func (db *DB) newMem(n int) (mem *memDB, err error) {
	num := db.s.allocFileNum()
	file := db.s.getJournalFile(num)
	w, err := file.Create()
	if err != nil {
		db.s.reuseFileNum(num)
		return
	}
	db.memMu.Lock()
	defer db.memMu.Unlock()
	if db.frozenMem != nil {
		panic("still has frozen mem")
	}
	if db.journal == nil {
		db.journal = journal.NewWriter(w)
	} else {
		db.journal.Reset(w)
		db.journalWriter.Close()
		db.frozenJournalFile = db.journalFile
	}
	db.journalWriter = w
	db.journalFile = file
	db.frozenMem = db.mem
	mdb := db.mpoolGet()
	if mdb == nil || mdb.Capacity() < n {
		mdb = memdb.New(db.s.icmp, maxInt(db.s.o.GetWriteBuffer(), n))
	}
	mem = &memDB{
		db:  db,
		DB:  mdb,
		ref: 2,
	}
	db.mem = mem
	db.frozenSeq = db.seq
	return
}
Create new memdb and froze the old one; need external synchronization.
newMem only called synchronously by the writer.
package leveldb
import (
	"sync/atomic"
	"time"
	"github.com/syndtr/goleveldb/leveldb/journal"
	"github.com/syndtr/goleveldb/leveldb/memdb"
)
type memDB struct {
	db *DB
	*memdb.DB
	ref int32
}
func (m *memDB) incref() {
	atomic.AddInt32(&m.ref, 1)
}
func (m *memDB) decref() {
	if ref := atomic.AddInt32(&m.ref, -1); ref == 0 {
		if m.Capacity() == m.db.s.o.GetWriteBuffer() {
			m.Reset()
			m.db.mpoolPut(m.DB)
		}
		m.db = nil
		m.DB = nil
	} else if ref < 0 {
		panic("negative memdb ref")
	}
}
func (db *DB) getSeq() uint64 {
	return atomic.LoadUint64(&db.seq)
}
func (db *DB) addSeq(delta uint64) {
	atomic.AddUint64(&db.seq, delta)
}
func (db *DB) sampleSeek(ikey iKey) {
	v := db.s.version()
	if v.sampleSeek(ikey) {
		db.compSendTrigger(db.tcompCmdC)
	}
	v.release()
}
func (db *DB) mpoolPut(mem *memdb.DB) {
	defer func() {
		recover()
	}()
	select {
	case db.memPool <- mem:
	default:
	}
}
func (db *DB) mpoolGet() *memdb.DB {
	select {
	case mem := <-db.memPool:
		return mem
	default:
		return nil
	}
}
func (db *DB) mpoolDrain() {
	ticker := time.NewTicker(30 * time.Second)
	for {
		select {
		case <-ticker.C:
			select {
			case <-db.memPool:
			default:
			}
		case _, _ = <-db.closeC:
			close(db.memPool)
			return
		}
	}
}