code: public Txn getTxn() throws Exception {
          mLock.lock();
          try {
              checkClosed();
              return mActive == null ? null : mActive.getTxn();
          } finally {
              mLock.unlock();
          }
      }
docs: Returns the implementation for the active transaction, or null if there
  is no active transaction.
  
  @throws Exception thrown by createTxn or reuseTxn