[TINKERPOP-2475] Barrier step touches one more element of next loop

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 3.4.8
• Fix Version/s: 3.5.0, 3.4.9
• Component/s: process
• Assignee: Stephen Mallette
• Reporter: Liu-Delin
• Created: 17/Nov/20 06:37
• Updated: 17/Nov/20 23:23
• Resolved: 17/Nov/20 20:45

Description

This bug will preload one more element in barrier action. For example, if we have 4 vertices, and we have a barrier(2) step, we expect to get the first 2 in a barrier then the last 2 in another barrier loop. But when we get the first 2 vertices, the third one has already been loaded.

The root cause is below code: 

@Override
public void processAllStarts() {
    while (this.starts.hasNext() && (this.maxBarrierSize == Integer.MAX_VALUE || this.barrier.size() < this.maxBarrierSize)) {
        final Traverser.Admin<S> traverser = this.starts.next();
        traverser.setStepId(this.getNextStep().getId()); // when barrier is reloaded, the traversers should be at the next step
        this.barrier.add(traverser);
    }
}

In while loop, we use this.starts.hasNext() at first, and it will cause the vertex loading action. It will affect the performance when the vertex is saved in DB instead of memory.