docs: synchronized public void prepareAcceptMastership(int partitionId) {
          // can't acquire mastership twice for the same partition id
          if (!m_masterOfPartitions.add(partitionId)) {
              return;
          }
          if (exportLog.isDebugEnabled()) {
              exportLog.debug("Export streams on local partition " + partitionId + " will become master.");
          }
      }