func (m *Nitro) collectDead() {
	buf1 := m.snapshots.MakeBuf()
	buf2 := m.snapshots.MakeBuf()
	defer m.snapshots.FreeBuf(buf1)
	defer m.snapshots.FreeBuf(buf2)

	iter := m.gcsnapshots.NewIterator(CompareSnapshot, buf1)
	defer iter.Close()

	for iter.SeekFirst(); iter.Valid(); iter.Next() {
		node := iter.GetNode()
		sn := (*Snapshot)(node.Item())
		if sn.sn != m.lastGCSn+1 {
			return
		}

		m.lastGCSn = sn.sn
		m.gcchan <- sn.gclist
		m.gcsnapshots.DeleteNode(node, CompareSnapshot, buf2, &m.gcsnapshots.Stats)
	}
}
func (m *Nitro) collectDead() {
	buf1 := m.snapshots.MakeBuf()
	buf2 := m.snapshots.MakeBuf()
	defer m.snapshots.FreeBuf(buf1)
	defer m.snapshots.FreeBuf(buf2)

	iter := m.gcsnapshots.NewIterator(CompareSnapshot, buf1)
	defer iter.Close()

	for iter.SeekFirst(); iter.Valid(); iter.Next() {
		node := iter.GetNode()
		sn := (*Snapshot)(node.Item())
		if sn.sn != m.lastGCSn+1 {
			return
		}

		m.lastGCSn = sn.sn
		m.gcchan <- sn.gclist
		m.gcsnapshots.DeleteNode(node, CompareSnapshot, buf2, &m.gcsnapshots.Stats)
	}
}
// Invariant: Each snapshot n is dependent on snapshot n-1.
// Unless snapshot n-1 is collected, snapshot n cannot be collected.