@Override
  public Procedure poll(long timeout, TimeUnit unit) {
    return poll(unit.toNanos(timeout));
  }
Fetch one Procedure from the queue NOTE: this method is called with the sched lock held.

@return the Procedure to execute, or null if nothing is available.
iotdb-core/confignode/src/main/java/org/apache/iotdb/confignode/procedure/scheduler/AbstractProcedureScheduler.java