presto-main/src/main/java/io/prestosql/execution/executor/TaskHandle.java/TaskHandle/pollNextSplit
class TaskHandle:    public synchronized PrioritizedSplitRunner pollNextSplit()
    {
        if (destroyed) {
            return null;
        }

        if (runningLeafSplits.size() >= concurrencyController.getTargetConcurrency()) {
            return null;
        }

        PrioritizedSplitRunner split = queuedLeafSplits.poll();
        if (split != null) {
            runningLeafSplits.add(split);
        }
        return split;
    }