func (o *oracle) readTs() uint64 {
	// 忽略 isManaged 部分逻辑

	var readTs uint64
	o.Lock()
	readTs = o.nextTxnTs - 1
	o.readMark.Begin(readTs)
	o.Unlock()

	// Wait for all txns which have no conflicts, have been assigned a commit
	// timestamp and are going through the write to value log and LSM tree
	// process. Not waiting here could mean that some txns which have been
	// committed would not be read.
	y.Check(o.txnMark.WaitForMark(context.Background(), readTs))
	return readTs
}

// NewIterator returns a new iterator. Depending upon the options, either only keys, or both
// key-value pairs would be fetched. The keys are returned in lexicographically sorted order.
// Using prefetch is recommended if you're doing a long running iteration, for performance.
//
// Multiple Iterators:
// For a read-only txn, multiple iterators can be running simultaneously.  However, for a read-write
// txn, iterators have the nuance of being a snapshot of the writes for the transaction at the time
// iterator was created. If writes are performed after an iterator is created, then that iterator
// will not be able to see those writes. Only writes performed before an iterator was created can be
// viewed.
func (txn *Txn) NewIterator(opt IteratorOptions) *Iterator {
	if txn.discarded {
		panic("Transaction has already been discarded")
	}
	if txn.db.IsClosed() {
		panic(ErrDBClosed.Error())
	}

	// Keep track of the number of active iterators.
	atomic.AddInt32(&txn.numIterators, 1)

	// TODO: If Prefix is set, only pick those memtables which have keys with
	// the prefix.
	tables, decr := txn.db.getMemTables()
	defer decr()
	txn.db.vlog.incrIteratorCount()
	var iters []y.Iterator
	if itr := txn.newPendingWritesIterator(opt.Reverse); itr != nil {
		iters = append(iters, itr)
	}
	for i := 0; i < len(tables); i++ {
		iters = append(iters, tables[i].sl.NewUniIterator(opt.Reverse))
	}
	iters = append(iters, txn.db.lc.iterators(&opt)...) // This will increment references.
	res := &Iterator{
		txn:    txn,
		iitr:   table.NewMergeIterator(iters, opt.Reverse),
		opt:    opt,
		readTs: txn.readTs,
	}
	return res
}

func (o *oracle) newCommitTs(txn *Txn) uint64 {
	o.Lock()
	defer o.Unlock()

	if o.hasConflict(txn) {
		return 0
	}

	var ts uint64
        o.doneRead(txn)
	o.cleanupCommittedTransactions()

	// This is the general case, when user doesn't specify the read and commit ts.
	ts = o.nextTxnTs
	o.nextTxnTs++
	o.txnMark.Begin(ts)

	y.AssertTrue(ts >= o.lastCleanupTs)

	if o.detectConflicts {
		// We should ensure that txns are not added to o.committedTxns slice when
		// conflict detection is disabled otherwise this slice would keep growing.
		o.committedTxns = append(o.committedTxns, committedTxn{
			ts:           ts,
			conflictKeys: txn.conflictKeys,
		})
	}

	return ts
}

// hasConflict must be called while having a lock.
  func (o *oracle) hasConflict(txn *Txn) bool {
      if len(txn.reads) == 0 {
          return false
      }
      for _, committedTxn := range o.committedTxns {
          // If the committedTxn.ts is less than txn.readTs that implies that the
          // committedTxn finished before the current transaction started.
          // We don't need to check for conflict in that case.
          // This change assumes linearizability. Lack of linearizability could
          // cause the read ts of a new txn to be lower than the commit ts of
          // a txn before it (@mrjn).
          if committedTxn.ts <= txn.readTs {
              continue
          }

          for _, ro := range txn.reads {
              if _, has := committedTxn.conflictKeys[ro]; has {
                  return true
              }
          }
      }

      return false
  }

func (o *oracle) cleanupCommittedTransactions() {
        // Must be called under o.Lock
	if !o.detectConflicts {
		// When detectConflicts is set to false, we do not store any
		// committedTxns and so there's nothing to clean up.
		return
	}
	// Same logic as discardAtOrBelow but unlocked
	var maxReadTs uint64
	if o.isManaged {
		maxReadTs = o.discardTs
	} else {
		maxReadTs = o.readMark.DoneUntil() // 在 readMark 堆中获取当前活跃事务的早 readTs
	}

	y.AssertTrue(maxReadTs >= o.lastCleanupTs)

	// do not run clean up if the maxReadTs (read timestamp of the
	// oldest transaction that is still in flight) has not increased
	if maxReadTs == o.lastCleanupTs {
		return
	}
	o.lastCleanupTs = maxReadTs

	tmp := o.committedTxns[:]
	for _, txn := range o.committedTxns {
		if txn.ts <= maxReadTs {
			continue
		}
		tmp = append(tmp, txn)
	}
	o.committedTxns = tmp
}