)
        at org.apache.tinkerpop.gremlin.server.op.AbstractOpProcessor.handleIterator(AbstractOpProcessor.java:167)
        at org.apache.tinkerpop.gremlin.server.op.AbstractEvalOpProcessor.lambda$evalOpInternal$5(AbstractEvalOpProcessor.java:263)
        at org.apache.tinkerpop.gremlin.groovy.engine.GremlinExecutor.lambda$eval$0(GremlinExecutor.java:283)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)

JanusGraph / janusgraph

Subgraph() step not working via GraphBinary #2969

Stack Trace (if you have one)