Github user mpollmeier commented on the issue:
https://github.com/apache/tinkerpop/pull/715
`emit` modifies the behaviour of the repeat traversal, and I am unsure why
that is. Let me explain what I know and hopefully you or someone else can fill
the blanks.
Let's take my toy graph `v5 <- v3 <- v1 <- v0 -> v2 -> v4 -> v6` since it's
easier to see where you are with a simple println of the vertex. Without emit,
this PR does what it's supposed to do: DFS. First the even numbers, then the
odds.
```
graph = TinkerGraph.open()
v0 = graph.addVertex("l0")
v1 = graph.addVertex("l1")
v2 = graph.addVertex("l1")
v3 = graph.addVertex("l2")
v4 = graph.addVertex("l2")
v5 = graph.addVertex("l3")
v6 = graph.addVertex("l3")
v0.addEdge("e", v2)
v2.addEdge("e", v4)
v4.addEdge("e", v6)
v0.addEdge("e", v1)
v1.addEdge("e", v3)
v3.addEdge("e", v5)
graph.traversal().V(v0).repeat(sideEffect{println("inside repeat at " +
it)}.out()).times(3)
inside repeat at v[0]
inside repeat at v[2]
inside repeat at v[4]
==>v[6]
inside repeat at v[1]
inside repeat at v[3]
==>v[5]
```
However if I add an `emit()` it becomes BFS again:
```
graph.traversal().V(v0).emit().repeat(sideEffect{println("inside repeat at
" + it)}.out()).times(3)
==>v[0]
inside repeat at v[0]
==>v[2]
inside repeat at v[2]
==>v[1]
inside repeat at v[1]
==>v[4]
inside repeat at v[4]
==>v[6]
==>v[3]
inside repeat at v[3]
==>v[5]
```
I tried to understand it in the debugger, and the core difference seems to
be the behaviour of `this.starts.hasNext()` in
`RepeatEndStep.standardAlgorithm`, which is an `ExpandableStepIterator`.
* without emit, `hasNext` finds that
`this.hostStep.getPreviousStep().hasNext()` is true, because it asks it's
`previousStep` (which is a `VertexStep(OUT)`), which in turns has some results
for `processNextStart`.
* with emit, the above chain returns false, which in turn leads to a
NoSuchElementException, which short-circuits and `while (true)` loop, which
results in losing DFS semantics.
I don't know where these different semantics derive from.
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